[PATCH v9 0/9] Add latency priority for CFS class

[PATCH v9 0/9] Add latency priority for CFS class

[Vincent Guittot]

This patchset restarts the work about adding a latency priority to describe
the latency tolerance of cfs tasks.

Patch [1] is a new one that has been added with v6. It fixes an
unfairness for low prio tasks because of wakeup_gran() being bigger
than the maximum vruntime credit that a waking task can keep after
sleeping.

The patches [2-4] have been done by Parth:
https://lore.kernel.org/lkml/20200228090755.22829-1-parth@linux.ibm.com/

I have just rebased and moved the set of latency priority outside the
priority update. I have removed the reviewed tag because the patches
are 2 years old.

This aims to be a generic interface and the following patches is one use
of it to improve the scheduling latency of cfs tasks.

Patch [5] uses latency nice priority to define a latency offset
and then decide if a cfs task can or should preempt the current
running task. The patch gives some tests results with cyclictests and
hackbench to highlight the benefit of latency priority for short
interactive task or long intensive tasks.

Patch [6] adds the support of latency nice priority to task group by
adding a cpu.latency.nice field. The range is [-20:19] as for setting task
latency priority.

Patch [7] makes sched_core taking into account the latency offset.

Patch [8] adds a rb tree to cover some corner cases where the latency
sensitive task (priority < 0) is preempted by high priority task (RT/DL)
or fails to preempt them. This patch ensures that tasks will have at least
a slice of sched_min_granularity in priority at wakeup.

Patch [9] removes useless check after adding a latency rb tree.

I have also backported the patchset on a dragonboard RB3 with an android
mainline kernel based on v5.18 for a quick test. I have used the
TouchLatency app which is part of AOSP and described to be a very good
test to highlight jitter and jank frame sources of a system [1].
In addition to the app, I have added some short running tasks waking-up
regularly (to use the 8 cpus for 4 ms every 37777us) to stress the system
without overloading it (and disabling EAS). The 1st results shows that the
patchset helps to reduce the missed deadline frames from 5% to less than
0.1% when the cpu.latency.nice of task group are set. I haven't rerun the
test with latest version.

I have also tested the patchset with the modified version of the alsa
latency test that has been shared by Tim. The test quickly xruns with
default latency nice priority 0 but is able to run without underuns with
a latency -20 and hackbench running simultaneously.

While preparing the version 8, I have evaluated the benefit of using an
augmented rbtree instead of adding a rbtree for latency sensitive entities,
which was a relevant suggestion done by PeterZ. Although the augmented
rbtree enables to sort additional information in the tree with a limited
overhead, it has more impact on legacy use cases (latency_nice >= 0)
because the augmented callbacks are always called to maintain this
additional information even when there is no sensitive tasks. In such
cases, the dedicated rbtree remains empty and the overhead is reduced to
loading a cached null node pointer. Nevertheless, we might want to
reconsider the augmented rbtree once the use of negative latency_nice will
be more widlely deployed. At now, the different tests that I have done,
have not shown improvements with augmented rbtree.

Below are some hackbench results:
        2 rbtrees               augmented rbtree        augmented rbtree	
                                sorted by vruntime      sorted by wakeup_vruntime
sched	pipe	
avg     26311,000               25976,667               25839,556
stdev   0,15 %                  0,28 %                  0,24 %
vs tip  0,50 %                  -0,78 %                 -1,31 %
hackbench	1 group	
avg     1,315                   1,344                   1,359
stdev   0,88 %                  1,55 %                  1,82 %
vs tip  -0,47 %                 -2,68 %                 -3,87 %
hackbench	4 groups
avg     1,339                   1,365                   1,367
stdev   2,39 %                  2,26 %                  3,58 %
vs tip  -0,08 %                 -2,01 %                 -2,22 %
hackbench	8 groups
avg     1,233                   1,286                   1,301
stdev   0,74 %                  1,09 %                  1,52 %
vs tip  0,29 %                  -4,05 %                 -5,27 %
hackbench	16 groups	
avg     1,268                   1,313                   1,319
stdev   0,85 %                  1,60 %                  0,68 %
vs tip  -0,02 %                 -3,56 %                 -4,01 %

[1] https://source.android.com/docs/core/debug/eval_perf#touchlatency

Change since v8:
- Rename get_sched_latency by get_sleep_latency
- move latency nice defines in sched/prio.h and fix latency_prio init value
- Fix typo and comments

Change since v7:
- Replaced se->on_latency by using RB_CLEAR_NODE() and RB_EMPTY_NODE()
- Clarify the limit behavior fo the cgroup cpu.latenyc_nice

Change since v6:
- Fix compilation error for !CONFIG_SCHED_DEBUG

Change since v5:
- Add patch 1 to fix unfairness for low prio task. This has been
  discovered while studying Youssef's tests results with latency nice
  which were hitting the same problem.
- Fixed latency_offset computation to take into account
  GENTLE_FAIR_SLEEPERS. This has diseappeared with v2and has been raised
  by Youssef's tests.
- Reworked and optimized how latency_offset in used to check for
  preempting current task at wakeup and tick. This cover more cases too.
- Add patch 9 to remove check_preempt_from_others() which is not needed
  anymore with the rb tree.

Change since v4:
- Removed permission checks to set latency priority. This enables user
  without elevated privilege like audio application to set their latency
  priority as requested by Tim.
- Removed cpu.latency and replaced it by cpu.latency.nice so we keep a
  generic interface not tied to latency_offset which can be used to
  implement other latency features.
- Added an entry in Documentation/admin-guide/cgroup-v2.rst to describe
  cpu.latency.nice.
- Fix some typos.

Change since v3:
- Fix 2 compilation warnings raised by kernel test robot <lkp@intel.com>

Change since v2:
- Set a latency_offset field instead of saving a weight and computing it
  on the fly.
- Make latency_offset available for task group: cpu.latency
- Fix some corner cases to make latency sensitive tasks schedule first and
  add a rb tree for latency sensitive task.

Change since v1:
- fix typo
- move some codes in the right patch to make bisect happy
- simplify and fixed how the weight is computed
- added support of sched core patch 7

Parth Shah (3):
  sched: Introduce latency-nice as a per-task attribute
  sched/core: Propagate parent task's latency requirements to the child
    task
  sched: Allow sched_{get,set}attr to change latency_nice of the task

Vincent Guittot (6):
  sched/fair: fix unfairness at wakeup
  sched/fair: Take into account latency priority at wakeup
  sched/fair: Add sched group latency support
  sched/core: Support latency priority with sched core
  sched/fair: Add latency list
  sched/fair: remove check_preempt_from_others

 Documentation/admin-guide/cgroup-v2.rst |  10 ++
 include/linux/sched.h                   |   4 +
 include/linux/sched/prio.h              |  27 +++
 include/uapi/linux/sched.h              |   4 +-
 include/uapi/linux/sched/types.h        |  19 +++
 init/init_task.c                        |   1 +
 kernel/sched/core.c                     | 106 ++++++++++++
 kernel/sched/debug.c                    |   1 +
 kernel/sched/fair.c                     | 209 ++++++++++++++++++++----
 kernel/sched/sched.h                    |  45 ++++-
 tools/include/uapi/linux/sched.h        |   4 +-
 11 files changed, 394 insertions(+), 36 deletions(-)

-- 
2.17.1

[PATCH v9 1/9] sched/fair: fix unfairness at wakeup

[Vincent Guittot]

At wake up, the vruntime of a task is updated to not be more older than
a sched_latency period behind the min_vruntime. This prevents long sleeping
task to get unlimited credit at wakeup.
Such waking task should preempt current one to use its CPU bandwidth but
wakeup_gran() can be larger than sched_latency, filter out the
wakeup preemption and as a results steals some CPU bandwidth to
the waking task.

Make sure that a task, which vruntime has been capped, will preempt current
task and use its CPU bandwidth even if wakeup_gran() is in the same range
as sched_latency.

If the waking task failed to preempt current it could to wait up to
sysctl_sched_min_granularity before preempting it during next tick.

Strictly speaking, we should use cfs->min_vruntime instead of
curr->vruntime but it doesn't worth the additional overhead and complexity
as the vruntime of current should be close to min_vruntime if not equal.

Reported-by: Youssef Esmat <youssefesmat@chromium.org>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c  | 46 ++++++++++++++++++++------------------------
 kernel/sched/sched.h | 34 +++++++++++++++++++++++++++++++-
 2 files changed, 54 insertions(+), 26 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4cc56c91e06e..c8a697f8db88 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4645,33 +4645,17 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
 {
 	u64 vruntime = cfs_rq->min_vruntime;
 
-	/*
-	 * The 'current' period is already promised to the current tasks,
-	 * however the extra weight of the new task will slow them down a
-	 * little, place the new task so that it fits in the slot that
-	 * stays open at the end.
-	 */
-	if (initial && sched_feat(START_DEBIT))
-		vruntime += sched_vslice(cfs_rq, se);
-
-	/* sleeps up to a single latency don't count. */
-	if (!initial) {
-		unsigned long thresh;
-
-		if (se_is_idle(se))
-			thresh = sysctl_sched_min_granularity;
-		else
-			thresh = sysctl_sched_latency;
-
+	if (!initial)
+		/* sleeps up to a single latency don't count. */
+		vruntime -= get_sleep_latency(se_is_idle(se));
+	else if (sched_feat(START_DEBIT))
 		/*
-		 * Halve their sleep time's effect, to allow
-		 * for a gentler effect of sleepers:
+		 * The 'current' period is already promised to the current tasks,
+		 * however the extra weight of the new task will slow them down a
+		 * little, place the new task so that it fits in the slot that
+		 * stays open at the end.
 		 */
-		if (sched_feat(GENTLE_FAIR_SLEEPERS))
-			thresh >>= 1;
-
-		vruntime -= thresh;
-	}
+		vruntime += sched_vslice(cfs_rq, se);
 
 	/* ensure we never gain time by being placed backwards. */
 	se->vruntime = max_vruntime(se->vruntime, vruntime);
@@ -7520,6 +7504,18 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
 		return -1;
 
 	gran = wakeup_gran(se);
+
+	/*
+	 * At wake up, the vruntime of a task is capped to not be older than
+	 * a sched_latency period compared to min_vruntime. This prevents long
+	 * sleeping task to get unlimited credit at wakeup. Such waking up task
+	 * has to preempt current in order to not lose its share of CPU
+	 * bandwidth but wakeup_gran() can become higher than scheduling period
+	 * for low priority task. Make sure that long sleeping task will get a
+	 * chance to preempt current.
+	 */
+	gran = min_t(s64, gran, get_latency_max());
+
 	if (vdiff > gran)
 		return 1;
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 771f8ddb7053..842ce0094d9c 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2453,9 +2453,9 @@ extern void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags);
 extern const_debug unsigned int sysctl_sched_nr_migrate;
 extern const_debug unsigned int sysctl_sched_migration_cost;
 
-#ifdef CONFIG_SCHED_DEBUG
 extern unsigned int sysctl_sched_latency;
 extern unsigned int sysctl_sched_min_granularity;
+#ifdef CONFIG_SCHED_DEBUG
 extern unsigned int sysctl_sched_idle_min_granularity;
 extern unsigned int sysctl_sched_wakeup_granularity;
 extern int sysctl_resched_latency_warn_ms;
@@ -2470,6 +2470,38 @@ extern unsigned int sysctl_numa_balancing_scan_size;
 extern unsigned int sysctl_numa_balancing_hot_threshold;
 #endif
 
+static inline unsigned long get_sleep_latency(bool idle)
+{
+	unsigned long thresh;
+
+	if (idle)
+		thresh = sysctl_sched_min_granularity;
+	else
+		thresh = sysctl_sched_latency;
+
+	/*
+	 * Halve their sleep time's effect, to allow
+	 * for a gentler effect of sleepers:
+	 */
+	if (sched_feat(GENTLE_FAIR_SLEEPERS))
+		thresh >>= 1;
+
+	return thresh;
+}
+
+static inline unsigned long get_latency_max(void)
+{
+	unsigned long thresh = get_sleep_latency(false);
+
+	 /*
+	  * If the waking task failed to preempt current it could to wait up to
+	  * sysctl_sched_min_granularity before preempting it during next tick.
+	  */
+	thresh -= sysctl_sched_min_granularity;
+
+	return thresh;
+}
+
 #ifdef CONFIG_SCHED_HRTICK
 
 /*
-- 
2.17.1

[PATCH v9 2/9] sched: Introduce latency-nice as a per-task attribute

[Vincent Guittot]

From: Parth Shah <parth@linux.ibm.com>

Latency-nice indicates the latency requirements of a task with respect
to the other tasks in the system. The value of the attribute can be within
the range of [-20, 19] both inclusive to be in-line with the values just
like task nice values.

latency_nice = -20 indicates the task to have the least latency as
compared to the tasks having latency_nice = +19.

The latency_nice may affect only the CFS SCHED_CLASS by getting
latency requirements from the userspace.

Additionally, add debugging bits for newly added latency_nice attribute.

Signed-off-by: Parth Shah <parth@linux.ibm.com>
[rebase, move defines in sched/prio.h]
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 include/linux/sched.h      |  1 +
 include/linux/sched/prio.h | 18 ++++++++++++++++++
 kernel/sched/debug.c       |  1 +
 3 files changed, 20 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 23de7fe86cc4..856240573300 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -784,6 +784,7 @@ struct task_struct {
 	int				static_prio;
 	int				normal_prio;
 	unsigned int			rt_priority;
+	int				latency_nice;
 
 	struct sched_entity		se;
 	struct sched_rt_entity		rt;
diff --git a/include/linux/sched/prio.h b/include/linux/sched/prio.h
index ab83d85e1183..bfcd7f1d1e11 100644
--- a/include/linux/sched/prio.h
+++ b/include/linux/sched/prio.h
@@ -42,4 +42,22 @@ static inline long rlimit_to_nice(long prio)
 	return (MAX_NICE - prio + 1);
 }
 
+/*
+ * Latency nice is meant to provide scheduler hints about the relative
+ * latency requirements of a task with respect to other tasks.
+ * Thus a task with latency_nice == 19 can be hinted as the task with no
+ * latency requirements, in contrast to the task with latency_nice == -20
+ * which should be given priority in terms of lower latency.
+ */
+#define MAX_LATENCY_NICE	19
+#define MIN_LATENCY_NICE	-20
+
+#define LATENCY_NICE_WIDTH	\
+	(MAX_LATENCY_NICE - MIN_LATENCY_NICE + 1)
+
+/*
+ * Default tasks should be treated as a task with latency_nice = 0.
+ */
+#define DEFAULT_LATENCY_NICE	0
+
 #endif /* _LINUX_SCHED_PRIO_H */
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 1637b65ba07a..68be7a3e42a3 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -1043,6 +1043,7 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
 #endif
 	P(policy);
 	P(prio);
+	P(latency_nice);
 	if (task_has_dl_policy(p)) {
 		P(dl.runtime);
 		P(dl.deadline);
-- 
2.17.1

[PATCH v9 3/9] sched/core: Propagate parent task's latency requirements to the child task

[Vincent Guittot]

From: Parth Shah <parth@linux.ibm.com>

Clone parent task's latency_nice attribute to the forked child task.

Reset the latency_nice value to default value when the child task is
set to sched_reset_on_fork.

Also, initialize init_task.latency_nice value with DEFAULT_LATENCY_NICE
value

Signed-off-by: Parth Shah <parth@linux.ibm.com>
[rebase]
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 init/init_task.c    | 1 +
 kernel/sched/core.c | 1 +
 2 files changed, 2 insertions(+)

diff --git a/init/init_task.c b/init/init_task.c
index ff6c4b9bfe6b..7dd71dd2d261 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -78,6 +78,7 @@ struct task_struct init_task
 	.prio		= MAX_PRIO - 20,
 	.static_prio	= MAX_PRIO - 20,
 	.normal_prio	= MAX_PRIO - 20,
+	.latency_nice	= DEFAULT_LATENCY_NICE,
 	.policy		= SCHED_NORMAL,
 	.cpus_ptr	= &init_task.cpus_mask,
 	.user_cpus_ptr	= NULL,
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 07ac08caf019..8c84c652853b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4592,6 +4592,7 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
 		p->prio = p->normal_prio = p->static_prio;
 		set_load_weight(p, false);
 
+		p->latency_nice = DEFAULT_LATENCY_NICE;
 		/*
 		 * We don't need the reset flag anymore after the fork. It has
 		 * fulfilled its duty:
-- 
2.17.1

[PATCH v9 4/9] sched: Allow sched_{get,set}attr to change latency_nice of the task

[Vincent Guittot]

From: Parth Shah <parth@linux.ibm.com>

Introduce the latency_nice attribute to sched_attr and provide a
mechanism to change the value with the use of sched_setattr/sched_getattr
syscall.

Also add new flag "SCHED_FLAG_LATENCY_NICE" to hint the change in
latency_nice of the task on every sched_setattr syscall.

Signed-off-by: Parth Shah <parth@linux.ibm.com>
[rebase and add a dedicated __setscheduler_latency ]
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 include/uapi/linux/sched.h       |  4 +++-
 include/uapi/linux/sched/types.h | 19 +++++++++++++++++++
 kernel/sched/core.c              | 24 ++++++++++++++++++++++++
 tools/include/uapi/linux/sched.h |  4 +++-
 4 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index 3bac0a8ceab2..b2e932c25be6 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -132,6 +132,7 @@ struct clone_args {
 #define SCHED_FLAG_KEEP_PARAMS		0x10
 #define SCHED_FLAG_UTIL_CLAMP_MIN	0x20
 #define SCHED_FLAG_UTIL_CLAMP_MAX	0x40
+#define SCHED_FLAG_LATENCY_NICE		0x80
 
 #define SCHED_FLAG_KEEP_ALL	(SCHED_FLAG_KEEP_POLICY | \
 				 SCHED_FLAG_KEEP_PARAMS)
@@ -143,6 +144,7 @@ struct clone_args {
 			 SCHED_FLAG_RECLAIM		| \
 			 SCHED_FLAG_DL_OVERRUN		| \
 			 SCHED_FLAG_KEEP_ALL		| \
-			 SCHED_FLAG_UTIL_CLAMP)
+			 SCHED_FLAG_UTIL_CLAMP		| \
+			 SCHED_FLAG_LATENCY_NICE)
 
 #endif /* _UAPI_LINUX_SCHED_H */
diff --git a/include/uapi/linux/sched/types.h b/include/uapi/linux/sched/types.h
index f2c4589d4dbf..db1e8199e8c8 100644
--- a/include/uapi/linux/sched/types.h
+++ b/include/uapi/linux/sched/types.h
@@ -10,6 +10,7 @@ struct sched_param {
 
 #define SCHED_ATTR_SIZE_VER0	48	/* sizeof first published struct */
 #define SCHED_ATTR_SIZE_VER1	56	/* add: util_{min,max} */
+#define SCHED_ATTR_SIZE_VER2	60	/* add: latency_nice */
 
 /*
  * Extended scheduling parameters data structure.
@@ -98,6 +99,22 @@ struct sched_param {
  * scheduled on a CPU with no more capacity than the specified value.
  *
  * A task utilization boundary can be reset by setting the attribute to -1.
+ *
+ * Latency Tolerance Attributes
+ * ===========================
+ *
+ * A subset of sched_attr attributes allows to specify the relative latency
+ * requirements of a task with respect to the other tasks running/queued in the
+ * system.
+ *
+ * @ sched_latency_nice	task's latency_nice value
+ *
+ * The latency_nice of a task can have any value in a range of
+ * [MIN_LATENCY_NICE..MAX_LATENCY_NICE].
+ *
+ * A task with latency_nice with the value of LATENCY_NICE_MIN can be
+ * taken for a task requiring a lower latency as opposed to the task with
+ * higher latency_nice.
  */
 struct sched_attr {
 	__u32 size;
@@ -120,6 +137,8 @@ struct sched_attr {
 	__u32 sched_util_min;
 	__u32 sched_util_max;
 
+	/* latency requirement hints */
+	__s32 sched_latency_nice;
 };
 
 #endif /* _UAPI_LINUX_SCHED_TYPES_H */
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8c84c652853b..18c31a68eb18 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7352,6 +7352,14 @@ static void __setscheduler_params(struct task_struct *p,
 	p->rt_priority = attr->sched_priority;
 	p->normal_prio = normal_prio(p);
 	set_load_weight(p, true);
+
+}
+
+static void __setscheduler_latency(struct task_struct *p,
+		const struct sched_attr *attr)
+{
+	if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE)
+		p->latency_nice = attr->sched_latency_nice;
 }
 
 /*
@@ -7494,6 +7502,13 @@ static int __sched_setscheduler(struct task_struct *p,
 			return retval;
 	}
 
+	if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE) {
+		if (attr->sched_latency_nice > MAX_LATENCY_NICE)
+			return -EINVAL;
+		if (attr->sched_latency_nice < MIN_LATENCY_NICE)
+			return -EINVAL;
+	}
+
 	if (pi)
 		cpuset_read_lock();
 
@@ -7528,6 +7543,9 @@ static int __sched_setscheduler(struct task_struct *p,
 			goto change;
 		if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
 			goto change;
+		if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE &&
+		    attr->sched_latency_nice != p->latency_nice)
+			goto change;
 
 		p->sched_reset_on_fork = reset_on_fork;
 		retval = 0;
@@ -7616,6 +7634,7 @@ static int __sched_setscheduler(struct task_struct *p,
 		__setscheduler_params(p, attr);
 		__setscheduler_prio(p, newprio);
 	}
+	__setscheduler_latency(p, attr);
 	__setscheduler_uclamp(p, attr);
 
 	if (queued) {
@@ -7826,6 +7845,9 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
 	    size < SCHED_ATTR_SIZE_VER1)
 		return -EINVAL;
 
+	if ((attr->sched_flags & SCHED_FLAG_LATENCY_NICE) &&
+	    size < SCHED_ATTR_SIZE_VER2)
+		return -EINVAL;
 	/*
 	 * XXX: Do we want to be lenient like existing syscalls; or do we want
 	 * to be strict and return an error on out-of-bounds values?
@@ -8063,6 +8085,8 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
 	get_params(p, &kattr);
 	kattr.sched_flags &= SCHED_FLAG_ALL;
 
+	kattr.sched_latency_nice = p->latency_nice;
+
 #ifdef CONFIG_UCLAMP_TASK
 	/*
 	 * This could race with another potential updater, but this is fine
diff --git a/tools/include/uapi/linux/sched.h b/tools/include/uapi/linux/sched.h
index 3bac0a8ceab2..b2e932c25be6 100644
--- a/tools/include/uapi/linux/sched.h
+++ b/tools/include/uapi/linux/sched.h
@@ -132,6 +132,7 @@ struct clone_args {
 #define SCHED_FLAG_KEEP_PARAMS		0x10
 #define SCHED_FLAG_UTIL_CLAMP_MIN	0x20
 #define SCHED_FLAG_UTIL_CLAMP_MAX	0x40
+#define SCHED_FLAG_LATENCY_NICE		0x80
 
 #define SCHED_FLAG_KEEP_ALL	(SCHED_FLAG_KEEP_POLICY | \
 				 SCHED_FLAG_KEEP_PARAMS)
@@ -143,6 +144,7 @@ struct clone_args {
 			 SCHED_FLAG_RECLAIM		| \
 			 SCHED_FLAG_DL_OVERRUN		| \
 			 SCHED_FLAG_KEEP_ALL		| \
-			 SCHED_FLAG_UTIL_CLAMP)
+			 SCHED_FLAG_UTIL_CLAMP		| \
+			 SCHED_FLAG_LATENCY_NICE)
 
 #endif /* _UAPI_LINUX_SCHED_H */
-- 
2.17.1

[PATCH 5/9] sched/fair: Take into account latency priority at wakeup

[Vincent Guittot]

Take into account the latency priority of a thread when deciding to
preempt the current running thread. We don't want to provide more CPU
bandwidth to a thread but reorder the scheduling to run latency sensitive
task first whenever possible.

As long as a thread didn't use its bandwidth, it will be able to preempt
the current thread.

At the opposite, a thread with a low latency priority will preempt current
thread at wakeup only to keep fair CPU bandwidth sharing. Otherwise it will
wait for the tick to get its sched slice.

                                   curr vruntime
                                       |
                      sysctl_sched_wakeup_granularity
                                   <-->
----------------------------------|----|-----------------------|---------------
                                  |    |<--------------------->
                                  |    .  sysctl_sched_latency
                                  |    .
default/current latency entity    |    .
                                  |    .
1111111111111111111111111111111111|0000|-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-
se preempts curr at wakeup ------>|<- se doesn't preempt curr -----------------
                                  |    .
                                  |    .
                                  |    .
low latency entity                |    .
                                   ---------------------->|
                               % of sysctl_sched_latency  |
1111111111111111111111111111111111111111111111111111111111|0000|-1-1-1-1-1-1-1-
preempt ------------------------------------------------->|<- do not preempt --
                                  |    .
                                  |    .
                                  |    .
high latency entity               |    .
         |<-----------------------|----.
         | % of sysctl_sched_latency   .
111111111|0000|-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1
preempt->|<- se doesn't preempt curr ------------------------------------------

Tests results of nice latency impact on heavy load like hackbench:

hackbench -l (2560 / group) -g group
group        latency 0             latency 19
1            1.378(+/-  1%)      1.337(+/- 1%) + 3%
4            1.393(+/-  3%)      1.312(+/- 3%) + 6%
8            1.308(+/-  2%)      1.279(+/- 1%) + 2%
16           1.347(+/-  1%)      1.317(+/- 1%) + 2%

hackbench -p -l (2560 / group) -g group
group
1            1.836(+/- 17%)      1.148(+/- 5%) +37%
4            1.586(+/-  6%)      1.109(+/- 8%) +30%
8            1.209(+/-  4%)      0.780(+/- 4%) +35%
16           0.805(+/-  5%)      0.728(+/- 4%) +10%

By deacreasing the latency prio, we reduce the number of preemption at
wakeup and help hackbench making progress.

Test results of nice latency impact on short live load like cyclictest
while competing with heavy load like hackbench:

hackbench -l 10000 -g $group &
cyclictest --policy other -D 5 -q -n
        latency 0           latency -20
group   min  avg    max     min  avg    max
0       16    19     29      17   18     29
1       43   299   7359      63   84   3422
4       56   449  14806      45   83    284
8       63   820  51123      63   83    283
16      64  1326  70684      41  157  26852

group = 0 means that hackbench is not running.

The avg is significantly improved with nice latency -20 especially with
large number of groups but min and max remain quite similar. If we add the
histogram parameter to get details of latency, we have :

hackbench -l 10000 -g 16 &
cyclictest --policy other -D 5 -q -n  -H 20000 --histfile data.txt
              latency 0    latency -20
Min Latencies:    64           62
Avg Latencies:  1170          107
Max Latencies: 88069        10417
50% latencies:   122           86
75% latencies:   614           91
85% latencies:   961           94
90% latencies:  1225           97
95% latencies:  6120          102
99% latencies: 18328          159

With percentile details, we see the benefit of nice latency -20 as
only 1% of the latencies are above 159us whereas the default latency
has got 15% around ~1ms or above and 5% over the 6ms.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 include/linux/sched.h      |  4 ++-
 include/linux/sched/prio.h |  9 ++++++
 init/init_task.c           |  2 +-
 kernel/sched/core.c        | 38 +++++++++++++++++++---
 kernel/sched/debug.c       |  2 +-
 kernel/sched/fair.c        | 66 ++++++++++++++++++++++++++++++++++----
 kernel/sched/sched.h       |  6 ++++
 7 files changed, 112 insertions(+), 15 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 856240573300..2f33326adb8d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -568,6 +568,8 @@ struct sched_entity {
 	/* cached value of my_q->h_nr_running */
 	unsigned long			runnable_weight;
 #endif
+	/* preemption offset in ns */
+	long				latency_offset;
 
 #ifdef CONFIG_SMP
 	/*
@@ -784,7 +786,7 @@ struct task_struct {
 	int				static_prio;
 	int				normal_prio;
 	unsigned int			rt_priority;
-	int				latency_nice;
+	int				latency_prio;
 
 	struct sched_entity		se;
 	struct sched_rt_entity		rt;
diff --git a/include/linux/sched/prio.h b/include/linux/sched/prio.h
index bfcd7f1d1e11..be79503d86af 100644
--- a/include/linux/sched/prio.h
+++ b/include/linux/sched/prio.h
@@ -59,5 +59,14 @@ static inline long rlimit_to_nice(long prio)
  * Default tasks should be treated as a task with latency_nice = 0.
  */
 #define DEFAULT_LATENCY_NICE	0
+#define DEFAULT_LATENCY_PRIO	(DEFAULT_LATENCY_NICE + LATENCY_NICE_WIDTH/2)
+
+/*
+ * Convert user-nice values [ -20 ... 0 ... 19 ]
+ * to static latency [ 0..39 ],
+ * and back.
+ */
+#define NICE_TO_LATENCY(nice)	((nice) + DEFAULT_LATENCY_PRIO)
+#define LATENCY_TO_NICE(prio)	((prio) - DEFAULT_LATENCY_PRIO)
 
 #endif /* _LINUX_SCHED_PRIO_H */
diff --git a/init/init_task.c b/init/init_task.c
index 7dd71dd2d261..071deff8dbd1 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -78,7 +78,7 @@ struct task_struct init_task
 	.prio		= MAX_PRIO - 20,
 	.static_prio	= MAX_PRIO - 20,
 	.normal_prio	= MAX_PRIO - 20,
-	.latency_nice	= DEFAULT_LATENCY_NICE,
+	.latency_prio	= DEFAULT_LATENCY_PRIO,
 	.policy		= SCHED_NORMAL,
 	.cpus_ptr	= &init_task.cpus_mask,
 	.user_cpus_ptr	= NULL,
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 18c31a68eb18..b2b8cb6c08cd 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1283,6 +1283,16 @@ static void set_load_weight(struct task_struct *p, bool update_load)
 	}
 }
 
+static void set_latency_offset(struct task_struct *p)
+{
+	long weight = sched_latency_to_weight[p->latency_prio];
+	s64 offset;
+
+	offset = weight * get_sleep_latency(false);
+	offset = div_s64(offset, NICE_LATENCY_WEIGHT_MAX);
+	p->se.latency_offset = (long)offset;
+}
+
 #ifdef CONFIG_UCLAMP_TASK
 /*
  * Serializes updates of utilization clamp values
@@ -4592,7 +4602,9 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
 		p->prio = p->normal_prio = p->static_prio;
 		set_load_weight(p, false);
 
-		p->latency_nice = DEFAULT_LATENCY_NICE;
+		p->latency_prio = NICE_TO_LATENCY(0);
+		set_latency_offset(p);
+
 		/*
 		 * We don't need the reset flag anymore after the fork. It has
 		 * fulfilled its duty:
@@ -7358,8 +7370,10 @@ static void __setscheduler_params(struct task_struct *p,
 static void __setscheduler_latency(struct task_struct *p,
 		const struct sched_attr *attr)
 {
-	if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE)
-		p->latency_nice = attr->sched_latency_nice;
+	if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE) {
+		p->latency_prio = NICE_TO_LATENCY(attr->sched_latency_nice);
+		set_latency_offset(p);
+	}
 }
 
 /*
@@ -7544,7 +7558,7 @@ static int __sched_setscheduler(struct task_struct *p,
 		if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
 			goto change;
 		if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE &&
-		    attr->sched_latency_nice != p->latency_nice)
+		    attr->sched_latency_nice != LATENCY_TO_NICE(p->latency_prio))
 			goto change;
 
 		p->sched_reset_on_fork = reset_on_fork;
@@ -8085,7 +8099,7 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
 	get_params(p, &kattr);
 	kattr.sched_flags &= SCHED_FLAG_ALL;
 
-	kattr.sched_latency_nice = p->latency_nice;
+	kattr.sched_latency_nice = LATENCY_TO_NICE(p->latency_prio);
 
 #ifdef CONFIG_UCLAMP_TASK
 	/*
@@ -11294,6 +11308,20 @@ const u32 sched_prio_to_wmult[40] = {
  /*  15 */ 119304647, 148102320, 186737708, 238609294, 286331153,
 };
 
+/*
+ * latency weight for wakeup preemption
+ */
+const int sched_latency_to_weight[40] = {
+ /* -20 */     -1024,     -973,     -922,      -870,      -819,
+ /* -15 */      -768,     -717,     -666,      -614,      -563,
+ /* -10 */      -512,     -461,     -410,      -358,      -307,
+ /*  -5 */      -256,     -205,     -154,      -102,       -51,
+ /*   0 */         0,       51,      102,       154,       205,
+ /*   5 */       256,      307,      358,       410,       461,
+ /*  10 */       512,      563,      614,       666,       717,
+ /*  15 */       768,      819,      870,       922,       973,
+};
+
 void call_trace_sched_update_nr_running(struct rq *rq, int count)
 {
         trace_sched_update_nr_running_tp(rq, count);
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 68be7a3e42a3..b3922184af91 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -1043,7 +1043,7 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
 #endif
 	P(policy);
 	P(prio);
-	P(latency_nice);
+	P(latency_prio);
 	if (task_has_dl_policy(p)) {
 		P(dl.runtime);
 		P(dl.deadline);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c8a697f8db88..0e80e65113bd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4858,6 +4858,8 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 		update_idle_cfs_rq_clock_pelt(cfs_rq);
 }
 
+static long wakeup_latency_gran(struct sched_entity *curr, struct sched_entity *se);
+
 /*
  * Preempt the current task with a newly woken task if needed:
  */
@@ -4866,7 +4868,7 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 {
 	unsigned long ideal_runtime, delta_exec;
 	struct sched_entity *se;
-	s64 delta;
+	s64 delta, offset;
 
 	ideal_runtime = sched_slice(cfs_rq, curr);
 	delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
@@ -4891,10 +4893,12 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 	se = __pick_first_entity(cfs_rq);
 	delta = curr->vruntime - se->vruntime;
 
-	if (delta < 0)
+	offset = wakeup_latency_gran(curr, se);
+	if (delta < offset)
 		return;
 
-	if (delta > ideal_runtime)
+	if ((delta > ideal_runtime) ||
+	    (delta > get_latency_max()))
 		resched_curr(rq_of(cfs_rq));
 }
 
@@ -6019,6 +6023,35 @@ static int sched_idle_cpu(int cpu)
 }
 #endif
 
+static void set_next_buddy(struct sched_entity *se);
+
+static void check_preempt_from_others(struct cfs_rq *cfs, struct sched_entity *se)
+{
+	struct sched_entity *next;
+
+	if (se->latency_offset >= 0)
+		return;
+
+	if (cfs->nr_running <= 1)
+		return;
+	/*
+	 * When waking from another class, we don't need to check to preempt at
+	 * wakeup and don't set next buddy as a candidate for being picked in
+	 * priority.
+	 * In case of simultaneous wakeup when current is another class, the
+	 * latency sensitive tasks lost opportunity to preempt non sensitive
+	 * tasks which woke up simultaneously.
+	 */
+
+	if (cfs->next)
+		next = cfs->next;
+	else
+		next = __pick_first_entity(cfs);
+
+	if (next && wakeup_preempt_entity(next, se) == 1)
+		set_next_buddy(se);
+}
+
 /*
  * The enqueue_task method is called before nr_running is
  * increased. Here we update the fair scheduling stats and
@@ -6105,14 +6138,15 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	if (!task_new)
 		update_overutilized_status(rq);
 
+	if (rq->curr->sched_class != &fair_sched_class)
+		check_preempt_from_others(cfs_rq_of(&p->se), &p->se);
+
 enqueue_throttle:
 	assert_list_leaf_cfs_rq(rq);
 
 	hrtick_update(rq);
 }
 
-static void set_next_buddy(struct sched_entity *se);
-
 /*
  * The dequeue_task method is called before nr_running is
  * decreased. We remove the task from the rbtree and
@@ -7461,6 +7495,23 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 }
 #endif /* CONFIG_SMP */
 
+static long wakeup_latency_gran(struct sched_entity *curr, struct sched_entity *se)
+{
+	long latency_offset = se->latency_offset;
+
+	/*
+	 * A negative latency offset means that the sched_entity has latency
+	 * requirement that needs to be evaluated versus other entity.
+	 * Otherwise, use the latency weight to evaluate how much scheduling
+	 * delay is acceptable by se.
+	 */
+	if ((latency_offset < 0) || (curr->latency_offset < 0))
+		latency_offset -= curr->latency_offset;
+	latency_offset = min_t(long, latency_offset, get_latency_max());
+
+	return latency_offset;
+}
+
 static unsigned long wakeup_gran(struct sched_entity *se)
 {
 	unsigned long gran = sysctl_sched_wakeup_granularity;
@@ -7499,11 +7550,12 @@ static int
 wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
 {
 	s64 gran, vdiff = curr->vruntime - se->vruntime;
+	s64 offset = wakeup_latency_gran(curr, se);
 
-	if (vdiff <= 0)
+	if (vdiff < offset)
 		return -1;
 
-	gran = wakeup_gran(se);
+	gran = offset + wakeup_gran(se);
 
 	/*
 	 * At wake up, the vruntime of a task is capped to not be older than
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 842ce0094d9c..7292652731d0 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -125,6 +125,11 @@ extern int sched_rr_timeslice;
  */
 #define NS_TO_JIFFIES(TIME)	((unsigned long)(TIME) / (NSEC_PER_SEC / HZ))
 
+/* Maximum nice latency weight used to scale the latency_offset */
+
+#define NICE_LATENCY_SHIFT	(SCHED_FIXEDPOINT_SHIFT)
+#define NICE_LATENCY_WEIGHT_MAX	(1L << NICE_LATENCY_SHIFT)
+
 /*
  * Increase resolution of nice-level calculations for 64-bit architectures.
  * The extra resolution improves shares distribution and load balancing of
@@ -2115,6 +2120,7 @@ static_assert(WF_TTWU == SD_BALANCE_WAKE);
 
 extern const int		sched_prio_to_weight[40];
 extern const u32		sched_prio_to_wmult[40];
+extern const int		sched_latency_to_weight[40];
 
 /*
  * {de,en}queue flags:
-- 
2.17.1

[PATCH v9 6/9] sched/fair: Add sched group latency support

[Vincent Guittot]

Task can set its latency priority with sched_setattr(), which is then used
to set the latency offset of its sched_enity, but sched group entities
still have the default latency offset value.

Add a latency.nice field in cpu cgroup controller to set the latency
priority of the group similarly to sched_setattr(). The latency priority
is then used to set the offset of the sched_entities of the group.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 Documentation/admin-guide/cgroup-v2.rst | 10 +++++
 kernel/sched/core.c                     | 52 +++++++++++++++++++++++++
 kernel/sched/fair.c                     | 33 ++++++++++++++++
 kernel/sched/sched.h                    |  4 ++
 4 files changed, 99 insertions(+)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index dc254a3cb956..93a73663a5f7 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1118,6 +1118,16 @@ All time durations are in microseconds.
         values similar to the sched_setattr(2). This maximum utilization
         value is used to clamp the task specific maximum utilization clamp.
 
+  cpu.latency.nice
+	A read-write single value file which exists on non-root
+	cgroups.  The default is "0".
+
+	The nice value is in the range [-20, 19].
+
+	This interface file allows reading and setting latency using the
+	same values used by sched_setattr(2). The latency_nice of a group is
+	used to limit the impact of the latency_nice of a task outside the
+	group.
 
 
 Memory
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b2b8cb6c08cd..9f6700f812ea 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10967,6 +10967,47 @@ static int cpu_idle_write_s64(struct cgroup_subsys_state *css,
 {
 	return sched_group_set_idle(css_tg(css), idle);
 }
+
+static s64 cpu_latency_nice_read_s64(struct cgroup_subsys_state *css,
+				    struct cftype *cft)
+{
+	int prio, delta, last_delta = INT_MAX;
+	s64 weight;
+
+	weight = css_tg(css)->latency_offset * NICE_LATENCY_WEIGHT_MAX;
+	weight = div_s64(weight, get_sleep_latency(false));
+
+	/* Find the closest nice value to the current weight */
+	for (prio = 0; prio < ARRAY_SIZE(sched_latency_to_weight); prio++) {
+		delta = abs(sched_latency_to_weight[prio] - weight);
+		if (delta >= last_delta)
+			break;
+		last_delta = delta;
+	}
+
+	return LATENCY_TO_NICE(prio-1);
+}
+
+static int cpu_latency_nice_write_s64(struct cgroup_subsys_state *css,
+				     struct cftype *cft, s64 nice)
+{
+	s64 latency_offset;
+	long weight;
+	int idx;
+
+	if (nice < MIN_LATENCY_NICE || nice > MAX_LATENCY_NICE)
+		return -ERANGE;
+
+	idx = NICE_TO_LATENCY(nice);
+	idx = array_index_nospec(idx, LATENCY_NICE_WIDTH);
+	weight = sched_latency_to_weight[idx];
+
+	latency_offset = weight * get_sleep_latency(false);
+	latency_offset = div_s64(latency_offset, NICE_LATENCY_WEIGHT_MAX);
+
+	return sched_group_set_latency(css_tg(css), latency_offset);
+}
+
 #endif
 
 static struct cftype cpu_legacy_files[] = {
@@ -10981,6 +11022,11 @@ static struct cftype cpu_legacy_files[] = {
 		.read_s64 = cpu_idle_read_s64,
 		.write_s64 = cpu_idle_write_s64,
 	},
+	{
+		.name = "latency.nice",
+		.read_s64 = cpu_latency_nice_read_s64,
+		.write_s64 = cpu_latency_nice_write_s64,
+	},
 #endif
 #ifdef CONFIG_CFS_BANDWIDTH
 	{
@@ -11198,6 +11244,12 @@ static struct cftype cpu_files[] = {
 		.read_s64 = cpu_idle_read_s64,
 		.write_s64 = cpu_idle_write_s64,
 	},
+	{
+		.name = "latency.nice",
+		.flags = CFTYPE_NOT_ON_ROOT,
+		.read_s64 = cpu_latency_nice_read_s64,
+		.write_s64 = cpu_latency_nice_write_s64,
+	},
 #endif
 #ifdef CONFIG_CFS_BANDWIDTH
 	{
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0e80e65113bd..75c0a8d203c3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12158,6 +12158,7 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
 		goto err;
 
 	tg->shares = NICE_0_LOAD;
+	tg->latency_offset = 0;
 
 	init_cfs_bandwidth(tg_cfs_bandwidth(tg));
 
@@ -12256,6 +12257,9 @@ void init_tg_cfs_entry(struct task_group *tg, struct cfs_rq *cfs_rq,
 	}
 
 	se->my_q = cfs_rq;
+
+	se->latency_offset = tg->latency_offset;
+
 	/* guarantee group entities always have weight */
 	update_load_set(&se->load, NICE_0_LOAD);
 	se->parent = parent;
@@ -12386,6 +12390,35 @@ int sched_group_set_idle(struct task_group *tg, long idle)
 	return 0;
 }
 
+int sched_group_set_latency(struct task_group *tg, s64 latency)
+{
+	int i;
+
+	if (tg == &root_task_group)
+		return -EINVAL;
+
+	if (abs(latency) > sysctl_sched_latency)
+		return -EINVAL;
+
+	mutex_lock(&shares_mutex);
+
+	if (tg->latency_offset == latency) {
+		mutex_unlock(&shares_mutex);
+		return 0;
+	}
+
+	tg->latency_offset = latency;
+
+	for_each_possible_cpu(i) {
+		struct sched_entity *se = tg->se[i];
+
+		WRITE_ONCE(se->latency_offset, latency);
+	}
+
+	mutex_unlock(&shares_mutex);
+	return 0;
+}
+
 #else /* CONFIG_FAIR_GROUP_SCHED */
 
 void free_fair_sched_group(struct task_group *tg) { }
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 7292652731d0..c3735a34d394 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -383,6 +383,8 @@ struct task_group {
 
 	/* A positive value indicates that this is a SCHED_IDLE group. */
 	int			idle;
+	/* latency constraint of the group. */
+	int			latency_offset;
 
 #ifdef	CONFIG_SMP
 	/*
@@ -493,6 +495,8 @@ extern int sched_group_set_shares(struct task_group *tg, unsigned long shares);
 
 extern int sched_group_set_idle(struct task_group *tg, long idle);
 
+extern int sched_group_set_latency(struct task_group *tg, s64 latency);
+
 #ifdef CONFIG_SMP
 extern void set_task_rq_fair(struct sched_entity *se,
 			     struct cfs_rq *prev, struct cfs_rq *next);
-- 
2.17.1

[PATCH v9 7/9] sched/core: Support latency priority with sched core

[Vincent Guittot]

Take into account wakeup_latency_gran() when ordering the cfs threads.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 75c0a8d203c3..be446dc58be7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11833,6 +11833,9 @@ bool cfs_prio_less(struct task_struct *a, struct task_struct *b, bool in_fi)
 	delta = (s64)(sea->vruntime - seb->vruntime) +
 		(s64)(cfs_rqb->min_vruntime_fi - cfs_rqa->min_vruntime_fi);
 
+	/* Take into account latency prio */
+	delta -= wakeup_latency_gran(sea, seb);
+
 	return delta > 0;
 }
 #else
-- 
2.17.1

[PATCH v9 8/9] sched/fair: Add latency list

[Vincent Guittot]

Add a rb tree for latency sensitive entities so we can schedule the most
sensitive one first even when it failed to preempt current at wakeup or
when it got quickly preempted by another entity of higher priority.

In order to keep fairness, the latency is used once at wakeup to get a
minimum slice and not during the following scheduling slice to prevent
long running entity to got more running time than allocated to his nice
priority.

The rb tree enables to cover the last corner case where latency
sensitive entity can't got schedule quickly after the wakeup.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 include/linux/sched.h |  1 +
 kernel/sched/core.c   |  1 +
 kernel/sched/fair.c   | 95 +++++++++++++++++++++++++++++++++++++++++--
 kernel/sched/sched.h  |  1 +
 4 files changed, 95 insertions(+), 3 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2f33326adb8d..5187114a9920 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -548,6 +548,7 @@ struct sched_entity {
 	/* For load-balancing: */
 	struct load_weight		load;
 	struct rb_node			run_node;
+	struct rb_node			latency_node;
 	struct list_head		group_node;
 	unsigned int			on_rq;
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9f6700f812ea..eaca0e34ab58 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4361,6 +4361,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 	p->se.nr_migrations		= 0;
 	p->se.vruntime			= 0;
 	INIT_LIST_HEAD(&p->se.group_node);
+	RB_CLEAR_NODE(&p->se.latency_node);
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	p->se.cfs_rq			= NULL;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index be446dc58be7..76da7c7a13ab 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -665,7 +665,76 @@ struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq)
 
 	return __node_2_se(last);
 }
+#endif
 
+/**************************************************************
+ * Scheduling class tree data structure manipulation methods:
+ * for latency
+ */
+
+static inline bool latency_before(struct sched_entity *a,
+				struct sched_entity *b)
+{
+	return (s64)(a->vruntime + a->latency_offset - b->vruntime - b->latency_offset) < 0;
+}
+
+#define __latency_node_2_se(node) \
+	rb_entry((node), struct sched_entity, latency_node)
+
+static inline bool __latency_less(struct rb_node *a, const struct rb_node *b)
+{
+	return latency_before(__latency_node_2_se(a), __latency_node_2_se(b));
+}
+
+/*
+ * Enqueue an entity into the latency rb-tree:
+ */
+static void __enqueue_latency(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
+{
+
+	/* Only latency sensitive entity can be added to the list */
+	if (se->latency_offset >= 0)
+		return;
+
+	if (!RB_EMPTY_NODE(&se->latency_node))
+		return;
+
+	/*
+	 * An execution time less than sysctl_sched_min_granularity means that
+	 * the entity has been preempted by a higher sched class or an entity
+	 * with higher latency constraint.
+	 * Put it back in the list so it gets a chance to run 1st during the
+	 * next slice.
+	 */
+	if (!(flags & ENQUEUE_WAKEUP)) {
+		u64 delta_exec = se->sum_exec_runtime - se->prev_sum_exec_runtime;
+
+		if (delta_exec >= sysctl_sched_min_granularity)
+			return;
+	}
+
+	rb_add_cached(&se->latency_node, &cfs_rq->latency_timeline, __latency_less);
+}
+
+static void __dequeue_latency(struct cfs_rq *cfs_rq, struct sched_entity *se)
+{
+	if (!RB_EMPTY_NODE(&se->latency_node)) {
+		rb_erase_cached(&se->latency_node, &cfs_rq->latency_timeline);
+		RB_CLEAR_NODE(&se->latency_node);
+	}
+}
+
+static struct sched_entity *__pick_first_latency(struct cfs_rq *cfs_rq)
+{
+	struct rb_node *left = rb_first_cached(&cfs_rq->latency_timeline);
+
+	if (!left)
+		return NULL;
+
+	return __latency_node_2_se(left);
+}
+
+#ifdef CONFIG_SCHED_DEBUG
 /**************************************************************
  * Scheduling class statistics methods:
  */
@@ -4739,8 +4808,10 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 	check_schedstat_required();
 	update_stats_enqueue_fair(cfs_rq, se, flags);
 	check_spread(cfs_rq, se);
-	if (!curr)
+	if (!curr) {
 		__enqueue_entity(cfs_rq, se);
+		__enqueue_latency(cfs_rq, se, flags);
+	}
 	se->on_rq = 1;
 
 	if (cfs_rq->nr_running == 1) {
@@ -4826,8 +4897,10 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 
 	clear_buddies(cfs_rq, se);
 
-	if (se != cfs_rq->curr)
+	if (se != cfs_rq->curr) {
 		__dequeue_entity(cfs_rq, se);
+		__dequeue_latency(cfs_rq, se);
+	}
 	se->on_rq = 0;
 	account_entity_dequeue(cfs_rq, se);
 
@@ -4916,6 +4989,7 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 		 */
 		update_stats_wait_end_fair(cfs_rq, se);
 		__dequeue_entity(cfs_rq, se);
+		__dequeue_latency(cfs_rq, se);
 		update_load_avg(cfs_rq, se, UPDATE_TG);
 	}
 
@@ -4954,7 +5028,7 @@ static struct sched_entity *
 pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 {
 	struct sched_entity *left = __pick_first_entity(cfs_rq);
-	struct sched_entity *se;
+	struct sched_entity *latency, *se;
 
 	/*
 	 * If curr is set we have to see if its left of the leftmost entity
@@ -4996,6 +5070,12 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 		se = cfs_rq->last;
 	}
 
+	/* Check for latency sensitive entity waiting for running */
+	latency = __pick_first_latency(cfs_rq);
+	if (latency && (latency != se) &&
+	    wakeup_preempt_entity(latency, se) < 1)
+		se = latency;
+
 	return se;
 }
 
@@ -5019,6 +5099,7 @@ static void put_prev_entity(struct cfs_rq *cfs_rq, struct sched_entity *prev)
 		update_stats_wait_start_fair(cfs_rq, prev);
 		/* Put 'current' back into the tree. */
 		__enqueue_entity(cfs_rq, prev);
+		__enqueue_latency(cfs_rq, prev, 0);
 		/* in !on_rq case, update occurred at dequeue */
 		update_load_avg(cfs_rq, prev, 0);
 	}
@@ -12106,6 +12187,7 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
 void init_cfs_rq(struct cfs_rq *cfs_rq)
 {
 	cfs_rq->tasks_timeline = RB_ROOT_CACHED;
+	cfs_rq->latency_timeline = RB_ROOT_CACHED;
 	u64_u32_store(cfs_rq->min_vruntime, (u64)(-(1LL << 20)));
 #ifdef CONFIG_SMP
 	raw_spin_lock_init(&cfs_rq->removed.lock);
@@ -12414,8 +12496,15 @@ int sched_group_set_latency(struct task_group *tg, s64 latency)
 
 	for_each_possible_cpu(i) {
 		struct sched_entity *se = tg->se[i];
+		struct rq *rq = cpu_rq(i);
+		struct rq_flags rf;
+
+		rq_lock_irqsave(rq, &rf);
 
+		__dequeue_latency(se->cfs_rq, se);
 		WRITE_ONCE(se->latency_offset, latency);
+
+		rq_unlock_irqrestore(rq, &rf);
 	}
 
 	mutex_unlock(&shares_mutex);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c3735a34d394..b81179c512e6 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -575,6 +575,7 @@ struct cfs_rq {
 #endif
 
 	struct rb_root_cached	tasks_timeline;
+	struct rb_root_cached	latency_timeline;
 
 	/*
 	 * 'curr' points to currently running entity on this cfs_rq.
-- 
2.17.1

[PATCH v9 9/9] sched/fair: remove check_preempt_from_others

[Vincent Guittot]

With the dedicated latency list, we don't have to take care of this special
case anymore as pick_next_entity checks for a runnable latency sensitive
task.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c | 34 ++--------------------------------
 1 file changed, 2 insertions(+), 32 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 76da7c7a13ab..466a2fee1592 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6104,35 +6104,6 @@ static int sched_idle_cpu(int cpu)
 }
 #endif
 
-static void set_next_buddy(struct sched_entity *se);
-
-static void check_preempt_from_others(struct cfs_rq *cfs, struct sched_entity *se)
-{
-	struct sched_entity *next;
-
-	if (se->latency_offset >= 0)
-		return;
-
-	if (cfs->nr_running <= 1)
-		return;
-	/*
-	 * When waking from another class, we don't need to check to preempt at
-	 * wakeup and don't set next buddy as a candidate for being picked in
-	 * priority.
-	 * In case of simultaneous wakeup when current is another class, the
-	 * latency sensitive tasks lost opportunity to preempt non sensitive
-	 * tasks which woke up simultaneously.
-	 */
-
-	if (cfs->next)
-		next = cfs->next;
-	else
-		next = __pick_first_entity(cfs);
-
-	if (next && wakeup_preempt_entity(next, se) == 1)
-		set_next_buddy(se);
-}
-
 /*
  * The enqueue_task method is called before nr_running is
  * increased. Here we update the fair scheduling stats and
@@ -6219,15 +6190,14 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	if (!task_new)
 		update_overutilized_status(rq);
 
-	if (rq->curr->sched_class != &fair_sched_class)
-		check_preempt_from_others(cfs_rq_of(&p->se), &p->se);
-
 enqueue_throttle:
 	assert_list_leaf_cfs_rq(rq);
 
 	hrtick_update(rq);
 }
 
+static void set_next_buddy(struct sched_entity *se);
+
 /*
  * The dequeue_task method is called before nr_running is
  * decreased. We remove the task from the rbtree and
-- 
2.17.1

Re: [PATCH v9 0/9] Add latency priority for CFS class

[K Prateek Nayak]

Hello Vincent,

Following are the test results on dual socket Zen3 machine (2 x 64C/128T)

tl;dr

o All benchmarks with DEFAULT_LATENCY_NICE value are comparable to tip.
  There is, however, a noticeable dip for unixbench-spawn test case.

o With the 2 rbtree approach, I do not see much difference in the
  hackbench results with varying latency nice value. Tests on v5 did
  yield noticeable improvements for hackbench.
  (https://lore.kernel.org/lkml/cd48ebbb-9724-985f-28e3-e558dea07827@amd.com/)

o For hackbench + cyclictest and hackbench + schbench, I see the
  expected behavior with different latency nice values.

o There are a few cases with hackbench and hackbench + cyclictest where
  the results are non-monotonic with different latency nice values.
  (Marked with "^").

I'll leave the detailed results below:

On 11/15/2022 10:48 PM, Vincent Guittot wrote:
> This patchset restarts the work about adding a latency priority to describe
> the latency tolerance of cfs tasks.
> 
> Patch [1] is a new one that has been added with v6. It fixes an
> unfairness for low prio tasks because of wakeup_gran() being bigger
> than the maximum vruntime credit that a waking task can keep after
> sleeping.
> 
> The patches [2-4] have been done by Parth:
> https://lore.kernel.org/lkml/20200228090755.22829-1-parth@linux.ibm.com/
> 
> I have just rebased and moved the set of latency priority outside the
> priority update. I have removed the reviewed tag because the patches
> are 2 years old.
> 
> This aims to be a generic interface and the following patches is one use
> of it to improve the scheduling latency of cfs tasks.
> 
> Patch [5] uses latency nice priority to define a latency offset
> and then decide if a cfs task can or should preempt the current
> running task. The patch gives some tests results with cyclictests and
> hackbench to highlight the benefit of latency priority for short
> interactive task or long intensive tasks.
> 
> Patch [6] adds the support of latency nice priority to task group by
> adding a cpu.latency.nice field. The range is [-20:19] as for setting task
> latency priority.
> 
> Patch [7] makes sched_core taking into account the latency offset.
> 
> Patch [8] adds a rb tree to cover some corner cases where the latency
> sensitive task (priority < 0) is preempted by high priority task (RT/DL)
> or fails to preempt them. This patch ensures that tasks will have at least
> a slice of sched_min_granularity in priority at wakeup.
> 
> Patch [9] removes useless check after adding a latency rb tree.
> 
> I have also backported the patchset on a dragonboard RB3 with an android
> mainline kernel based on v5.18 for a quick test. I have used the
> TouchLatency app which is part of AOSP and described to be a very good
> test to highlight jitter and jank frame sources of a system [1].
> In addition to the app, I have added some short running tasks waking-up
> regularly (to use the 8 cpus for 4 ms every 37777us) to stress the system
> without overloading it (and disabling EAS). The 1st results shows that the
> patchset helps to reduce the missed deadline frames from 5% to less than
> 0.1% when the cpu.latency.nice of task group are set. I haven't rerun the
> test with latest version.
> 
> I have also tested the patchset with the modified version of the alsa
> latency test that has been shared by Tim. The test quickly xruns with
> default latency nice priority 0 but is able to run without underuns with
> a latency -20 and hackbench running simultaneously.
> 
> While preparing the version 8, I have evaluated the benefit of using an
> augmented rbtree instead of adding a rbtree for latency sensitive entities,
> which was a relevant suggestion done by PeterZ. Although the augmented
> rbtree enables to sort additional information in the tree with a limited
> overhead, it has more impact on legacy use cases (latency_nice >= 0)
> because the augmented callbacks are always called to maintain this
> additional information even when there is no sensitive tasks. In such
> cases, the dedicated rbtree remains empty and the overhead is reduced to
> loading a cached null node pointer. Nevertheless, we might want to
> reconsider the augmented rbtree once the use of negative latency_nice will
> be more widlely deployed. At now, the different tests that I have done,
> have not shown improvements with augmented rbtree.
> 
> Below are some hackbench results:
>         2 rbtrees               augmented rbtree        augmented rbtree	
>                                 sorted by vruntime      sorted by wakeup_vruntime
> sched	pipe	
> avg     26311,000               25976,667               25839,556
> stdev   0,15 %                  0,28 %                  0,24 %
> vs tip  0,50 %                  -0,78 %                 -1,31 %
> hackbench	1 group	
> avg     1,315                   1,344                   1,359
> stdev   0,88 %                  1,55 %                  1,82 %
> vs tip  -0,47 %                 -2,68 %                 -3,87 %
> hackbench	4 groups
> avg     1,339                   1,365                   1,367
> stdev   2,39 %                  2,26 %                  3,58 %
> vs tip  -0,08 %                 -2,01 %                 -2,22 %
> hackbench	8 groups
> avg     1,233                   1,286                   1,301
> stdev   0,74 %                  1,09 %                  1,52 %
> vs tip  0,29 %                  -4,05 %                 -5,27 %
> hackbench	16 groups	
> avg     1,268                   1,313                   1,319
> stdev   0,85 %                  1,60 %                  0,68 %
> vs tip  -0,02 %                 -3,56 %                 -4,01 %

Following are the results from running standard benchmarks on a
dual socket Zen3 (2 x 64C/128T) machine configured in different
NPS modes.

NPS Modes are used to logically divide single socket into
multiple NUMA region.
Following is the NUMA configuration for each NPS mode on the system:

NPS1: Each socket is a NUMA node.
    Total 2 NUMA nodes in the dual socket machine.

    Node 0: 0-63,   128-191
    Node 1: 64-127, 192-255

NPS2: Each socket is further logically divided into 2 NUMA regions.
    Total 4 NUMA nodes exist over 2 socket.
   
    Node 0: 0-31,   128-159
    Node 1: 32-63,  160-191
    Node 2: 64-95,  192-223
    Node 3: 96-127, 223-255

NPS4: Each socket is logically divided into 4 NUMA regions.
    Total 8 NUMA nodes exist over 2 socket.
   
    Node 0: 0-15,    128-143
    Node 1: 16-31,   144-159
    Node 2: 32-47,   160-175
    Node 3: 48-63,   176-191
    Node 4: 64-79,   192-207
    Node 5: 80-95,   208-223
    Node 6: 96-111,  223-231
    Node 7: 112-127, 232-255

Benchmark Results:

Kernel versions:
- tip:          6.1.0 tip sched/core
- latency_nice: 6.1.0 tip sched/core + this series

When we started testing, the tip was at:
commit d6962c4fe8f9 "sched: Clear ttwu_pending after enqueue_task()"


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~ hackbench - DEFAULT_LATENCY_NICE ~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

NPS1

Test:			tip			latency_nice
 1-groups:	   4.25 (0.00 pct)	   4.14 (2.58 pct)
 2-groups:	   4.95 (0.00 pct)	   4.92 (0.60 pct)
 4-groups:	   5.19 (0.00 pct)	   5.18 (0.19 pct)
 8-groups:	   5.45 (0.00 pct)	   5.44 (0.18 pct)
16-groups:	   7.33 (0.00 pct)	   7.32 (0.13 pct)

NPS2

Test:			tip			latency_nice
 1-groups:	   4.09 (0.00 pct)	   4.08 (0.24 pct)
 2-groups:	   4.68 (0.00 pct)	   4.72 (-0.85 pct)
 4-groups:	   5.05 (0.00 pct)	   4.97 (1.58 pct)
 8-groups:	   5.37 (0.00 pct)	   5.34 (0.55 pct)
16-groups:	   6.69 (0.00 pct)	   6.74 (-0.74 pct)

NPS4

Test:			tip			latency_nice
 1-groups:	   4.28 (0.00 pct)	   4.35 (-1.63 pct)
 2-groups:	   4.78 (0.00 pct)	   4.76 (0.41 pct)
 4-groups:	   5.11 (0.00 pct)	   5.06 (0.97 pct)
 8-groups:	   5.48 (0.00 pct)	   5.40 (1.45 pct)
16-groups:	   7.07 (0.00 pct)	   6.70 (5.23 pct)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~ schbench - DEFAULT_LATENCY_NICE ~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

NPS1

#workers:	tip			latency_nice
  1:	  31.00 (0.00 pct)	  32.00 (-3.22 pct)
  2:	  33.00 (0.00 pct)	  34.00 (-3.03 pct)
  4:	  39.00 (0.00 pct)	  38.00 (2.56 pct)
  8:	  45.00 (0.00 pct)	  46.00 (-2.22 pct)
 16:	  61.00 (0.00 pct)	  66.00 (-8.19 pct)
 32:	 108.00 (0.00 pct)	 110.00 (-1.85 pct)
 64:	 212.00 (0.00 pct)	 216.00 (-1.88 pct)
128:	 475.00 (0.00 pct)	 701.00 (-47.57 pct)    *
128:     429.00 (0.00 pct)       441.00 (-2.79 pct)      [Verification Run]
256:	 44736.00 (0.00 pct)	 45632.00 (-2.00 pct)
512:	 77184.00 (0.00 pct)	 78720.00 (-1.99 pct)

NPS2

#workers:	tip			latency_nice
  1:	  28.00 (0.00 pct)	  33.00 (-17.85 pct)
  2:	  34.00 (0.00 pct)	  31.00 (8.82 pct)
  4:	  36.00 (0.00 pct)	  36.00 (0.00 pct)
  8:	  51.00 (0.00 pct)	  49.00 (3.92 pct)
 16:	  68.00 (0.00 pct)	  64.00 (5.88 pct)
 32:	 113.00 (0.00 pct)	 115.00 (-1.76 pct)
 64:	 221.00 (0.00 pct)	 219.00 (0.90 pct)
128:	 553.00 (0.00 pct)	 531.00 (3.97 pct)
256:	 43840.00 (0.00 pct)	 48192.00 (-9.92 pct)   *
256:	 50427.00 (0.00 pct)	 48351.00 (4.11 pct)    [Verification Run]
512:	 76672.00 (0.00 pct)	 81024.00 (-5.67 pct)

NPS4

#workers:	tip			latency_nice
  1:	  33.00 (0.00 pct)	  28.00 (15.15 pct)
  2:	  29.00 (0.00 pct)	  34.00 (-17.24 pct)
  4:	  39.00 (0.00 pct)	  36.00 (7.69 pct)
  8:	  58.00 (0.00 pct)	  55.00 (5.17 pct)
 16:	  66.00 (0.00 pct)	  67.00 (-1.51 pct)
 32:	 112.00 (0.00 pct)	 116.00 (-3.57 pct)
 64:	 215.00 (0.00 pct)	 213.00 (0.93 pct)
128:	 689.00 (0.00 pct)	 571.00 (17.12 pct)
256:	 45120.00 (0.00 pct)	 46400.00 (-2.83 pct)
512:	 77440.00 (0.00 pct)	 76160.00 (1.65 pct)


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~ tbench - DEFAULT_LATENCY_NICE ~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

NPS1

Clients:	tip			latency_nice
    1	 581.75 (0.00 pct)	 586.52 (0.81 pct)
    2	 1145.75 (0.00 pct)	 1160.69 (1.30 pct)
    4	 2127.94 (0.00 pct)	 2141.49 (0.63 pct)
    8	 3838.27 (0.00 pct)	 3721.10 (-3.05 pct)
   16	 6272.71 (0.00 pct)	 6539.82 (4.25 pct)
   32	 11400.12 (0.00 pct)	 12079.49 (5.95 pct)
   64	 21605.96 (0.00 pct)	 22908.83 (6.03 pct)
  128	 30715.43 (0.00 pct)	 31736.95 (3.32 pct)
  256	 55580.78 (0.00 pct)	 54786.29 (-1.42 pct)
  512	 56528.79 (0.00 pct)	 56453.54 (-0.13 pct)
 1024	 56520.40 (0.00 pct)	 56369.93 (-0.26 pct)

NPS2

Clients:	tip			latency_nice
    1	 584.13 (0.00 pct)	 582.53 (-0.27 pct)
    2	 1153.63 (0.00 pct)	 1140.27 (-1.15 pct)
    4	 2212.89 (0.00 pct)	 2159.49 (-2.41 pct)
    8	 3871.35 (0.00 pct)	 3840.77 (-0.78 pct)
   16	 6216.72 (0.00 pct)	 6437.98 (3.55 pct)
   32	 11766.98 (0.00 pct)	 11663.53 (-0.87 pct)
   64	 22000.93 (0.00 pct)	 21882.88 (-0.53 pct)
  128	 31520.53 (0.00 pct)	 31147.05 (-1.18 pct)
  256	 51420.11 (0.00 pct)	 55216.39 (7.38 pct)
  512	 53935.90 (0.00 pct)	 55407.60 (2.72 pct)
 1024	 55239.73 (0.00 pct)	 55997.25 (1.37 pct)

NPS4

Clients:	tip			latency_nice
    1	 585.83 (0.00 pct)	 578.17 (-1.30 pct)
    2	 1141.59 (0.00 pct)	 1131.14 (-0.91 pct)
    4	 2174.79 (0.00 pct)	 2086.52 (-4.05 pct)
    8	 3887.56 (0.00 pct)	 3778.47 (-2.80 pct)
   16	 6441.59 (0.00 pct)	 6364.30 (-1.19 pct)
   32	 12133.60 (0.00 pct)	 11465.26 (-5.50 pct)   *
   32    11677.16 (0.00 pct)     12662.09 (8.43 pct)    [Verification Run]
   64	 21769.15 (0.00 pct)	 19488.45 (-10.47 pct)  *
   64    20305.64 (0.00 pct)     21002.90 (3.43 pct)    [Verification Run]
  128	 31396.31 (0.00 pct)	 31177.37 (-0.69 pct)
  256	 52792.39 (0.00 pct)	 52890.41 (0.18 pct)
  512	 55315.44 (0.00 pct)	 53572.65 (-3.15 pct)
 1024	 52150.27 (0.00 pct)	 54079.48 (3.69 pct)


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~ stream - DEFAULT_LATENCY_NICE ~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

NPS1

10 Runs:

Test:		tip			latency_nice
 Copy:   307827.79 (0.00 pct)    330524.48 (7.37 pct)
Scale:   208872.28 (0.00 pct)    215002.06 (2.93 pct)
  Add:   239404.64 (0.00 pct)    230334.74 (-3.78 pct)
Triad:   247258.30 (0.00 pct)    238505.06 (-3.54 pct)

100 Runs:

Test:		tip			latency_nice
 Copy:   317217.55 (0.00 pct)    314467.62 (-0.86 pct)
Scale:   208740.82 (0.00 pct)    210452.00 (0.81 pct)
  Add:   240550.63 (0.00 pct)    232376.03 (-3.39 pct)
Triad:   249594.21 (0.00 pct)    242460.83 (-2.85 pct)

NPS2

10 Runs:

Test:		tip			latency_nice
 Copy:   340877.18 (0.00 pct)    339441.26 (-0.42 pct)
Scale:   217318.16 (0.00 pct)    216905.49 (-0.18 pct)
  Add:   259078.93 (0.00 pct)    261686.67 (1.00 pct)
Triad:   274500.78 (0.00 pct)    271699.83 (-1.02 pct)

100 Runs:

Test:		tip			latency_nice
 Copy:   341860.73 (0.00 pct)    335826.36 (-1.76 pct)
Scale:   218043.00 (0.00 pct)    216451.84 (-0.72 pct)
  Add:   253698.22 (0.00 pct)    257317.72 (1.42 pct)
Triad:   265011.84 (0.00 pct)    267769.93 (1.04 pct)

NPS4

10 Runs:

Test:		tip			latency_nice
 Copy:   340877.18 (0.00 pct)    365921.51 (7.34 pct)
Scale:   217318.16 (0.00 pct)    239408.65 (10.16 pct)
  Add:   259078.93 (0.00 pct)    264859.31 (2.23 pct)
Triad:   274500.78 (0.00 pct)    281543.65 (2.56 pct)

100 Runs:

Test:		tip			latency_nice
 Copy:   341860.73 (0.00 pct)    359255.16 (5.08 pct)
Scale:   218043.00 (0.00 pct)    238154.15 (9.22 pct)
  Add:   253698.22 (0.00 pct)    269223.49 (6.11 pct)
Triad:   265011.84 (0.00 pct)    278473.85 (5.07 pct)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~ ycsb-mongodb - DEFAULT_LATENCY_NICE ~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

o NPS1

tip:                    131244.00 (var: 2.67%)
latency_nice:           132118.00 (var: 3.62%) (+0.66%)

o NPS2

tip:                    127663.33 (var: 2.08%)
latency_nice:           129148.00 (var: 4.29%) (+1.16%)

o NPS4

tip:                    133295.00 (var: 1.58%)
latency_nice:           129975.33 (var: 1.10%) (-2.49%)


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~ Unixbench - DEFAULT_LATENCY_NICE ~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

o NPS1

Test			Metric	  Parallelism			tip		      latency_nice
unixbench-dhry2reg      Hmean     unixbench-dhry2reg-1      48929419.48 (   0.00%)    49137039.06 (   0.42%)
unixbench-dhry2reg      Hmean     unixbench-dhry2reg-512  6275526953.25 (   0.00%)  6265580479.15 (  -0.16%)
unixbench-syscall       Amean     unixbench-syscall-1        2994319.73 (   0.00%)     3008596.83 *  -0.48%*
unixbench-syscall       Amean     unixbench-syscall-512      7349715.87 (   0.00%)     7420994.50 *  -0.97%*
unixbench-pipe          Hmean     unixbench-pipe-1           2830206.03 (   0.00%)     2854405.99 *   0.86%*
unixbench-pipe          Hmean     unixbench-pipe-512       326207828.01 (   0.00%)   328997804.52 *   0.86%*
unixbench-spawn         Hmean     unixbench-spawn-1             6394.21 (   0.00%)        6367.75 (  -0.41%)
unixbench-spawn         Hmean     unixbench-spawn-512          72700.64 (   0.00%)       71454.19 *  -1.71%*
unixbench-execl         Hmean     unixbench-execl-1             4723.61 (   0.00%)        4750.59 (   0.57%)
unixbench-execl         Hmean     unixbench-execl-512          11212.05 (   0.00%)       11262.13 (   0.45%)

o NPS2

Test			Metric	  Parallelism			tip		      latency_nice
unixbench-dhry2reg      Hmean     unixbench-dhry2reg-1      49271512.85 (   0.00%)    49245260.43 (  -0.05%)
unixbench-dhry2reg      Hmean     unixbench-dhry2reg-512  6267992483.03 (   0.00%)  6264951100.67 (  -0.05%)
unixbench-syscall       Amean     unixbench-syscall-1        2995885.93 (   0.00%)     3005975.10 *  -0.34%*
unixbench-syscall       Amean     unixbench-syscall-512      7388865.77 (   0.00%)     7276275.63 *   1.52%*
unixbench-pipe          Hmean     unixbench-pipe-1           2828971.95 (   0.00%)     2856578.72 *   0.98%*
unixbench-pipe          Hmean     unixbench-pipe-512       326225385.37 (   0.00%)   328941270.81 *   0.83%*
unixbench-spawn         Hmean     unixbench-spawn-1             6958.71 (   0.00%)        6954.21 (  -0.06%)
unixbench-spawn         Hmean     unixbench-spawn-512          85443.56 (   0.00%)       70536.42 * -17.45%* (0.67% vs 0.93% - CoEff var)
unixbench-execl         Hmean     unixbench-execl-1             4767.99 (   0.00%)        4752.63 *  -0.32%*
unixbench-execl         Hmean     unixbench-execl-512          11250.72 (   0.00%)       11320.97 (   0.62%)

o NPS4

Test			Metric	  Parallelism			tip		      latency_nice
unixbench-dhry2reg      Hmean     unixbench-dhry2reg-1      49041932.68 (   0.00%)    49156671.05 (   0.23%)
unixbench-dhry2reg      Hmean     unixbench-dhry2reg-512  6286981589.85 (   0.00%)  6285248711.40 (  -0.03%)
unixbench-syscall       Amean     unixbench-syscall-1        2992405.60 (   0.00%)     3008933.03 *  -0.55%*
unixbench-syscall       Amean     unixbench-syscall-512      7971789.70 (   0.00%)     7814622.23 *   1.97%*
unixbench-pipe          Hmean     unixbench-pipe-1           2822892.54 (   0.00%)     2852615.11 *   1.05%*
unixbench-pipe          Hmean     unixbench-pipe-512       326408309.83 (   0.00%)   329617202.56 *   0.98%*
unixbench-spawn         Hmean     unixbench-spawn-1             7685.31 (   0.00%)        7243.54 (  -5.75%)
unixbench-spawn         Hmean     unixbench-spawn-512          72245.56 (   0.00%)       77000.81 *   6.58%*
unixbench-execl         Hmean     unixbench-execl-1             4761.42 (   0.00%)        4733.12 *  -0.59%*
unixbench-execl         Hmean     unixbench-execl-512          11533.53 (   0.00%)       11660.17 (   1.10%)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~ Hackbench - Various Latency Nice Values ~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

o 100000 loops

- pipe (process)

Test:                   LN: 0                   LN: 19                  LN: -20
 1-groups:         3.91 (0.00 pct)         3.91 (0.00 pct)         3.81 (2.55 pct)
 2-groups:         4.48 (0.00 pct)         4.52 (-0.89 pct)        4.53 (-1.11 pct)
 4-groups:         4.83 (0.00 pct)         4.83 (0.00 pct)         4.87 (-0.82 pct)
 8-groups:         5.09 (0.00 pct)         5.00 (1.76 pct)         5.07 (0.39 pct)
16-groups:         6.92 (0.00 pct)         6.79 (1.87 pct)         6.96 (-0.57 pct)

- pipe (thread)

 1-groups:         4.13 (0.00 pct)         4.08 (1.21 pct)         4.11 (0.48 pct)
 2-groups:         4.78 (0.00 pct)         4.90 (-2.51 pct)        4.79 (-0.20 pct)
 4-groups:         5.12 (0.00 pct)         5.08 (0.78 pct)         5.16 (-0.78 pct)
 8-groups:         5.31 (0.00 pct)         5.28 (0.56 pct)         5.33 (-0.37 pct)
16-groups:         7.34 (0.00 pct)         7.27 (0.95 pct)         7.33 (0.13 pct)

- socket (process)

Test:                   LN: 0                   LN: 19                  LN: -20
 1-groups:         6.61 (0.00 pct)         6.38 (3.47 pct)         6.54 (1.05 pct)
 2-groups:         6.59 (0.00 pct)         6.67 (-1.21 pct)        6.11 (7.28 pct)
 4-groups:         6.77 (0.00 pct)         6.78 (-0.14 pct)        6.79 (-0.29 pct)
 8-groups:         8.29 (0.00 pct)         8.39 (-1.20 pct)        8.36 (-0.84 pct)
16-groups:        12.21 (0.00 pct)        12.03 (1.47 pct)        12.35 (-1.14 pct)

- socket (thread)

Test:                   LN: 0                   LN: 19                  LN: -20
 1-groups:         6.50 (0.00 pct)         5.99 (7.84 pct)         6.02 (7.38 pct)	^
 2-groups:         6.07 (0.00 pct)         6.20 (-2.14 pct)        6.23 (-2.63 pct)
 4-groups:         6.61 (0.00 pct)         6.64 (-0.45 pct)        6.63 (-0.30 pct)
 8-groups:         8.87 (0.00 pct)         8.67 (2.25 pct)         8.78 (1.01 pct)
16-groups:        12.63 (0.00 pct)        12.54 (0.71 pct)        12.59 (0.31 pct)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~ Hackbench + Cyclictest - Various Latency Nice Values ~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

- Hackbench: 32 Groups

perf bench sched messaging -p -l 100000 -g 32&
cyclictest --policy other -D 5 -q -n -h 2000

o NPS1

----------------------------------------------------------------------------------------------------------
| Hackbench   |      Cyclictest LN = 19      |      Cyclictest LN = 0        |    Cyclictest LN = -20    |
| LN          |------------------------------|-------------------------------|---------------------------|
|             |   Min  |   Avg   |  Max      |   Min  |   Avg   |   Max      |   Min  |  Avg  |   Max    |
|-------------|--------|---------|-----------|--------|---------|------------|--------|-------|----------|
| 19          | 52.00  | 71.00   | 5191.00   | 29.00  | 68.00   |  4477.00   | 53.00  | 60.00 |  753.00  |
| 0           | 53.00  | 150.00  | 7300.00   | 53.00  | 105.00  |  7730.00   | 53.00  | 64.00 |  2067.00 |
| -20         | 33.00  | 159.00  | 98492.00  | 53.00  | 149.00  |  9608.00   | 53.00  | 91.00 |  5349.00 |
----------------------------------------------------------------------------------------------------------

o NPS4

----------------------------------------------------------------------------------------------------------
| Hackbench   |      Cyclictest LN = 19      |      Cyclictest LN = 0        |    Cyclictest LN = -20    |
| LN          |------------------------------|-------------------------------|---------------------------|
|             |   Min  |   Avg   |  Max      |   Min  |   Avg   |   Max      |   Min  |  Avg  |   Max    |
|-------------|--------|---------|-----------|--------|---------|------------|--------|-------|----------|
| 19          | 53.00  |  84.00  |  4790.00  | 53.00  |  72.00  |  3456.00   | 53.00  | 58.00 |  1271.00 |
| 0           | 53.00  |  99.00  |  5494.00  | 52.00  |  74.00  |  5813.00   | 53.00  | 59.00 |  1004.00 |
| -20         | 45.00  |  84.00  |  3592.00  | 53.00  |  91.00  |  15222.00  | 53.00  | 74.00 |  5232.00 |	^
----------------------------------------------------------------------------------------------------------

- Hackbench: 128 Groups

perf bench sched messaging -p -l 500000 -g 128&
cyclictest --policy other -D 5 -q -n -h 2000

o NPS1

----------------------------------------------------------------------------------------------------------
| Hackbench   |      Cyclictest LN = 19      |      Cyclictest LN = 0        |    Cyclictest LN = -20    |
| LN          |------------------------------|-------------------------------|---------------------------|
|             |   Min  |   Avg   |  Max      |   Min  |   Avg   |   Max      |   Min  |  Avg  |   Max    |
|-------------|--------|---------|-----------|--------|---------|------------|--------|-------|----------|
| 19          | 53.00  | 274.00  | 11294.00  | 33.00  | 130.00  |  20071.00  | 53.00  | 56.00 |  244.00  |	^
| 0           | 53.00  | 125.00  | 10014.00  | 53.00  | 113.00  |  15857.00  | 53.00  | 57.00 |  250.00  |
| -20         | 53.00  | 187.00  | 49565.00  | 53.00  | 230.00  |  73353.00  | 53.00  | 118.00|  8816.00 |
----------------------------------------------------------------------------------------------------------

o NPS4

----------------------------------------------------------------------------------------------------------
| Hackbench   |      Cyclictest LN = 19      |      Cyclictest LN = 0        |    Cyclictest LN = -20    |
| LN          |------------------------------|-------------------------------|---------------------------|
|             |   Min  |   Avg   |  Max      |   Min  |   Avg   |   Max      |   Min  |  Avg  |   Max    |
|-------------|--------|---------|-----------|--------|---------|------------|--------|-------|----------|
| 19          | 53.00  | 271.00  | 11411.00  | 53.00  | 82.00   |  5486.00   | 25.00  | 57.00 | 1256.00  |
| 0           | 53.00  | 148.00  | 8374.00   | 52.00  | 109.00  |  11074.00  | 52.00  | 59.00 | 1068.00  |
| -20         | 53.00  | 202.00  | 52537.00  | 53.00  | 205.00  |  22265.00  | 52.00  | 87.00 | 14151.00 |
----------------------------------------------------------------------------------------------------------

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~ Hackbench + schbench - Various Latency Nice Values ~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

perf bench sched messaging -p -l 400000 -g 128
schbench -m 2 -t 1 -s 30

o NPS1

-------------------------------------------------------------------------------------------------
| Hackbench |     schbench LN = 19       |      schbench LN = 0      |     schbench LN = -20    |
| LN        |----------------------------|---------------------------|--------------------------|
|           |  90th  |  95th  |  99th    |  90th  |  95th  |  99th   |  90th  |  95th  |  99th  |
|-----------|--------|--------|----------|--------|--------|---------|--------|--------|--------|
| 19        |   38   |   131  |   1458   |   46   |   151  |  2636   |   11   |   19   |  410   |	^
| 0         |   45   |   98   |   1758   |   25   |   50   |  1670   |   16   |   30   |  1042  |
| -20       |   47   |   348  |   29280  |   40   |   109  |  16144  |   35   |   63   |  9104  |
-------------------------------------------------------------------------------------------------

o NPS4

-------------------------------------------------------------------------------------------------
| Hackbench |     schbench LN = 19       |      schbench LN = 0      |     schbench LN = -20    |
| LN        |----------------------------|---------------------------|--------------------------|
|           |  90th  |  95th  |  99th    |  90th  |  95th  |  99th   |  90th  |  95th  |  99th  |
|-----------|--------|--------|----------|--------|--------|---------|--------|--------|--------|
| 19        |   19   |  60    |  1886    |   17   |  29    |  621    |   10   |   18   |  227   |
| 0         |   51   |  141   |  8120    |   37   |  78    |  8880   |   33   |   55   |  474   |	^
| -20       |   48   |  1494  |  27296   |   51   |  469   |  40384  |   31   |   64   |  4092  |	^
-------------------------------------------------------------------------------------------------

^ Note: There are cases where the Max, 99th percentile latency is
non-monotonic but I've also seen a good amount of run to run variation
there with a single bad sample polluting the results. In such cases,
the averages are more representative.

> 
> [1] https://source.android.com/docs/core/debug/eval_perf#touchlatency
> 
> [..snip..]
> 

Apart from couple of anomalies, latency nice reduces wait time, especially
when the system is heavily loaded. If there is any data, or any specific
workload you would like me to run on the test system, please do let me know.
Meanwhile, I'll try to get some numbers for larger workloads like SpecJBB
that did see improvements with latency nice on v5.
--
Thanks and Regards,
Prateek

Re: [PATCH v9 0/9] Add latency priority for CFS class

[Vincent Guittot]

Hi Prateek,

On Mon, 28 Nov 2022 at 12:52, K Prateek Nayak <kprateek.nayak@amd.com> wrote:
>
> Hello Vincent,
>
> Following are the test results on dual socket Zen3 machine (2 x 64C/128T)
>
> tl;dr
>
> o All benchmarks with DEFAULT_LATENCY_NICE value are comparable to tip.
>   There is, however, a noticeable dip for unixbench-spawn test case.
>
> o With the 2 rbtree approach, I do not see much difference in the
>   hackbench results with varying latency nice value. Tests on v5 did
>   yield noticeable improvements for hackbench.
>   (https://lore.kernel.org/lkml/cd48ebbb-9724-985f-28e3-e558dea07827@amd.com/)

The 2 rbtree approach is the one that was already used in v5. I just
rerun hackbench tests with latest tip and v6.2-rc7 and I can see large
performance improvement for pipe tests on my system (8 cores system).
Could you try witha larger number of group ? like 64, 128 and 256
groups

>
> o For hackbench + cyclictest and hackbench + schbench, I see the
>   expected behavior with different latency nice values.
>
> o There are a few cases with hackbench and hackbench + cyclictest where
>   the results are non-monotonic with different latency nice values.
>   (Marked with "^").
>
> I'll leave the detailed results below:
>
> On 11/15/2022 10:48 PM, Vincent Guittot wrote:
> > This patchset restarts the work about adding a latency priority to describe
> > the latency tolerance of cfs tasks.
> >
> > Patch [1] is a new one that has been added with v6. It fixes an
> > unfairness for low prio tasks because of wakeup_gran() being bigger
> > than the maximum vruntime credit that a waking task can keep after
> > sleeping.
> >
> > The patches [2-4] have been done by Parth:
> > https://lore.kernel.org/lkml/20200228090755.22829-1-parth@linux.ibm.com/
> >
> > I have just rebased and moved the set of latency priority outside the
> > priority update. I have removed the reviewed tag because the patches
> > are 2 years old.
> >
> > This aims to be a generic interface and the following patches is one use
> > of it to improve the scheduling latency of cfs tasks.
> >
> > Patch [5] uses latency nice priority to define a latency offset
> > and then decide if a cfs task can or should preempt the current
> > running task. The patch gives some tests results with cyclictests and
> > hackbench to highlight the benefit of latency priority for short
> > interactive task or long intensive tasks.
> >
> > Patch [6] adds the support of latency nice priority to task group by
> > adding a cpu.latency.nice field. The range is [-20:19] as for setting task
> > latency priority.
> >
> > Patch [7] makes sched_core taking into account the latency offset.
> >
> > Patch [8] adds a rb tree to cover some corner cases where the latency
> > sensitive task (priority < 0) is preempted by high priority task (RT/DL)
> > or fails to preempt them. This patch ensures that tasks will have at least
> > a slice of sched_min_granularity in priority at wakeup.
> >
> > Patch [9] removes useless check after adding a latency rb tree.
> >
> > I have also backported the patchset on a dragonboard RB3 with an android
> > mainline kernel based on v5.18 for a quick test. I have used the
> > TouchLatency app which is part of AOSP and described to be a very good
> > test to highlight jitter and jank frame sources of a system [1].
> > In addition to the app, I have added some short running tasks waking-up
> > regularly (to use the 8 cpus for 4 ms every 37777us) to stress the system
> > without overloading it (and disabling EAS). The 1st results shows that the
> > patchset helps to reduce the missed deadline frames from 5% to less than
> > 0.1% when the cpu.latency.nice of task group are set. I haven't rerun the
> > test with latest version.
> >
> > I have also tested the patchset with the modified version of the alsa
> > latency test that has been shared by Tim. The test quickly xruns with
> > default latency nice priority 0 but is able to run without underuns with
> > a latency -20 and hackbench running simultaneously.
> >
> > While preparing the version 8, I have evaluated the benefit of using an
> > augmented rbtree instead of adding a rbtree for latency sensitive entities,
> > which was a relevant suggestion done by PeterZ. Although the augmented
> > rbtree enables to sort additional information in the tree with a limited
> > overhead, it has more impact on legacy use cases (latency_nice >= 0)
> > because the augmented callbacks are always called to maintain this
> > additional information even when there is no sensitive tasks. In such
> > cases, the dedicated rbtree remains empty and the overhead is reduced to
> > loading a cached null node pointer. Nevertheless, we might want to
> > reconsider the augmented rbtree once the use of negative latency_nice will
> > be more widlely deployed. At now, the different tests that I have done,
> > have not shown improvements with augmented rbtree.
> >
> > Below are some hackbench results:
> >         2 rbtrees               augmented rbtree        augmented rbtree
> >                                 sorted by vruntime      sorted by wakeup_vruntime
> > sched pipe
> > avg     26311,000               25976,667               25839,556
> > stdev   0,15 %                  0,28 %                  0,24 %
> > vs tip  0,50 %                  -0,78 %                 -1,31 %
> > hackbench     1 group
> > avg     1,315                   1,344                   1,359
> > stdev   0,88 %                  1,55 %                  1,82 %
> > vs tip  -0,47 %                 -2,68 %                 -3,87 %
> > hackbench     4 groups
> > avg     1,339                   1,365                   1,367
> > stdev   2,39 %                  2,26 %                  3,58 %
> > vs tip  -0,08 %                 -2,01 %                 -2,22 %
> > hackbench     8 groups
> > avg     1,233                   1,286                   1,301
> > stdev   0,74 %                  1,09 %                  1,52 %
> > vs tip  0,29 %                  -4,05 %                 -5,27 %
> > hackbench     16 groups
> > avg     1,268                   1,313                   1,319
> > stdev   0,85 %                  1,60 %                  0,68 %
> > vs tip  -0,02 %                 -3,56 %                 -4,01 %
>
> Following are the results from running standard benchmarks on a
> dual socket Zen3 (2 x 64C/128T) machine configured in different
> NPS modes.
>
> NPS Modes are used to logically divide single socket into
> multiple NUMA region.
> Following is the NUMA configuration for each NPS mode on the system:
>
> NPS1: Each socket is a NUMA node.
>     Total 2 NUMA nodes in the dual socket machine.
>
>     Node 0: 0-63,   128-191
>     Node 1: 64-127, 192-255
>
> NPS2: Each socket is further logically divided into 2 NUMA regions.
>     Total 4 NUMA nodes exist over 2 socket.
>
>     Node 0: 0-31,   128-159
>     Node 1: 32-63,  160-191
>     Node 2: 64-95,  192-223
>     Node 3: 96-127, 223-255
>
> NPS4: Each socket is logically divided into 4 NUMA regions.
>     Total 8 NUMA nodes exist over 2 socket.
>
>     Node 0: 0-15,    128-143
>     Node 1: 16-31,   144-159
>     Node 2: 32-47,   160-175
>     Node 3: 48-63,   176-191
>     Node 4: 64-79,   192-207
>     Node 5: 80-95,   208-223
>     Node 6: 96-111,  223-231
>     Node 7: 112-127, 232-255
>
> Benchmark Results:
>
> Kernel versions:
> - tip:          6.1.0 tip sched/core
> - latency_nice: 6.1.0 tip sched/core + this series
>
> When we started testing, the tip was at:
> commit d6962c4fe8f9 "sched: Clear ttwu_pending after enqueue_task()"
>
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~ hackbench - DEFAULT_LATENCY_NICE ~
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> NPS1
>
> Test:                   tip                     latency_nice
>  1-groups:         4.25 (0.00 pct)         4.14 (2.58 pct)
>  2-groups:         4.95 (0.00 pct)         4.92 (0.60 pct)
>  4-groups:         5.19 (0.00 pct)         5.18 (0.19 pct)
>  8-groups:         5.45 (0.00 pct)         5.44 (0.18 pct)
> 16-groups:         7.33 (0.00 pct)         7.32 (0.13 pct)
>
> NPS2
>
> Test:                   tip                     latency_nice
>  1-groups:         4.09 (0.00 pct)         4.08 (0.24 pct)
>  2-groups:         4.68 (0.00 pct)         4.72 (-0.85 pct)
>  4-groups:         5.05 (0.00 pct)         4.97 (1.58 pct)
>  8-groups:         5.37 (0.00 pct)         5.34 (0.55 pct)
> 16-groups:         6.69 (0.00 pct)         6.74 (-0.74 pct)
>
> NPS4
>
> Test:                   tip                     latency_nice
>  1-groups:         4.28 (0.00 pct)         4.35 (-1.63 pct)
>  2-groups:         4.78 (0.00 pct)         4.76 (0.41 pct)
>  4-groups:         5.11 (0.00 pct)         5.06 (0.97 pct)
>  8-groups:         5.48 (0.00 pct)         5.40 (1.45 pct)
> 16-groups:         7.07 (0.00 pct)         6.70 (5.23 pct)
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~ schbench - DEFAULT_LATENCY_NICE ~
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> NPS1
>
> #workers:       tip                     latency_nice
>   1:      31.00 (0.00 pct)        32.00 (-3.22 pct)
>   2:      33.00 (0.00 pct)        34.00 (-3.03 pct)
>   4:      39.00 (0.00 pct)        38.00 (2.56 pct)
>   8:      45.00 (0.00 pct)        46.00 (-2.22 pct)
>  16:      61.00 (0.00 pct)        66.00 (-8.19 pct)
>  32:     108.00 (0.00 pct)       110.00 (-1.85 pct)
>  64:     212.00 (0.00 pct)       216.00 (-1.88 pct)
> 128:     475.00 (0.00 pct)       701.00 (-47.57 pct)    *
> 128:     429.00 (0.00 pct)       441.00 (-2.79 pct)      [Verification Run]
> 256:     44736.00 (0.00 pct)     45632.00 (-2.00 pct)
> 512:     77184.00 (0.00 pct)     78720.00 (-1.99 pct)
>
> NPS2
>
> #workers:       tip                     latency_nice
>   1:      28.00 (0.00 pct)        33.00 (-17.85 pct)
>   2:      34.00 (0.00 pct)        31.00 (8.82 pct)
>   4:      36.00 (0.00 pct)        36.00 (0.00 pct)
>   8:      51.00 (0.00 pct)        49.00 (3.92 pct)
>  16:      68.00 (0.00 pct)        64.00 (5.88 pct)
>  32:     113.00 (0.00 pct)       115.00 (-1.76 pct)
>  64:     221.00 (0.00 pct)       219.00 (0.90 pct)
> 128:     553.00 (0.00 pct)       531.00 (3.97 pct)
> 256:     43840.00 (0.00 pct)     48192.00 (-9.92 pct)   *
> 256:     50427.00 (0.00 pct)     48351.00 (4.11 pct)    [Verification Run]
> 512:     76672.00 (0.00 pct)     81024.00 (-5.67 pct)
>
> NPS4
>
> #workers:       tip                     latency_nice
>   1:      33.00 (0.00 pct)        28.00 (15.15 pct)
>   2:      29.00 (0.00 pct)        34.00 (-17.24 pct)
>   4:      39.00 (0.00 pct)        36.00 (7.69 pct)
>   8:      58.00 (0.00 pct)        55.00 (5.17 pct)
>  16:      66.00 (0.00 pct)        67.00 (-1.51 pct)
>  32:     112.00 (0.00 pct)       116.00 (-3.57 pct)
>  64:     215.00 (0.00 pct)       213.00 (0.93 pct)
> 128:     689.00 (0.00 pct)       571.00 (17.12 pct)
> 256:     45120.00 (0.00 pct)     46400.00 (-2.83 pct)
> 512:     77440.00 (0.00 pct)     76160.00 (1.65 pct)
>
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~ tbench - DEFAULT_LATENCY_NICE ~
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> NPS1
>
> Clients:        tip                     latency_nice
>     1    581.75 (0.00 pct)       586.52 (0.81 pct)
>     2    1145.75 (0.00 pct)      1160.69 (1.30 pct)
>     4    2127.94 (0.00 pct)      2141.49 (0.63 pct)
>     8    3838.27 (0.00 pct)      3721.10 (-3.05 pct)
>    16    6272.71 (0.00 pct)      6539.82 (4.25 pct)
>    32    11400.12 (0.00 pct)     12079.49 (5.95 pct)
>    64    21605.96 (0.00 pct)     22908.83 (6.03 pct)
>   128    30715.43 (0.00 pct)     31736.95 (3.32 pct)
>   256    55580.78 (0.00 pct)     54786.29 (-1.42 pct)
>   512    56528.79 (0.00 pct)     56453.54 (-0.13 pct)
>  1024    56520.40 (0.00 pct)     56369.93 (-0.26 pct)
>
> NPS2
>
> Clients:        tip                     latency_nice
>     1    584.13 (0.00 pct)       582.53 (-0.27 pct)
>     2    1153.63 (0.00 pct)      1140.27 (-1.15 pct)
>     4    2212.89 (0.00 pct)      2159.49 (-2.41 pct)
>     8    3871.35 (0.00 pct)      3840.77 (-0.78 pct)
>    16    6216.72 (0.00 pct)      6437.98 (3.55 pct)
>    32    11766.98 (0.00 pct)     11663.53 (-0.87 pct)
>    64    22000.93 (0.00 pct)     21882.88 (-0.53 pct)
>   128    31520.53 (0.00 pct)     31147.05 (-1.18 pct)
>   256    51420.11 (0.00 pct)     55216.39 (7.38 pct)
>   512    53935.90 (0.00 pct)     55407.60 (2.72 pct)
>  1024    55239.73 (0.00 pct)     55997.25 (1.37 pct)
>
> NPS4
>
> Clients:        tip                     latency_nice
>     1    585.83 (0.00 pct)       578.17 (-1.30 pct)
>     2    1141.59 (0.00 pct)      1131.14 (-0.91 pct)
>     4    2174.79 (0.00 pct)      2086.52 (-4.05 pct)
>     8    3887.56 (0.00 pct)      3778.47 (-2.80 pct)
>    16    6441.59 (0.00 pct)      6364.30 (-1.19 pct)
>    32    12133.60 (0.00 pct)     11465.26 (-5.50 pct)   *
>    32    11677.16 (0.00 pct)     12662.09 (8.43 pct)    [Verification Run]
>    64    21769.15 (0.00 pct)     19488.45 (-10.47 pct)  *
>    64    20305.64 (0.00 pct)     21002.90 (3.43 pct)    [Verification Run]
>   128    31396.31 (0.00 pct)     31177.37 (-0.69 pct)
>   256    52792.39 (0.00 pct)     52890.41 (0.18 pct)
>   512    55315.44 (0.00 pct)     53572.65 (-3.15 pct)
>  1024    52150.27 (0.00 pct)     54079.48 (3.69 pct)
>
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~ stream - DEFAULT_LATENCY_NICE ~
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> NPS1
>
> 10 Runs:
>
> Test:           tip                     latency_nice
>  Copy:   307827.79 (0.00 pct)    330524.48 (7.37 pct)
> Scale:   208872.28 (0.00 pct)    215002.06 (2.93 pct)
>   Add:   239404.64 (0.00 pct)    230334.74 (-3.78 pct)
> Triad:   247258.30 (0.00 pct)    238505.06 (-3.54 pct)
>
> 100 Runs:
>
> Test:           tip                     latency_nice
>  Copy:   317217.55 (0.00 pct)    314467.62 (-0.86 pct)
> Scale:   208740.82 (0.00 pct)    210452.00 (0.81 pct)
>   Add:   240550.63 (0.00 pct)    232376.03 (-3.39 pct)
> Triad:   249594.21 (0.00 pct)    242460.83 (-2.85 pct)
>
> NPS2
>
> 10 Runs:
>
> Test:           tip                     latency_nice
>  Copy:   340877.18 (0.00 pct)    339441.26 (-0.42 pct)
> Scale:   217318.16 (0.00 pct)    216905.49 (-0.18 pct)
>   Add:   259078.93 (0.00 pct)    261686.67 (1.00 pct)
> Triad:   274500.78 (0.00 pct)    271699.83 (-1.02 pct)
>
> 100 Runs:
>
> Test:           tip                     latency_nice
>  Copy:   341860.73 (0.00 pct)    335826.36 (-1.76 pct)
> Scale:   218043.00 (0.00 pct)    216451.84 (-0.72 pct)
>   Add:   253698.22 (0.00 pct)    257317.72 (1.42 pct)
> Triad:   265011.84 (0.00 pct)    267769.93 (1.04 pct)
>
> NPS4
>
> 10 Runs:
>
> Test:           tip                     latency_nice
>  Copy:   340877.18 (0.00 pct)    365921.51 (7.34 pct)
> Scale:   217318.16 (0.00 pct)    239408.65 (10.16 pct)
>   Add:   259078.93 (0.00 pct)    264859.31 (2.23 pct)
> Triad:   274500.78 (0.00 pct)    281543.65 (2.56 pct)
>
> 100 Runs:
>
> Test:           tip                     latency_nice
>  Copy:   341860.73 (0.00 pct)    359255.16 (5.08 pct)
> Scale:   218043.00 (0.00 pct)    238154.15 (9.22 pct)
>   Add:   253698.22 (0.00 pct)    269223.49 (6.11 pct)
> Triad:   265011.84 (0.00 pct)    278473.85 (5.07 pct)
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~ ycsb-mongodb - DEFAULT_LATENCY_NICE ~
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> o NPS1
>
> tip:                    131244.00 (var: 2.67%)
> latency_nice:           132118.00 (var: 3.62%) (+0.66%)
>
> o NPS2
>
> tip:                    127663.33 (var: 2.08%)
> latency_nice:           129148.00 (var: 4.29%) (+1.16%)
>
> o NPS4
>
> tip:                    133295.00 (var: 1.58%)
> latency_nice:           129975.33 (var: 1.10%) (-2.49%)
>
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~ Unixbench - DEFAULT_LATENCY_NICE ~
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> o NPS1
>
> Test                    Metric    Parallelism                   tip                   latency_nice
> unixbench-dhry2reg      Hmean     unixbench-dhry2reg-1      48929419.48 (   0.00%)    49137039.06 (   0.42%)
> unixbench-dhry2reg      Hmean     unixbench-dhry2reg-512  6275526953.25 (   0.00%)  6265580479.15 (  -0.16%)
> unixbench-syscall       Amean     unixbench-syscall-1        2994319.73 (   0.00%)     3008596.83 *  -0.48%*
> unixbench-syscall       Amean     unixbench-syscall-512      7349715.87 (   0.00%)     7420994.50 *  -0.97%*
> unixbench-pipe          Hmean     unixbench-pipe-1           2830206.03 (   0.00%)     2854405.99 *   0.86%*
> unixbench-pipe          Hmean     unixbench-pipe-512       326207828.01 (   0.00%)   328997804.52 *   0.86%*
> unixbench-spawn         Hmean     unixbench-spawn-1             6394.21 (   0.00%)        6367.75 (  -0.41%)
> unixbench-spawn         Hmean     unixbench-spawn-512          72700.64 (   0.00%)       71454.19 *  -1.71%*
> unixbench-execl         Hmean     unixbench-execl-1             4723.61 (   0.00%)        4750.59 (   0.57%)
> unixbench-execl         Hmean     unixbench-execl-512          11212.05 (   0.00%)       11262.13 (   0.45%)
>
> o NPS2
>
> Test                    Metric    Parallelism                   tip                   latency_nice
> unixbench-dhry2reg      Hmean     unixbench-dhry2reg-1      49271512.85 (   0.00%)    49245260.43 (  -0.05%)
> unixbench-dhry2reg      Hmean     unixbench-dhry2reg-512  6267992483.03 (   0.00%)  6264951100.67 (  -0.05%)
> unixbench-syscall       Amean     unixbench-syscall-1        2995885.93 (   0.00%)     3005975.10 *  -0.34%*
> unixbench-syscall       Amean     unixbench-syscall-512      7388865.77 (   0.00%)     7276275.63 *   1.52%*
> unixbench-pipe          Hmean     unixbench-pipe-1           2828971.95 (   0.00%)     2856578.72 *   0.98%*
> unixbench-pipe          Hmean     unixbench-pipe-512       326225385.37 (   0.00%)   328941270.81 *   0.83%*
> unixbench-spawn         Hmean     unixbench-spawn-1             6958.71 (   0.00%)        6954.21 (  -0.06%)
> unixbench-spawn         Hmean     unixbench-spawn-512          85443.56 (   0.00%)       70536.42 * -17.45%* (0.67% vs 0.93% - CoEff var)

I don't expect any perf improvement or regression when the latency
nice is not changed

> unixbench-execl         Hmean     unixbench-execl-1             4767.99 (   0.00%)        4752.63 *  -0.32%*
> unixbench-execl         Hmean     unixbench-execl-512          11250.72 (   0.00%)       11320.97 (   0.62%)
>
> o NPS4
>
> Test                    Metric    Parallelism                   tip                   latency_nice
> unixbench-dhry2reg      Hmean     unixbench-dhry2reg-1      49041932.68 (   0.00%)    49156671.05 (   0.23%)
> unixbench-dhry2reg      Hmean     unixbench-dhry2reg-512  6286981589.85 (   0.00%)  6285248711.40 (  -0.03%)
> unixbench-syscall       Amean     unixbench-syscall-1        2992405.60 (   0.00%)     3008933.03 *  -0.55%*
> unixbench-syscall       Amean     unixbench-syscall-512      7971789.70 (   0.00%)     7814622.23 *   1.97%*
> unixbench-pipe          Hmean     unixbench-pipe-1           2822892.54 (   0.00%)     2852615.11 *   1.05%*
> unixbench-pipe          Hmean     unixbench-pipe-512       326408309.83 (   0.00%)   329617202.56 *   0.98%*
> unixbench-spawn         Hmean     unixbench-spawn-1             7685.31 (   0.00%)        7243.54 (  -5.75%)
> unixbench-spawn         Hmean     unixbench-spawn-512          72245.56 (   0.00%)       77000.81 *   6.58%*
> unixbench-execl         Hmean     unixbench-execl-1             4761.42 (   0.00%)        4733.12 *  -0.59%*
> unixbench-execl         Hmean     unixbench-execl-512          11533.53 (   0.00%)       11660.17 (   1.10%)
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~ Hackbench - Various Latency Nice Values ~
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> o 100000 loops
>
> - pipe (process)
>
> Test:                   LN: 0                   LN: 19                  LN: -20
>  1-groups:         3.91 (0.00 pct)         3.91 (0.00 pct)         3.81 (2.55 pct)
>  2-groups:         4.48 (0.00 pct)         4.52 (-0.89 pct)        4.53 (-1.11 pct)
>  4-groups:         4.83 (0.00 pct)         4.83 (0.00 pct)         4.87 (-0.82 pct)
>  8-groups:         5.09 (0.00 pct)         5.00 (1.76 pct)         5.07 (0.39 pct)
> 16-groups:         6.92 (0.00 pct)         6.79 (1.87 pct)         6.96 (-0.57 pct)
>
> - pipe (thread)
>
>  1-groups:         4.13 (0.00 pct)         4.08 (1.21 pct)         4.11 (0.48 pct)
>  2-groups:         4.78 (0.00 pct)         4.90 (-2.51 pct)        4.79 (-0.20 pct)
>  4-groups:         5.12 (0.00 pct)         5.08 (0.78 pct)         5.16 (-0.78 pct)
>  8-groups:         5.31 (0.00 pct)         5.28 (0.56 pct)         5.33 (-0.37 pct)
> 16-groups:         7.34 (0.00 pct)         7.27 (0.95 pct)         7.33 (0.13 pct)
>
> - socket (process)
>
> Test:                   LN: 0                   LN: 19                  LN: -20
>  1-groups:         6.61 (0.00 pct)         6.38 (3.47 pct)         6.54 (1.05 pct)
>  2-groups:         6.59 (0.00 pct)         6.67 (-1.21 pct)        6.11 (7.28 pct)
>  4-groups:         6.77 (0.00 pct)         6.78 (-0.14 pct)        6.79 (-0.29 pct)
>  8-groups:         8.29 (0.00 pct)         8.39 (-1.20 pct)        8.36 (-0.84 pct)
> 16-groups:        12.21 (0.00 pct)        12.03 (1.47 pct)        12.35 (-1.14 pct)
>
> - socket (thread)
>
> Test:                   LN: 0                   LN: 19                  LN: -20
>  1-groups:         6.50 (0.00 pct)         5.99 (7.84 pct)         6.02 (7.38 pct)      ^
>  2-groups:         6.07 (0.00 pct)         6.20 (-2.14 pct)        6.23 (-2.63 pct)
>  4-groups:         6.61 (0.00 pct)         6.64 (-0.45 pct)        6.63 (-0.30 pct)
>  8-groups:         8.87 (0.00 pct)         8.67 (2.25 pct)         8.78 (1.01 pct)
> 16-groups:        12.63 (0.00 pct)        12.54 (0.71 pct)        12.59 (0.31 pct)
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~ Hackbench + Cyclictest - Various Latency Nice Values ~
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> - Hackbench: 32 Groups
>
> perf bench sched messaging -p -l 100000 -g 32&
> cyclictest --policy other -D 5 -q -n -h 2000
>
> o NPS1
>
> ----------------------------------------------------------------------------------------------------------
> | Hackbench   |      Cyclictest LN = 19      |      Cyclictest LN = 0        |    Cyclictest LN = -20    |
> | LN          |------------------------------|-------------------------------|---------------------------|
> |             |   Min  |   Avg   |  Max      |   Min  |   Avg   |   Max      |   Min  |  Avg  |   Max    |
> |-------------|--------|---------|-----------|--------|---------|------------|--------|-------|----------|
> | 19          | 52.00  | 71.00   | 5191.00   | 29.00  | 68.00   |  4477.00   | 53.00  | 60.00 |  753.00  |
> | 0           | 53.00  | 150.00  | 7300.00   | 53.00  | 105.00  |  7730.00   | 53.00  | 64.00 |  2067.00 |
> | -20         | 33.00  | 159.00  | 98492.00  | 53.00  | 149.00  |  9608.00   | 53.00  | 91.00 |  5349.00 |
> ----------------------------------------------------------------------------------------------------------
>
> o NPS4
>
> ----------------------------------------------------------------------------------------------------------
> | Hackbench   |      Cyclictest LN = 19      |      Cyclictest LN = 0        |    Cyclictest LN = -20    |
> | LN          |------------------------------|-------------------------------|---------------------------|
> |             |   Min  |   Avg   |  Max      |   Min  |   Avg   |   Max      |   Min  |  Avg  |   Max    |
> |-------------|--------|---------|-----------|--------|---------|------------|--------|-------|----------|
> | 19          | 53.00  |  84.00  |  4790.00  | 53.00  |  72.00  |  3456.00   | 53.00  | 58.00 |  1271.00 |
> | 0           | 53.00  |  99.00  |  5494.00  | 52.00  |  74.00  |  5813.00   | 53.00  | 59.00 |  1004.00 |
> | -20         | 45.00  |  84.00  |  3592.00  | 53.00  |  91.00  |  15222.00  | 53.00  | 74.00 |  5232.00 |      ^
> ----------------------------------------------------------------------------------------------------------
>
> - Hackbench: 128 Groups
>
> perf bench sched messaging -p -l 500000 -g 128&
> cyclictest --policy other -D 5 -q -n -h 2000
>
> o NPS1
>
> ----------------------------------------------------------------------------------------------------------
> | Hackbench   |      Cyclictest LN = 19      |      Cyclictest LN = 0        |    Cyclictest LN = -20    |
> | LN          |------------------------------|-------------------------------|---------------------------|
> |             |   Min  |   Avg   |  Max      |   Min  |   Avg   |   Max      |   Min  |  Avg  |   Max    |
> |-------------|--------|---------|-----------|--------|---------|------------|--------|-------|----------|
> | 19          | 53.00  | 274.00  | 11294.00  | 33.00  | 130.00  |  20071.00  | 53.00  | 56.00 |  244.00  |      ^
> | 0           | 53.00  | 125.00  | 10014.00  | 53.00  | 113.00  |  15857.00  | 53.00  | 57.00 |  250.00  |
> | -20         | 53.00  | 187.00  | 49565.00  | 53.00  | 230.00  |  73353.00  | 53.00  | 118.00|  8816.00 |
> ----------------------------------------------------------------------------------------------------------
>
> o NPS4
>
> ----------------------------------------------------------------------------------------------------------
> | Hackbench   |      Cyclictest LN = 19      |      Cyclictest LN = 0        |    Cyclictest LN = -20    |
> | LN          |------------------------------|-------------------------------|---------------------------|
> |             |   Min  |   Avg   |  Max      |   Min  |   Avg   |   Max      |   Min  |  Avg  |   Max    |
> |-------------|--------|---------|-----------|--------|---------|------------|--------|-------|----------|
> | 19          | 53.00  | 271.00  | 11411.00  | 53.00  | 82.00   |  5486.00   | 25.00  | 57.00 | 1256.00  |
> | 0           | 53.00  | 148.00  | 8374.00   | 52.00  | 109.00  |  11074.00  | 52.00  | 59.00 | 1068.00  |
> | -20         | 53.00  | 202.00  | 52537.00  | 53.00  | 205.00  |  22265.00  | 52.00  | 87.00 | 14151.00 |
> ----------------------------------------------------------------------------------------------------------
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~ Hackbench + schbench - Various Latency Nice Values ~
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> perf bench sched messaging -p -l 400000 -g 128
> schbench -m 2 -t 1 -s 30
>
> o NPS1
>
> -------------------------------------------------------------------------------------------------
> | Hackbench |     schbench LN = 19       |      schbench LN = 0      |     schbench LN = -20    |
> | LN        |----------------------------|---------------------------|--------------------------|
> |           |  90th  |  95th  |  99th    |  90th  |  95th  |  99th   |  90th  |  95th  |  99th  |
> |-----------|--------|--------|----------|--------|--------|---------|--------|--------|--------|
> | 19        |   38   |   131  |   1458   |   46   |   151  |  2636   |   11   |   19   |  410   |       ^
> | 0         |   45   |   98   |   1758   |   25   |   50   |  1670   |   16   |   30   |  1042  |
> | -20       |   47   |   348  |   29280  |   40   |   109  |  16144  |   35   |   63   |  9104  |
> -------------------------------------------------------------------------------------------------
>
> o NPS4
>
> -------------------------------------------------------------------------------------------------
> | Hackbench |     schbench LN = 19       |      schbench LN = 0      |     schbench LN = -20    |
> | LN        |----------------------------|---------------------------|--------------------------|
> |           |  90th  |  95th  |  99th    |  90th  |  95th  |  99th   |  90th  |  95th  |  99th  |
> |-----------|--------|--------|----------|--------|--------|---------|--------|--------|--------|
> | 19        |   19   |  60    |  1886    |   17   |  29    |  621    |   10   |   18   |  227   |
> | 0         |   51   |  141   |  8120    |   37   |  78    |  8880   |   33   |   55   |  474   |       ^
> | -20       |   48   |  1494  |  27296   |   51   |  469   |  40384  |   31   |   64   |  4092  |       ^
> -------------------------------------------------------------------------------------------------
>
> ^ Note: There are cases where the Max, 99th percentile latency is
> non-monotonic but I've also seen a good amount of run to run variation
> there with a single bad sample polluting the results. In such cases,
> the averages are more representative.
>
> >
> > [1] https://source.android.com/docs/core/debug/eval_perf#touchlatency
> >
> > [..snip..]
> >
>
> Apart from couple of anomalies, latency nice reduces wait time, especially
> when the system is heavily loaded. If there is any data, or any specific
> workload you would like me to run on the test system, please do let me know.
> Meanwhile, I'll try to get some numbers for larger workloads like SpecJBB
> that did see improvements with latency nice on v5.

Thanks for your tests

Vincent

> --
> Thanks and Regards,
> Prateek

Re: [PATCH 5/9] sched/fair: Take into account latency priority at wakeup

[Joel Fernandes]

On Tue, Nov 15, 2022 at 12:19 PM Vincent Guittot
<vincent.guittot@linaro.org> wrote:
>
> Take into account the latency priority of a thread when deciding to
> preempt the current running thread. We don't want to provide more CPU
> bandwidth to a thread but reorder the scheduling to run latency sensitive
> task first whenever possible.
>
> As long as a thread didn't use its bandwidth, it will be able to preempt
> the current thread.
>
> At the opposite, a thread with a low latency priority will preempt current
> thread at wakeup only to keep fair CPU bandwidth sharing. Otherwise it will
> wait for the tick to get its sched slice.
>
>                                    curr vruntime
>                                        |
>                       sysctl_sched_wakeup_granularity
>                                    <-->
> ----------------------------------|----|-----------------------|---------------
>                                   |    |<--------------------->
>                                   |    .  sysctl_sched_latency
>                                   |    .
> default/current latency entity    |    .
>                                   |    .
> 1111111111111111111111111111111111|0000|-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-
> se preempts curr at wakeup ------>|<- se doesn't preempt curr -----------------
>                                   |    .
>                                   |    .
>                                   |    .
> low latency entity                |    .
>                                    ---------------------->|
>                                % of sysctl_sched_latency  |
> 1111111111111111111111111111111111111111111111111111111111|0000|-1-1-1-1-1-1-1-
> preempt ------------------------------------------------->|<- do not preempt --
>                                   |    .
>                                   |    .
>                                   |    .
> high latency entity               |    .
>          |<-----------------------|----.
>          | % of sysctl_sched_latency   .
> 111111111|0000|-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1
> preempt->|<- se doesn't preempt curr ------------------------------------------
> Tests results of nice latency impact on heavy load like hackbench:
>
> hackbench -l (2560 / group) -g group
> group        latency 0             latency 19
> 1            1.378(+/-  1%)      1.337(+/- 1%) + 3%
> 4            1.393(+/-  3%)      1.312(+/- 3%) + 6%
> 8            1.308(+/-  2%)      1.279(+/- 1%) + 2%
> 16           1.347(+/-  1%)      1.317(+/- 1%) + 2%
>
> hackbench -p -l (2560 / group) -g group
> group
> 1            1.836(+/- 17%)      1.148(+/- 5%) +37%
> 4            1.586(+/-  6%)      1.109(+/- 8%) +30%
> 8            1.209(+/-  4%)      0.780(+/- 4%) +35%
> 16           0.805(+/-  5%)      0.728(+/- 4%) +10%
>
> By deacreasing the latency prio, we reduce the number of preemption at
> wakeup and help hackbench making progress.
>
> Test results of nice latency impact on short live load like cyclictest
> while competing with heavy load like hackbench:
>
> hackbench -l 10000 -g $group &
> cyclictest --policy other -D 5 -q -n
>         latency 0           latency -20
> group   min  avg    max     min  avg    max
> 0       16    19     29      17   18     29
> 1       43   299   7359      63   84   3422
> 4       56   449  14806      45   83    284
> 8       63   820  51123      63   83    283
> 16      64  1326  70684      41  157  26852
>
> group = 0 means that hackbench is not running.
>
> The avg is significantly improved with nice latency -20 especially with
> large number of groups but min and max remain quite similar. If we add the
> histogram parameter to get details of latency, we have :
>
> hackbench -l 10000 -g 16 &
> cyclictest --policy other -D 5 -q -n  -H 20000 --histfile data.txt
>               latency 0    latency -20
> Min Latencies:    64           62
> Avg Latencies:  1170          107
> Max Latencies: 88069        10417
> 50% latencies:   122           86
> 75% latencies:   614           91
> 85% latencies:   961           94
> 90% latencies:  1225           97
> 95% latencies:  6120          102
> 99% latencies: 18328          159
>
> With percentile details, we see the benefit of nice latency -20 as
> only 1% of the latencies are above 159us whereas the default latency
> has got 15% around ~1ms or above and 5% over the 6ms.
>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  include/linux/sched.h      |  4 ++-
>  include/linux/sched/prio.h |  9 ++++++
>  init/init_task.c           |  2 +-
>  kernel/sched/core.c        | 38 +++++++++++++++++++---
>  kernel/sched/debug.c       |  2 +-
>  kernel/sched/fair.c        | 66 ++++++++++++++++++++++++++++++++++----
>  kernel/sched/sched.h       |  6 ++++
>  7 files changed, 112 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 856240573300..2f33326adb8d 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -568,6 +568,8 @@ struct sched_entity {
>         /* cached value of my_q->h_nr_running */
>         unsigned long                   runnable_weight;
>  #endif
> +       /* preemption offset in ns */
> +       long                            latency_offset;
>
>  #ifdef CONFIG_SMP
>         /*
> @@ -784,7 +786,7 @@ struct task_struct {
>         int                             static_prio;
>         int                             normal_prio;
>         unsigned int                    rt_priority;
> -       int                             latency_nice;
> +       int                             latency_prio;
>
>         struct sched_entity             se;
>         struct sched_rt_entity          rt;
> diff --git a/include/linux/sched/prio.h b/include/linux/sched/prio.h
> index bfcd7f1d1e11..be79503d86af 100644
> --- a/include/linux/sched/prio.h
> +++ b/include/linux/sched/prio.h
> @@ -59,5 +59,14 @@ static inline long rlimit_to_nice(long prio)
>   * Default tasks should be treated as a task with latency_nice = 0.
>   */
>  #define DEFAULT_LATENCY_NICE   0
> +#define DEFAULT_LATENCY_PRIO   (DEFAULT_LATENCY_NICE + LATENCY_NICE_WIDTH/2)
> +
> +/*
> + * Convert user-nice values [ -20 ... 0 ... 19 ]
> + * to static latency [ 0..39 ],
> + * and back.
> + */
> +#define NICE_TO_LATENCY(nice)  ((nice) + DEFAULT_LATENCY_PRIO)
> +#define LATENCY_TO_NICE(prio)  ((prio) - DEFAULT_LATENCY_PRIO)
>
>  #endif /* _LINUX_SCHED_PRIO_H */
> diff --git a/init/init_task.c b/init/init_task.c
> index 7dd71dd2d261..071deff8dbd1 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -78,7 +78,7 @@ struct task_struct init_task
>         .prio           = MAX_PRIO - 20,
>         .static_prio    = MAX_PRIO - 20,
>         .normal_prio    = MAX_PRIO - 20,
> -       .latency_nice   = DEFAULT_LATENCY_NICE,
> +       .latency_prio   = DEFAULT_LATENCY_PRIO,
>         .policy         = SCHED_NORMAL,
>         .cpus_ptr       = &init_task.cpus_mask,
>         .user_cpus_ptr  = NULL,
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 18c31a68eb18..b2b8cb6c08cd 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1283,6 +1283,16 @@ static void set_load_weight(struct task_struct *p, bool update_load)
>         }
>  }
>
> +static void set_latency_offset(struct task_struct *p)
> +{
> +       long weight = sched_latency_to_weight[p->latency_prio];
> +       s64 offset;
> +
> +       offset = weight * get_sleep_latency(false);
> +       offset = div_s64(offset, NICE_LATENCY_WEIGHT_MAX);
> +       p->se.latency_offset = (long)offset;
> +}
> +
>  #ifdef CONFIG_UCLAMP_TASK
>  /*
>   * Serializes updates of utilization clamp values
> @@ -4592,7 +4602,9 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
>                 p->prio = p->normal_prio = p->static_prio;
>                 set_load_weight(p, false);
>
> -               p->latency_nice = DEFAULT_LATENCY_NICE;
> +               p->latency_prio = NICE_TO_LATENCY(0);
> +               set_latency_offset(p);
> +
>                 /*
>                  * We don't need the reset flag anymore after the fork. It has
>                  * fulfilled its duty:
> @@ -7358,8 +7370,10 @@ static void __setscheduler_params(struct task_struct *p,
>  static void __setscheduler_latency(struct task_struct *p,
>                 const struct sched_attr *attr)
>  {
> -       if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE)
> -               p->latency_nice = attr->sched_latency_nice;
> +       if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE) {
> +               p->latency_prio = NICE_TO_LATENCY(attr->sched_latency_nice);
> +               set_latency_offset(p);
> +       }
>  }
>
>  /*
> @@ -7544,7 +7558,7 @@ static int __sched_setscheduler(struct task_struct *p,
>                 if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
>                         goto change;
>                 if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE &&
> -                   attr->sched_latency_nice != p->latency_nice)
> +                   attr->sched_latency_nice != LATENCY_TO_NICE(p->latency_prio))
>                         goto change;
>
>                 p->sched_reset_on_fork = reset_on_fork;
> @@ -8085,7 +8099,7 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
>         get_params(p, &kattr);
>         kattr.sched_flags &= SCHED_FLAG_ALL;
>
> -       kattr.sched_latency_nice = p->latency_nice;
> +       kattr.sched_latency_nice = LATENCY_TO_NICE(p->latency_prio);
>
>  #ifdef CONFIG_UCLAMP_TASK
>         /*
> @@ -11294,6 +11308,20 @@ const u32 sched_prio_to_wmult[40] = {
>   /*  15 */ 119304647, 148102320, 186737708, 238609294, 286331153,
>  };
>
> +/*
> + * latency weight for wakeup preemption
> + */
> +const int sched_latency_to_weight[40] = {
> + /* -20 */     -1024,     -973,     -922,      -870,      -819,
> + /* -15 */      -768,     -717,     -666,      -614,      -563,
> + /* -10 */      -512,     -461,     -410,      -358,      -307,
> + /*  -5 */      -256,     -205,     -154,      -102,       -51,
> + /*   0 */         0,       51,      102,       154,       205,
> + /*   5 */       256,      307,      358,       410,       461,
> + /*  10 */       512,      563,      614,       666,       717,
> + /*  15 */       768,      819,      870,       922,       973,
> +};
> +

The table is linear. You could approximate this as: weight = nice * 51
since it is a linear scale and do the conversion in place.

Or, since the only place you are using the latency_to_weight is in
set_latency_offset(), can we drop the sched_latency_to_weight array
and simplify as follows?

static void set_latency_offset(struct task_struct *p)
{
  s64 offset = p->latency_prio * get_sleep_latency(false);
  p->latency_prio = (long)div_s64(offset, 40);
}

>  void call_trace_sched_update_nr_running(struct rq *rq, int count)
>  {
>          trace_sched_update_nr_running_tp(rq, count);
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 68be7a3e42a3..b3922184af91 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -1043,7 +1043,7 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
>  #endif
>         P(policy);
>         P(prio);
> -       P(latency_nice);
> +       P(latency_prio);
>         if (task_has_dl_policy(p)) {
>                 P(dl.runtime);
>                 P(dl.deadline);
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c8a697f8db88..0e80e65113bd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4858,6 +4858,8 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>                 update_idle_cfs_rq_clock_pelt(cfs_rq);
>  }
>
> +static long wakeup_latency_gran(struct sched_entity *curr, struct sched_entity *se);
> +
>  /*
>   * Preempt the current task with a newly woken task if needed:
>   */
> @@ -4866,7 +4868,7 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>  {
>         unsigned long ideal_runtime, delta_exec;
>         struct sched_entity *se;
> -       s64 delta;
> +       s64 delta, offset;
>
>         ideal_runtime = sched_slice(cfs_rq, curr);
>         delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
> @@ -4891,10 +4893,12 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>         se = __pick_first_entity(cfs_rq);
>         delta = curr->vruntime - se->vruntime;
>
> -       if (delta < 0)
> +       offset = wakeup_latency_gran(curr, se);
> +       if (delta < offset)
>                 return;

Agreed.

> -       if (delta > ideal_runtime)
> +       if ((delta > ideal_runtime) ||
> +           (delta > get_latency_max()))
>                 resched_curr(rq_of(cfs_rq));
>  }
>
> @@ -6019,6 +6023,35 @@ static int sched_idle_cpu(int cpu)
>  }
>  #endif
>
> +static void set_next_buddy(struct sched_entity *se);
> +
> +static void check_preempt_from_others(struct cfs_rq *cfs, struct sched_entity *se)
> +{
> +       struct sched_entity *next;
> +
> +       if (se->latency_offset >= 0)
> +               return;
> +
> +       if (cfs->nr_running <= 1)
> +               return;
> +       /*
> +        * When waking from another class, we don't need to check to preempt at
> +        * wakeup and don't set next buddy as a candidate for being picked in
> +        * priority.
> +        * In case of simultaneous wakeup when current is another class, the
> +        * latency sensitive tasks lost opportunity to preempt non sensitive
> +        * tasks which woke up simultaneously.
> +        */
> +
> +       if (cfs->next)
> +               next = cfs->next;
> +       else
> +               next = __pick_first_entity(cfs);
> +
> +       if (next && wakeup_preempt_entity(next, se) == 1)
> +               set_next_buddy(se);
> +}
> +
>  /*
>   * The enqueue_task method is called before nr_running is
>   * increased. Here we update the fair scheduling stats and
> @@ -6105,14 +6138,15 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>         if (!task_new)
>                 update_overutilized_status(rq);
>
> +       if (rq->curr->sched_class != &fair_sched_class)
> +               check_preempt_from_others(cfs_rq_of(&p->se), &p->se);
> +
>  enqueue_throttle:
>         assert_list_leaf_cfs_rq(rq);
>
>         hrtick_update(rq);
>  }
>
> -static void set_next_buddy(struct sched_entity *se);
> -
>  /*
>   * The dequeue_task method is called before nr_running is
>   * decreased. We remove the task from the rbtree and
> @@ -7461,6 +7495,23 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  }
>  #endif /* CONFIG_SMP */
>
> +static long wakeup_latency_gran(struct sched_entity *curr, struct sched_entity *se)
> +{
> +       long latency_offset = se->latency_offset;
> +
> +       /*
> +        * A negative latency offset means that the sched_entity has latency
> +        * requirement that needs to be evaluated versus other entity.
> +        * Otherwise, use the latency weight to evaluate how much scheduling
> +        * delay is acceptable by se.
> +        */
> +       if ((latency_offset < 0) || (curr->latency_offset < 0))
> +               latency_offset -= curr->latency_offset;
> +       latency_offset = min_t(long, latency_offset, get_latency_max());

Over here can we make positive latency offsets also be evaluated
"versus other entity"?

It feels strange to have different rules for positive latency_offset
when comparing curr and se. IMO we should also factor in latency
requirements by comparing 2 positive nice values. It should be
relative even for positive values, just like regular nice IMO and not
have hidden meaning. If there is hidden meaning, it confuses the user
and requires documentation that most users will not read. Especially
because latency_nice shares the word "nice" with regular nice values.

Thanks,

 - Joel
.

> +
> +       return latency_offset;
> +}
> +
>  static unsigned long wakeup_gran(struct sched_entity *se)
>  {
>         unsigned long gran = sysctl_sched_wakeup_granularity;
> @@ -7499,11 +7550,12 @@ static int
>  wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
>  {
>         s64 gran, vdiff = curr->vruntime - se->vruntime;
> +       s64 offset = wakeup_latency_gran(curr, se);
>
> -       if (vdiff <= 0)
> +       if (vdiff < offset)
>                 return -1;
>
> -       gran = wakeup_gran(se);
> +       gran = offset + wakeup_gran(se);
>
>         /*
>          * At wake up, the vruntime of a task is capped to not be older than
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 842ce0094d9c..7292652731d0 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -125,6 +125,11 @@ extern int sched_rr_timeslice;
>   */
>  #define NS_TO_JIFFIES(TIME)    ((unsigned long)(TIME) / (NSEC_PER_SEC / HZ))
>
> +/* Maximum nice latency weight used to scale the latency_offset */
> +
> +#define NICE_LATENCY_SHIFT     (SCHED_FIXEDPOINT_SHIFT)
> +#define NICE_LATENCY_WEIGHT_MAX        (1L << NICE_LATENCY_SHIFT)
> +
>  /*
>   * Increase resolution of nice-level calculations for 64-bit architectures.
>   * The extra resolution improves shares distribution and load balancing of
> @@ -2115,6 +2120,7 @@ static_assert(WF_TTWU == SD_BALANCE_WAKE);
>
>  extern const int               sched_prio_to_weight[40];
>  extern const u32               sched_prio_to_wmult[40];
> +extern const int               sched_latency_to_weight[40];
>
>  /*
>   * {de,en}queue flags:
> --
> 2.17.1
>

Re: [PATCH 5/9] sched/fair: Take into account latency priority at wakeup

[Vincent Guittot]

On Tue, 29 Nov 2022 at 05:25, Joel Fernandes <joel@joelfernandes.org> wrote:
>
> On Tue, Nov 15, 2022 at 12:19 PM Vincent Guittot
> <vincent.guittot@linaro.org> wrote:
> >
> > Take into account the latency priority of a thread when deciding to
> > preempt the current running thread. We don't want to provide more CPU
> > bandwidth to a thread but reorder the scheduling to run latency sensitive
> > task first whenever possible.
> >
> > As long as a thread didn't use its bandwidth, it will be able to preempt
> > the current thread.
> >
> > At the opposite, a thread with a low latency priority will preempt current
> > thread at wakeup only to keep fair CPU bandwidth sharing. Otherwise it will
> > wait for the tick to get its sched slice.
> >
> >                                    curr vruntime
> >                                        |
> >                       sysctl_sched_wakeup_granularity
> >                                    <-->
> > ----------------------------------|----|-----------------------|---------------
> >                                   |    |<--------------------->
> >                                   |    .  sysctl_sched_latency
> >                                   |    .
> > default/current latency entity    |    .
> >                                   |    .
> > 1111111111111111111111111111111111|0000|-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-
> > se preempts curr at wakeup ------>|<- se doesn't preempt curr -----------------
> >                                   |    .
> >                                   |    .
> >                                   |    .
> > low latency entity                |    .
> >                                    ---------------------->|
> >                                % of sysctl_sched_latency  |
> > 1111111111111111111111111111111111111111111111111111111111|0000|-1-1-1-1-1-1-1-
> > preempt ------------------------------------------------->|<- do not preempt --
> >                                   |    .
> >                                   |    .
> >                                   |    .
> > high latency entity               |    .
> >          |<-----------------------|----.
> >          | % of sysctl_sched_latency   .
> > 111111111|0000|-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1
> > preempt->|<- se doesn't preempt curr ------------------------------------------
> > Tests results of nice latency impact on heavy load like hackbench:
> >
> > hackbench -l (2560 / group) -g group
> > group        latency 0             latency 19
> > 1            1.378(+/-  1%)      1.337(+/- 1%) + 3%
> > 4            1.393(+/-  3%)      1.312(+/- 3%) + 6%
> > 8            1.308(+/-  2%)      1.279(+/- 1%) + 2%
> > 16           1.347(+/-  1%)      1.317(+/- 1%) + 2%
> >
> > hackbench -p -l (2560 / group) -g group
> > group
> > 1            1.836(+/- 17%)      1.148(+/- 5%) +37%
> > 4            1.586(+/-  6%)      1.109(+/- 8%) +30%
> > 8            1.209(+/-  4%)      0.780(+/- 4%) +35%
> > 16           0.805(+/-  5%)      0.728(+/- 4%) +10%
> >
> > By deacreasing the latency prio, we reduce the number of preemption at
> > wakeup and help hackbench making progress.
> >
> > Test results of nice latency impact on short live load like cyclictest
> > while competing with heavy load like hackbench:
> >
> > hackbench -l 10000 -g $group &
> > cyclictest --policy other -D 5 -q -n
> >         latency 0           latency -20
> > group   min  avg    max     min  avg    max
> > 0       16    19     29      17   18     29
> > 1       43   299   7359      63   84   3422
> > 4       56   449  14806      45   83    284
> > 8       63   820  51123      63   83    283
> > 16      64  1326  70684      41  157  26852
> >
> > group = 0 means that hackbench is not running.
> >
> > The avg is significantly improved with nice latency -20 especially with
> > large number of groups but min and max remain quite similar. If we add the
> > histogram parameter to get details of latency, we have :
> >
> > hackbench -l 10000 -g 16 &
> > cyclictest --policy other -D 5 -q -n  -H 20000 --histfile data.txt
> >               latency 0    latency -20
> > Min Latencies:    64           62
> > Avg Latencies:  1170          107
> > Max Latencies: 88069        10417
> > 50% latencies:   122           86
> > 75% latencies:   614           91
> > 85% latencies:   961           94
> > 90% latencies:  1225           97
> > 95% latencies:  6120          102
> > 99% latencies: 18328          159
> >
> > With percentile details, we see the benefit of nice latency -20 as
> > only 1% of the latencies are above 159us whereas the default latency
> > has got 15% around ~1ms or above and 5% over the 6ms.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> > ---
> >  include/linux/sched.h      |  4 ++-
> >  include/linux/sched/prio.h |  9 ++++++
> >  init/init_task.c           |  2 +-
> >  kernel/sched/core.c        | 38 +++++++++++++++++++---
> >  kernel/sched/debug.c       |  2 +-
> >  kernel/sched/fair.c        | 66 ++++++++++++++++++++++++++++++++++----
> >  kernel/sched/sched.h       |  6 ++++
> >  7 files changed, 112 insertions(+), 15 deletions(-)
> >
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 856240573300..2f33326adb8d 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -568,6 +568,8 @@ struct sched_entity {
> >         /* cached value of my_q->h_nr_running */
> >         unsigned long                   runnable_weight;
> >  #endif
> > +       /* preemption offset in ns */
> > +       long                            latency_offset;
> >
> >  #ifdef CONFIG_SMP
> >         /*
> > @@ -784,7 +786,7 @@ struct task_struct {
> >         int                             static_prio;
> >         int                             normal_prio;
> >         unsigned int                    rt_priority;
> > -       int                             latency_nice;
> > +       int                             latency_prio;
> >
> >         struct sched_entity             se;
> >         struct sched_rt_entity          rt;
> > diff --git a/include/linux/sched/prio.h b/include/linux/sched/prio.h
> > index bfcd7f1d1e11..be79503d86af 100644
> > --- a/include/linux/sched/prio.h
> > +++ b/include/linux/sched/prio.h
> > @@ -59,5 +59,14 @@ static inline long rlimit_to_nice(long prio)
> >   * Default tasks should be treated as a task with latency_nice = 0.
> >   */
> >  #define DEFAULT_LATENCY_NICE   0
> > +#define DEFAULT_LATENCY_PRIO   (DEFAULT_LATENCY_NICE + LATENCY_NICE_WIDTH/2)
> > +
> > +/*
> > + * Convert user-nice values [ -20 ... 0 ... 19 ]
> > + * to static latency [ 0..39 ],
> > + * and back.
> > + */
> > +#define NICE_TO_LATENCY(nice)  ((nice) + DEFAULT_LATENCY_PRIO)
> > +#define LATENCY_TO_NICE(prio)  ((prio) - DEFAULT_LATENCY_PRIO)
> >
> >  #endif /* _LINUX_SCHED_PRIO_H */
> > diff --git a/init/init_task.c b/init/init_task.c
> > index 7dd71dd2d261..071deff8dbd1 100644
> > --- a/init/init_task.c
> > +++ b/init/init_task.c
> > @@ -78,7 +78,7 @@ struct task_struct init_task
> >         .prio           = MAX_PRIO - 20,
> >         .static_prio    = MAX_PRIO - 20,
> >         .normal_prio    = MAX_PRIO - 20,
> > -       .latency_nice   = DEFAULT_LATENCY_NICE,
> > +       .latency_prio   = DEFAULT_LATENCY_PRIO,
> >         .policy         = SCHED_NORMAL,
> >         .cpus_ptr       = &init_task.cpus_mask,
> >         .user_cpus_ptr  = NULL,
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 18c31a68eb18..b2b8cb6c08cd 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1283,6 +1283,16 @@ static void set_load_weight(struct task_struct *p, bool update_load)
> >         }
> >  }
> >
> > +static void set_latency_offset(struct task_struct *p)
> > +{
> > +       long weight = sched_latency_to_weight[p->latency_prio];
> > +       s64 offset;
> > +
> > +       offset = weight * get_sleep_latency(false);
> > +       offset = div_s64(offset, NICE_LATENCY_WEIGHT_MAX);
> > +       p->se.latency_offset = (long)offset;
> > +}
> > +
> >  #ifdef CONFIG_UCLAMP_TASK
> >  /*
> >   * Serializes updates of utilization clamp values
> > @@ -4592,7 +4602,9 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
> >                 p->prio = p->normal_prio = p->static_prio;
> >                 set_load_weight(p, false);
> >
> > -               p->latency_nice = DEFAULT_LATENCY_NICE;
> > +               p->latency_prio = NICE_TO_LATENCY(0);
> > +               set_latency_offset(p);
> > +
> >                 /*
> >                  * We don't need the reset flag anymore after the fork. It has
> >                  * fulfilled its duty:
> > @@ -7358,8 +7370,10 @@ static void __setscheduler_params(struct task_struct *p,
> >  static void __setscheduler_latency(struct task_struct *p,
> >                 const struct sched_attr *attr)
> >  {
> > -       if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE)
> > -               p->latency_nice = attr->sched_latency_nice;
> > +       if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE) {
> > +               p->latency_prio = NICE_TO_LATENCY(attr->sched_latency_nice);
> > +               set_latency_offset(p);
> > +       }
> >  }
> >
> >  /*
> > @@ -7544,7 +7558,7 @@ static int __sched_setscheduler(struct task_struct *p,
> >                 if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
> >                         goto change;
> >                 if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE &&
> > -                   attr->sched_latency_nice != p->latency_nice)
> > +                   attr->sched_latency_nice != LATENCY_TO_NICE(p->latency_prio))
> >                         goto change;
> >
> >                 p->sched_reset_on_fork = reset_on_fork;
> > @@ -8085,7 +8099,7 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
> >         get_params(p, &kattr);
> >         kattr.sched_flags &= SCHED_FLAG_ALL;
> >
> > -       kattr.sched_latency_nice = p->latency_nice;
> > +       kattr.sched_latency_nice = LATENCY_TO_NICE(p->latency_prio);
> >
> >  #ifdef CONFIG_UCLAMP_TASK
> >         /*
> > @@ -11294,6 +11308,20 @@ const u32 sched_prio_to_wmult[40] = {
> >   /*  15 */ 119304647, 148102320, 186737708, 238609294, 286331153,
> >  };
> >
> > +/*
> > + * latency weight for wakeup preemption
> > + */
> > +const int sched_latency_to_weight[40] = {
> > + /* -20 */     -1024,     -973,     -922,      -870,      -819,
> > + /* -15 */      -768,     -717,     -666,      -614,      -563,
> > + /* -10 */      -512,     -461,     -410,      -358,      -307,
> > + /*  -5 */      -256,     -205,     -154,      -102,       -51,
> > + /*   0 */         0,       51,      102,       154,       205,
> > + /*   5 */       256,      307,      358,       410,       461,
> > + /*  10 */       512,      563,      614,       666,       717,
> > + /*  15 */       768,      819,      870,       922,       973,
> > +};
> > +
>
> The table is linear. You could approximate this as: weight = nice * 51
> since it is a linear scale and do the conversion in place.
>
> Or, since the only place you are using the latency_to_weight is in
> set_latency_offset(), can we drop the sched_latency_to_weight array
> and simplify as follows?

It's also used in cgroup patch and keeps a coherency between
nice/weight an latency_nice/offset so I prefer keeping current
implementation

>
> static void set_latency_offset(struct task_struct *p)
> {
>   s64 offset = p->latency_prio * get_sleep_latency(false);
>   p->latency_prio = (long)div_s64(offset, 40);
> }
>
> >  void call_trace_sched_update_nr_running(struct rq *rq, int count)
> >  {
> >          trace_sched_update_nr_running_tp(rq, count);
> > diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> > index 68be7a3e42a3..b3922184af91 100644
> > --- a/kernel/sched/debug.c
> > +++ b/kernel/sched/debug.c
> > @@ -1043,7 +1043,7 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
> >  #endif
> >         P(policy);
> >         P(prio);
> > -       P(latency_nice);
> > +       P(latency_prio);
> >         if (task_has_dl_policy(p)) {
> >                 P(dl.runtime);
> >                 P(dl.deadline);
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index c8a697f8db88..0e80e65113bd 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4858,6 +4858,8 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> >                 update_idle_cfs_rq_clock_pelt(cfs_rq);
> >  }
> >
> > +static long wakeup_latency_gran(struct sched_entity *curr, struct sched_entity *se);
> > +
> >  /*
> >   * Preempt the current task with a newly woken task if needed:
> >   */
> > @@ -4866,7 +4868,7 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> >  {
> >         unsigned long ideal_runtime, delta_exec;
> >         struct sched_entity *se;
> > -       s64 delta;
> > +       s64 delta, offset;
> >
> >         ideal_runtime = sched_slice(cfs_rq, curr);
> >         delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
> > @@ -4891,10 +4893,12 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> >         se = __pick_first_entity(cfs_rq);
> >         delta = curr->vruntime - se->vruntime;
> >
> > -       if (delta < 0)
> > +       offset = wakeup_latency_gran(curr, se);
> > +       if (delta < offset)
> >                 return;
>
> Agreed.
>
> > -       if (delta > ideal_runtime)
> > +       if ((delta > ideal_runtime) ||
> > +           (delta > get_latency_max()))
> >                 resched_curr(rq_of(cfs_rq));
> >  }
> >
> > @@ -6019,6 +6023,35 @@ static int sched_idle_cpu(int cpu)
> >  }
> >  #endif
> >
> > +static void set_next_buddy(struct sched_entity *se);
> > +
> > +static void check_preempt_from_others(struct cfs_rq *cfs, struct sched_entity *se)
> > +{
> > +       struct sched_entity *next;
> > +
> > +       if (se->latency_offset >= 0)
> > +               return;
> > +
> > +       if (cfs->nr_running <= 1)
> > +               return;
> > +       /*
> > +        * When waking from another class, we don't need to check to preempt at
> > +        * wakeup and don't set next buddy as a candidate for being picked in
> > +        * priority.
> > +        * In case of simultaneous wakeup when current is another class, the
> > +        * latency sensitive tasks lost opportunity to preempt non sensitive
> > +        * tasks which woke up simultaneously.
> > +        */
> > +
> > +       if (cfs->next)
> > +               next = cfs->next;
> > +       else
> > +               next = __pick_first_entity(cfs);
> > +
> > +       if (next && wakeup_preempt_entity(next, se) == 1)
> > +               set_next_buddy(se);
> > +}
> > +
> >  /*
> >   * The enqueue_task method is called before nr_running is
> >   * increased. Here we update the fair scheduling stats and
> > @@ -6105,14 +6138,15 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> >         if (!task_new)
> >                 update_overutilized_status(rq);
> >
> > +       if (rq->curr->sched_class != &fair_sched_class)
> > +               check_preempt_from_others(cfs_rq_of(&p->se), &p->se);
> > +
> >  enqueue_throttle:
> >         assert_list_leaf_cfs_rq(rq);
> >
> >         hrtick_update(rq);
> >  }
> >
> > -static void set_next_buddy(struct sched_entity *se);
> > -
> >  /*
> >   * The dequeue_task method is called before nr_running is
> >   * decreased. We remove the task from the rbtree and
> > @@ -7461,6 +7495,23 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> >  }
> >  #endif /* CONFIG_SMP */
> >
> > +static long wakeup_latency_gran(struct sched_entity *curr, struct sched_entity *se)
> > +{
> > +       long latency_offset = se->latency_offset;
> > +
> > +       /*
> > +        * A negative latency offset means that the sched_entity has latency
> > +        * requirement that needs to be evaluated versus other entity.
> > +        * Otherwise, use the latency weight to evaluate how much scheduling
> > +        * delay is acceptable by se.
> > +        */
> > +       if ((latency_offset < 0) || (curr->latency_offset < 0))
> > +               latency_offset -= curr->latency_offset;
> > +       latency_offset = min_t(long, latency_offset, get_latency_max());
>
> Over here can we make positive latency offsets also be evaluated
> "versus other entity"?
>
> It feels strange to have different rules for positive latency_offset
> when comparing curr and se. IMO we should also factor in latency
> requirements by comparing 2 positive nice values. It should be
> relative even for positive values, just like regular nice IMO and not
> have hidden meaning. If there is hidden meaning, it confuses the user
> and requires documentation that most users will not read. Especially
> because latency_nice shares the word "nice" with regular nice values.

This has already been discussed in the previous revisions. This is not
a hidden behavior but the normal behavior.

A negative latency nice, means that the task are not tolerant to
scheduling delay and it want to preempt current and run now. Or, if
the task is current, it doesn't want to be preempted and finish its
slice. In this case, we compare current and wake up task in case there
is 2 latency sensitive tasks that are fighting to run 1st.

Whereas a positive latency nice means that the task is tolerant to
scheduling delay and you don't care preempting current as long as it's
in an acceptable vruntime range. Why would the latency nice of the
current task make the wakeup task less tolerant to the scheduling
delay ? As an example, If current is latency_nice 19 and the wakeup
task is latency nice 19 too, both are tolerant to scheduling delay and
the waking up task should preempt current only if there is an
unfairness problem. By comparing their positive latency nice values,
you are back to the normal behavior which defeats the purpose of the
feature.

Thanks
Vincent

>
> Thanks,
>
>  - Joel
> .
>
> > +
> > +       return latency_offset;
> > +}
> > +
> >  static unsigned long wakeup_gran(struct sched_entity *se)
> >  {
> >         unsigned long gran = sysctl_sched_wakeup_granularity;
> > @@ -7499,11 +7550,12 @@ static int
> >  wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
> >  {
> >         s64 gran, vdiff = curr->vruntime - se->vruntime;
> > +       s64 offset = wakeup_latency_gran(curr, se);
> >
> > -       if (vdiff <= 0)
> > +       if (vdiff < offset)
> >                 return -1;
> >
> > -       gran = wakeup_gran(se);
> > +       gran = offset + wakeup_gran(se);
> >
> >         /*
> >          * At wake up, the vruntime of a task is capped to not be older than
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 842ce0094d9c..7292652731d0 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -125,6 +125,11 @@ extern int sched_rr_timeslice;
> >   */
> >  #define NS_TO_JIFFIES(TIME)    ((unsigned long)(TIME) / (NSEC_PER_SEC / HZ))
> >
> > +/* Maximum nice latency weight used to scale the latency_offset */
> > +
> > +#define NICE_LATENCY_SHIFT     (SCHED_FIXEDPOINT_SHIFT)
> > +#define NICE_LATENCY_WEIGHT_MAX        (1L << NICE_LATENCY_SHIFT)
> > +
> >  /*
> >   * Increase resolution of nice-level calculations for 64-bit architectures.
> >   * The extra resolution improves shares distribution and load balancing of
> > @@ -2115,6 +2120,7 @@ static_assert(WF_TTWU == SD_BALANCE_WAKE);
> >
> >  extern const int               sched_prio_to_weight[40];
> >  extern const u32               sched_prio_to_wmult[40];
> > +extern const int               sched_latency_to_weight[40];
> >
> >  /*
> >   * {de,en}queue flags:
> > --
> > 2.17.1
> >

Re: [PATCH 5/9] sched/fair: Take into account latency priority at wakeup

[Joel Fernandes]

Hi Vincent,
Thank you so much for your quick reply.

> On Nov 29, 2022, at 3:58 AM, Vincent Guittot <vincent.guittot@linaro.org> wrote:
> 
> On Tue, 29 Nov 2022 at 05:25, Joel Fernandes <joel@joelfernandes.org> wrote:
>> 
>>> On Tue, Nov 15, 2022 at 12:19 PM Vincent Guittot
>>> <vincent.guittot@linaro.org> wrote:
>>> 
>>> Take into account the latency priority of a thread when deciding to
>>> preempt the current running thread. We don't want to provide more CPU
>>> bandwidth to a thread but reorder the scheduling to run latency sensitive
>>> task first whenever possible.
>>> 
>>> As long as a thread didn't use its bandwidth, it will be able to preempt
>>> the current thread.
>>> 
>>> At the opposite, a thread with a low latency priority will preempt current
>>> thread at wakeup only to keep fair CPU bandwidth sharing. Otherwise it will
>>> wait for the tick to get its sched slice.
>>> 
>>>                                   curr vruntime
>>>                                       |
>>>                      sysctl_sched_wakeup_granularity
>>>                                   <-->
>>> ----------------------------------|----|-----------------------|---------------
>>>                                  |    |<--------------------->
>>>                                  |    .  sysctl_sched_latency
>>>                                  |    .
>>> default/current latency entity    |    .
>>>                                  |    .
>>> 1111111111111111111111111111111111|0000|-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-
>>> se preempts curr at wakeup ------>|<- se doesn't preempt curr -----------------
>>>                                  |    .
>>>                                  |    .
>>>                                  |    .
>>> low latency entity                |    .
>>>                                   ---------------------->|
>>>                               % of sysctl_sched_latency  |
>>> 1111111111111111111111111111111111111111111111111111111111|0000|-1-1-1-1-1-1-1-
>>> preempt ------------------------------------------------->|<- do not preempt --
>>>                                  |    .
>>>                                  |    .
>>>                                  |    .
>>> high latency entity               |    .
>>>         |<-----------------------|----.
>>>         | % of sysctl_sched_latency   .
>>> 111111111|0000|-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1
>>> preempt->|<- se doesn't preempt curr ------------------------------------------
>>> Tests results of nice latency impact on heavy load like hackbench:
>>> 
>>> hackbench -l (2560 / group) -g group
>>> group        latency 0             latency 19
>>> 1            1.378(+/-  1%)      1.337(+/- 1%) + 3%
>>> 4            1.393(+/-  3%)      1.312(+/- 3%) + 6%
>>> 8            1.308(+/-  2%)      1.279(+/- 1%) + 2%
>>> 16           1.347(+/-  1%)      1.317(+/- 1%) + 2%
>>> 
>>> hackbench -p -l (2560 / group) -g group
>>> group
>>> 1            1.836(+/- 17%)      1.148(+/- 5%) +37%
>>> 4            1.586(+/-  6%)      1.109(+/- 8%) +30%
>>> 8            1.209(+/-  4%)      0.780(+/- 4%) +35%
>>> 16           0.805(+/-  5%)      0.728(+/- 4%) +10%
>>> 
>>> By deacreasing the latency prio, we reduce the number of preemption at
>>> wakeup and help hackbench making progress.
>>> 
>>> Test results of nice latency impact on short live load like cyclictest
>>> while competing with heavy load like hackbench:
>>> 
>>> hackbench -l 10000 -g $group &
>>> cyclictest --policy other -D 5 -q -n
>>>        latency 0           latency -20
>>> group   min  avg    max     min  avg    max
>>> 0       16    19     29      17   18     29
>>> 1       43   299   7359      63   84   3422
>>> 4       56   449  14806      45   83    284
>>> 8       63   820  51123      63   83    283
>>> 16      64  1326  70684      41  157  26852
>>> 
>>> group = 0 means that hackbench is not running.
>>> 
>>> The avg is significantly improved with nice latency -20 especially with
>>> large number of groups but min and max remain quite similar. If we add the
>>> histogram parameter to get details of latency, we have :
>>> 
>>> hackbench -l 10000 -g 16 &
>>> cyclictest --policy other -D 5 -q -n  -H 20000 --histfile data.txt
>>>              latency 0    latency -20
>>> Min Latencies:    64           62
>>> Avg Latencies:  1170          107
>>> Max Latencies: 88069        10417
>>> 50% latencies:   122           86
>>> 75% latencies:   614           91
>>> 85% latencies:   961           94
>>> 90% latencies:  1225           97
>>> 95% latencies:  6120          102
>>> 99% latencies: 18328          159
>>> 
>>> With percentile details, we see the benefit of nice latency -20 as
>>> only 1% of the latencies are above 159us whereas the default latency
>>> has got 15% around ~1ms or above and 5% over the 6ms.
>>> 
>>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
>>> ---
>>> include/linux/sched.h      |  4 ++-
>>> include/linux/sched/prio.h |  9 ++++++
>>> init/init_task.c           |  2 +-
>>> kernel/sched/core.c        | 38 +++++++++++++++++++---
>>> kernel/sched/debug.c       |  2 +-
>>> kernel/sched/fair.c        | 66 ++++++++++++++++++++++++++++++++++----
>>> kernel/sched/sched.h       |  6 ++++
>>> 7 files changed, 112 insertions(+), 15 deletions(-)
>>> 
>>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>>> index 856240573300..2f33326adb8d 100644
>>> --- a/include/linux/sched.h
>>> +++ b/include/linux/sched.h
>>> @@ -568,6 +568,8 @@ struct sched_entity {
>>>        /* cached value of my_q->h_nr_running */
>>>        unsigned long                   runnable_weight;
>>> #endif
>>> +       /* preemption offset in ns */
>>> +       long                            latency_offset;
>>> 
>>> #ifdef CONFIG_SMP
>>>        /*
>>> @@ -784,7 +786,7 @@ struct task_struct {
>>>        int                             static_prio;
>>>        int                             normal_prio;
>>>        unsigned int                    rt_priority;
>>> -       int                             latency_nice;
>>> +       int                             latency_prio;
>>> 
>>>        struct sched_entity             se;
>>>        struct sched_rt_entity          rt;
>>> diff --git a/include/linux/sched/prio.h b/include/linux/sched/prio.h
>>> index bfcd7f1d1e11..be79503d86af 100644
>>> --- a/include/linux/sched/prio.h
>>> +++ b/include/linux/sched/prio.h
>>> @@ -59,5 +59,14 @@ static inline long rlimit_to_nice(long prio)
>>>  * Default tasks should be treated as a task with latency_nice = 0.
>>>  */
>>> #define DEFAULT_LATENCY_NICE   0
>>> +#define DEFAULT_LATENCY_PRIO   (DEFAULT_LATENCY_NICE + LATENCY_NICE_WIDTH/2)
>>> +
>>> +/*
>>> + * Convert user-nice values [ -20 ... 0 ... 19 ]
>>> + * to static latency [ 0..39 ],
>>> + * and back.
>>> + */
>>> +#define NICE_TO_LATENCY(nice)  ((nice) + DEFAULT_LATENCY_PRIO)
>>> +#define LATENCY_TO_NICE(prio)  ((prio) - DEFAULT_LATENCY_PRIO)
>>> 
>>> #endif /* _LINUX_SCHED_PRIO_H */
>>> diff --git a/init/init_task.c b/init/init_task.c
>>> index 7dd71dd2d261..071deff8dbd1 100644
>>> --- a/init/init_task.c
>>> +++ b/init/init_task.c
>>> @@ -78,7 +78,7 @@ struct task_struct init_task
>>>        .prio           = MAX_PRIO - 20,
>>>        .static_prio    = MAX_PRIO - 20,
>>>        .normal_prio    = MAX_PRIO - 20,
>>> -       .latency_nice   = DEFAULT_LATENCY_NICE,
>>> +       .latency_prio   = DEFAULT_LATENCY_PRIO,
>>>        .policy         = SCHED_NORMAL,
>>>        .cpus_ptr       = &init_task.cpus_mask,
>>>        .user_cpus_ptr  = NULL,
>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>> index 18c31a68eb18..b2b8cb6c08cd 100644
>>> --- a/kernel/sched/core.c
>>> +++ b/kernel/sched/core.c
>>> @@ -1283,6 +1283,16 @@ static void set_load_weight(struct task_struct *p, bool update_load)
>>>        }
>>> }
>>> 
>>> +static void set_latency_offset(struct task_struct *p)
>>> +{
>>> +       long weight = sched_latency_to_weight[p->latency_prio];
>>> +       s64 offset;
>>> +
>>> +       offset = weight * get_sleep_latency(false);
>>> +       offset = div_s64(offset, NICE_LATENCY_WEIGHT_MAX);
>>> +       p->se.latency_offset = (long)offset;
>>> +}
>>> +
>>> #ifdef CONFIG_UCLAMP_TASK
>>> /*
>>>  * Serializes updates of utilization clamp values
>>> @@ -4592,7 +4602,9 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
>>>                p->prio = p->normal_prio = p->static_prio;
>>>                set_load_weight(p, false);
>>> 
>>> -               p->latency_nice = DEFAULT_LATENCY_NICE;
>>> +               p->latency_prio = NICE_TO_LATENCY(0);
>>> +               set_latency_offset(p);
>>> +
>>>                /*
>>>                 * We don't need the reset flag anymore after the fork. It has
>>>                 * fulfilled its duty:
>>> @@ -7358,8 +7370,10 @@ static void __setscheduler_params(struct task_struct *p,
>>> static void __setscheduler_latency(struct task_struct *p,
>>>                const struct sched_attr *attr)
>>> {
>>> -       if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE)
>>> -               p->latency_nice = attr->sched_latency_nice;
>>> +       if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE) {
>>> +               p->latency_prio = NICE_TO_LATENCY(attr->sched_latency_nice);
>>> +               set_latency_offset(p);
>>> +       }
>>> }
>>> 
>>> /*
>>> @@ -7544,7 +7558,7 @@ static int __sched_setscheduler(struct task_struct *p,
>>>                if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
>>>                        goto change;
>>>                if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE &&
>>> -                   attr->sched_latency_nice != p->latency_nice)
>>> +                   attr->sched_latency_nice != LATENCY_TO_NICE(p->latency_prio))
>>>                        goto change;
>>> 
>>>                p->sched_reset_on_fork = reset_on_fork;
>>> @@ -8085,7 +8099,7 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
>>>        get_params(p, &kattr);
>>>        kattr.sched_flags &= SCHED_FLAG_ALL;
>>> 
>>> -       kattr.sched_latency_nice = p->latency_nice;
>>> +       kattr.sched_latency_nice = LATENCY_TO_NICE(p->latency_prio);
>>> 
>>> #ifdef CONFIG_UCLAMP_TASK
>>>        /*
>>> @@ -11294,6 +11308,20 @@ const u32 sched_prio_to_wmult[40] = {
>>>  /*  15 */ 119304647, 148102320, 186737708, 238609294, 286331153,
>>> };
>>> 
>>> +/*
>>> + * latency weight for wakeup preemption
>>> + */
>>> +const int sched_latency_to_weight[40] = {
>>> + /* -20 */     -1024,     -973,     -922,      -870,      -819,
>>> + /* -15 */      -768,     -717,     -666,      -614,      -563,
>>> + /* -10 */      -512,     -461,     -410,      -358,      -307,
>>> + /*  -5 */      -256,     -205,     -154,      -102,       -51,
>>> + /*   0 */         0,       51,      102,       154,       205,
>>> + /*   5 */       256,      307,      358,       410,       461,
>>> + /*  10 */       512,      563,      614,       666,       717,
>>> + /*  15 */       768,      819,      870,       922,       973,
>>> +};
>>> +
>> 
>> The table is linear. You could approximate this as: weight = nice * 51
>> since it is a linear scale and do the conversion in place.
>> 
>> Or, since the only place you are using the latency_to_weight is in
>> set_latency_offset(), can we drop the sched_latency_to_weight array
>> and simplify as follows?
> 
> It's also used in cgroup patch and keeps a coherency between
> nice/weight an latency_nice/offset so I prefer

I dont think it’s a valid comparison as nice/weight conversion are non linear and over there a table makes sense: weight = 1024 / 1.25 ^ nice

> keeping current
> implementation

I could be missing something, but, since its a linear scale, why does cgroup need weight at all? Just store nice directly. Why would that not work?

In the end the TG and SE has the latency offset in the struct, that is all you care about. All the conversion back and forth is unnecessary, as it is a linear scale and just increases LOC and takes more memory to store linear arrays.

Again I could be missing something and I will try to play with your series and see if I can show you what I mean (or convince myself it’s needed).

>> static void set_latency_offset(struct task_struct *p)
>> {
>>  s64 offset = p->latency_prio * get_sleep_latency(false);
>>  p->latency_prio = (long)div_s64(offset, 40);
>> }
>> 
>>> void call_trace_sched_update_nr_running(struct rq *rq, int count)
>>> {
>>>         trace_sched_update_nr_running_tp(rq, count);
>>> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
>>> index 68be7a3e42a3..b3922184af91 100644
>>> --- a/kernel/sched/debug.c
>>> +++ b/kernel/sched/debug.c
>>> @@ -1043,7 +1043,7 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
>>> #endif
>>>        P(policy);
>>>        P(prio);
>>> -       P(latency_nice);
>>> +       P(latency_prio);
>>>        if (task_has_dl_policy(p)) {
>>>                P(dl.runtime);
>>>                P(dl.deadline);
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index c8a697f8db88..0e80e65113bd 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -4858,6 +4858,8 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>>>                update_idle_cfs_rq_clock_pelt(cfs_rq);
>>> }
>>> 
>>> +static long wakeup_latency_gran(struct sched_entity *curr, struct sched_entity *se);
>>> +
>>> /*
>>>  * Preempt the current task with a newly woken task if needed:
>>>  */
>>> @@ -4866,7 +4868,7 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>>> {
>>>        unsigned long ideal_runtime, delta_exec;
>>>        struct sched_entity *se;
>>> -       s64 delta;
>>> +       s64 delta, offset;
>>> 
>>>        ideal_runtime = sched_slice(cfs_rq, curr);
>>>        delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
>>> @@ -4891,10 +4893,12 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>>>        se = __pick_first_entity(cfs_rq);
>>>        delta = curr->vruntime - se->vruntime;
>>> 
>>> -       if (delta < 0)
>>> +       offset = wakeup_latency_gran(curr, se);
>>> +       if (delta < offset)
>>>                return;
>> 
>> Agreed.
>> 
>>> -       if (delta > ideal_runtime)
>>> +       if ((delta > ideal_runtime) ||
>>> +           (delta > get_latency_max()))
>>>                resched_curr(rq_of(cfs_rq));
>>> }
>>> 
>>> @@ -6019,6 +6023,35 @@ static int sched_idle_cpu(int cpu)
>>> }
>>> #endif
>>> 
>>> +static void set_next_buddy(struct sched_entity *se);
>>> +
>>> +static void check_preempt_from_others(struct cfs_rq *cfs, struct sched_entity *se)
>>> +{
>>> +       struct sched_entity *next;
>>> +
>>> +       if (se->latency_offset >= 0)
>>> +               return;
>>> +
>>> +       if (cfs->nr_running <= 1)
>>> +               return;
>>> +       /*
>>> +        * When waking from another class, we don't need to check to preempt at
>>> +        * wakeup and don't set next buddy as a candidate for being picked in
>>> +        * priority.
>>> +        * In case of simultaneous wakeup when current is another class, the
>>> +        * latency sensitive tasks lost opportunity to preempt non sensitive
>>> +        * tasks which woke up simultaneously.
>>> +        */
>>> +
>>> +       if (cfs->next)
>>> +               next = cfs->next;
>>> +       else
>>> +               next = __pick_first_entity(cfs);
>>> +
>>> +       if (next && wakeup_preempt_entity(next, se) == 1)
>>> +               set_next_buddy(se);
>>> +}
>>> +
>>> /*
>>>  * The enqueue_task method is called before nr_running is
>>>  * increased. Here we update the fair scheduling stats and
>>> @@ -6105,14 +6138,15 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>>>        if (!task_new)
>>>                update_overutilized_status(rq);
>>> 
>>> +       if (rq->curr->sched_class != &fair_sched_class)
>>> +               check_preempt_from_others(cfs_rq_of(&p->se), &p->se);
>>> +
>>> enqueue_throttle:
>>>        assert_list_leaf_cfs_rq(rq);
>>> 
>>>        hrtick_update(rq);
>>> }
>>> 
>>> -static void set_next_buddy(struct sched_entity *se);
>>> -
>>> /*
>>>  * The dequeue_task method is called before nr_running is
>>>  * decreased. We remove the task from the rbtree and
>>> @@ -7461,6 +7495,23 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>>> }
>>> #endif /* CONFIG_SMP */
>>> 
>>> +static long wakeup_latency_gran(struct sched_entity *curr, struct sched_entity *se)
>>> +{
>>> +       long latency_offset = se->latency_offset;
>>> +
>>> +       /*
>>> +        * A negative latency offset means that the sched_entity has latency
>>> +        * requirement that needs to be evaluated versus other entity.
>>> +        * Otherwise, use the latency weight to evaluate how much scheduling
>>> +        * delay is acceptable by se.
>>> +        */
>>> +       if ((latency_offset < 0) || (curr->latency_offset < 0))
>>> +               latency_offset -= curr->latency_offset;
>>> +       latency_offset = min_t(long, latency_offset, get_latency_max());
>> 
>> Over here can we make positive latency offsets also be evaluated
>> "versus other entity"?
>> 
>> It feels strange to have different rules for positive latency_offset
>> when comparing curr and se. IMO we should also factor in latency
>> requirements by comparing 2 positive nice values. It should be
>> relative even for positive values, just like regular nice IMO and not
>> have hidden meaning. If there is hidden meaning, it confuses the user
>> and requires documentation that most users will not read. Especially
>> because latency_nice shares the word "nice" with regular nice values.
> 
> This has already been discussed in the previous revisions.

Sorry to be late to the party.

> This is not
> a hidden behavior but the normal behavior.
> 
> A negative latency nice, means that the task are not tolerant to
> scheduling delay and it want to preempt current and run now. Or, if
> the task is current, it doesn't want to be preempted and finish its
> slice. In this case, we compare current and wake up task in case there
> is 2 latency sensitive tasks that are fighting to run 1st.
> 
> Whereas a positive latency nice means that the task is tolerant to
> scheduling delay and you don't care preempting current as long as it's
> in an acceptable vruntime range. Why would the latency nice of the
> current task make the wakeup task less tolerant to the scheduling
> delay ? As an example, If current is latency_nice 19 and the wakeup
> task is latency nice 19 too, both are tolerant to scheduling delay and
> the waking up task should preempt current only if there is an
> unfairness problem. By comparing their positive latency nice values,
> you are back to the normal behavior which defeats the purpose of the
> feature.

I see it as, if 2 tasks are latency tolerant, then they will have higher latency with respect to a third tasks that is latency in tolerant. But I am ok with your definition as well…

Thanks!

 - Joel

> 
> Thanks
> Vincent
> 
>> 
>> Thanks,
>> 
>> - Joel
>> .
>> 
>>> +
>>> +       return latency_offset;
>>> +}
>>> +
>>> static unsigned long wakeup_gran(struct sched_entity *se)
>>> {
>>>        unsigned long gran = sysctl_sched_wakeup_granularity;
>>> @@ -7499,11 +7550,12 @@ static int
>>> wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
>>> {
>>>        s64 gran, vdiff = curr->vruntime - se->vruntime;
>>> +       s64 offset = wakeup_latency_gran(curr, se);
>>> 
>>> -       if (vdiff <= 0)
>>> +       if (vdiff < offset)
>>>                return -1;
>>> 
>>> -       gran = wakeup_gran(se);
>>> +       gran = offset + wakeup_gran(se);
>>> 
>>>        /*
>>>         * At wake up, the vruntime of a task is capped to not be older than
>>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>>> index 842ce0094d9c..7292652731d0 100644
>>> --- a/kernel/sched/sched.h
>>> +++ b/kernel/sched/sched.h
>>> @@ -125,6 +125,11 @@ extern int sched_rr_timeslice;
>>>  */
>>> #define NS_TO_JIFFIES(TIME)    ((unsigned long)(TIME) / (NSEC_PER_SEC / HZ))
>>> 
>>> +/* Maximum nice latency weight used to scale the latency_offset */
>>> +
>>> +#define NICE_LATENCY_SHIFT     (SCHED_FIXEDPOINT_SHIFT)
>>> +#define NICE_LATENCY_WEIGHT_MAX        (1L << NICE_LATENCY_SHIFT)
>>> +
>>> /*
>>>  * Increase resolution of nice-level calculations for 64-bit architectures.
>>>  * The extra resolution improves shares distribution and load balancing of
>>> @@ -2115,6 +2120,7 @@ static_assert(WF_TTWU == SD_BALANCE_WAKE);
>>> 
>>> extern const int               sched_prio_to_weight[40];
>>> extern const u32               sched_prio_to_wmult[40];
>>> +extern const int               sched_latency_to_weight[40];
>>> 
>>> /*
>>>  * {de,en}queue flags:
>>> --
>>> 2.17.1
>>> 

Re: [PATCH 5/9] sched/fair: Take into account latency priority at wakeup

[Vincent Guittot]

On Tue, 29 Nov 2022 at 16:45, Joel Fernandes <joel@joelfernandes.org> wrote:
>
> Hi Vincent,
> Thank you so much for your quick reply.
>
> > On Nov 29, 2022, at 3:58 AM, Vincent Guittot <vincent.guittot@linaro.org> wrote:
> >
> > On Tue, 29 Nov 2022 at 05:25, Joel Fernandes <joel@joelfernandes.org> wrote:
> >>
> >>> On Tue, Nov 15, 2022 at 12:19 PM Vincent Guittot
> >>> <vincent.guittot@linaro.org> wrote:
> >>>
> >>> Take into account the latency priority of a thread when deciding to
> >>> preempt the current running thread. We don't want to provide more CPU
> >>> bandwidth to a thread but reorder the scheduling to run latency sensitive
> >>> task first whenever possible.
> >>>
> >>> As long as a thread didn't use its bandwidth, it will be able to preempt
> >>> the current thread.
> >>>
> >>> At the opposite, a thread with a low latency priority will preempt current
> >>> thread at wakeup only to keep fair CPU bandwidth sharing. Otherwise it will
> >>> wait for the tick to get its sched slice.
> >>>
> >>>                                   curr vruntime
> >>>                                       |
> >>>                      sysctl_sched_wakeup_granularity
> >>>                                   <-->
> >>> ----------------------------------|----|-----------------------|---------------
> >>>                                  |    |<--------------------->
> >>>                                  |    .  sysctl_sched_latency
> >>>                                  |    .
> >>> default/current latency entity    |    .
> >>>                                  |    .
> >>> 1111111111111111111111111111111111|0000|-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-
> >>> se preempts curr at wakeup ------>|<- se doesn't preempt curr -----------------
> >>>                                  |    .
> >>>                                  |    .
> >>>                                  |    .
> >>> low latency entity                |    .
> >>>                                   ---------------------->|
> >>>                               % of sysctl_sched_latency  |
> >>> 1111111111111111111111111111111111111111111111111111111111|0000|-1-1-1-1-1-1-1-
> >>> preempt ------------------------------------------------->|<- do not preempt --
> >>>                                  |    .
> >>>                                  |    .
> >>>                                  |    .
> >>> high latency entity               |    .
> >>>         |<-----------------------|----.
> >>>         | % of sysctl_sched_latency   .
> >>> 111111111|0000|-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1
> >>> preempt->|<- se doesn't preempt curr ------------------------------------------
> >>> Tests results of nice latency impact on heavy load like hackbench:
> >>>
> >>> hackbench -l (2560 / group) -g group
> >>> group        latency 0             latency 19
> >>> 1            1.378(+/-  1%)      1.337(+/- 1%) + 3%
> >>> 4            1.393(+/-  3%)      1.312(+/- 3%) + 6%
> >>> 8            1.308(+/-  2%)      1.279(+/- 1%) + 2%
> >>> 16           1.347(+/-  1%)      1.317(+/- 1%) + 2%
> >>>
> >>> hackbench -p -l (2560 / group) -g group
> >>> group
> >>> 1            1.836(+/- 17%)      1.148(+/- 5%) +37%
> >>> 4            1.586(+/-  6%)      1.109(+/- 8%) +30%
> >>> 8            1.209(+/-  4%)      0.780(+/- 4%) +35%
> >>> 16           0.805(+/-  5%)      0.728(+/- 4%) +10%
> >>>
> >>> By deacreasing the latency prio, we reduce the number of preemption at
> >>> wakeup and help hackbench making progress.
> >>>
> >>> Test results of nice latency impact on short live load like cyclictest
> >>> while competing with heavy load like hackbench:
> >>>
> >>> hackbench -l 10000 -g $group &
> >>> cyclictest --policy other -D 5 -q -n
> >>>        latency 0           latency -20
> >>> group   min  avg    max     min  avg    max
> >>> 0       16    19     29      17   18     29
> >>> 1       43   299   7359      63   84   3422
> >>> 4       56   449  14806      45   83    284
> >>> 8       63   820  51123      63   83    283
> >>> 16      64  1326  70684      41  157  26852
> >>>
> >>> group = 0 means that hackbench is not running.
> >>>
> >>> The avg is significantly improved with nice latency -20 especially with
> >>> large number of groups but min and max remain quite similar. If we add the
> >>> histogram parameter to get details of latency, we have :
> >>>
> >>> hackbench -l 10000 -g 16 &
> >>> cyclictest --policy other -D 5 -q -n  -H 20000 --histfile data.txt
> >>>              latency 0    latency -20
> >>> Min Latencies:    64           62
> >>> Avg Latencies:  1170          107
> >>> Max Latencies: 88069        10417
> >>> 50% latencies:   122           86
> >>> 75% latencies:   614           91
> >>> 85% latencies:   961           94
> >>> 90% latencies:  1225           97
> >>> 95% latencies:  6120          102
> >>> 99% latencies: 18328          159
> >>>
> >>> With percentile details, we see the benefit of nice latency -20 as
> >>> only 1% of the latencies are above 159us whereas the default latency
> >>> has got 15% around ~1ms or above and 5% over the 6ms.
> >>>
> >>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> >>> ---
> >>> include/linux/sched.h      |  4 ++-
> >>> include/linux/sched/prio.h |  9 ++++++
> >>> init/init_task.c           |  2 +-
> >>> kernel/sched/core.c        | 38 +++++++++++++++++++---
> >>> kernel/sched/debug.c       |  2 +-
> >>> kernel/sched/fair.c        | 66 ++++++++++++++++++++++++++++++++++----
> >>> kernel/sched/sched.h       |  6 ++++
> >>> 7 files changed, 112 insertions(+), 15 deletions(-)
> >>>
> >>> diff --git a/include/linux/sched.h b/include/linux/sched.h
> >>> index 856240573300..2f33326adb8d 100644
> >>> --- a/include/linux/sched.h
> >>> +++ b/include/linux/sched.h
> >>> @@ -568,6 +568,8 @@ struct sched_entity {
> >>>        /* cached value of my_q->h_nr_running */
> >>>        unsigned long                   runnable_weight;
> >>> #endif
> >>> +       /* preemption offset in ns */
> >>> +       long                            latency_offset;
> >>>
> >>> #ifdef CONFIG_SMP
> >>>        /*
> >>> @@ -784,7 +786,7 @@ struct task_struct {
> >>>        int                             static_prio;
> >>>        int                             normal_prio;
> >>>        unsigned int                    rt_priority;
> >>> -       int                             latency_nice;
> >>> +       int                             latency_prio;
> >>>
> >>>        struct sched_entity             se;
> >>>        struct sched_rt_entity          rt;
> >>> diff --git a/include/linux/sched/prio.h b/include/linux/sched/prio.h
> >>> index bfcd7f1d1e11..be79503d86af 100644
> >>> --- a/include/linux/sched/prio.h
> >>> +++ b/include/linux/sched/prio.h
> >>> @@ -59,5 +59,14 @@ static inline long rlimit_to_nice(long prio)
> >>>  * Default tasks should be treated as a task with latency_nice = 0.
> >>>  */
> >>> #define DEFAULT_LATENCY_NICE   0
> >>> +#define DEFAULT_LATENCY_PRIO   (DEFAULT_LATENCY_NICE + LATENCY_NICE_WIDTH/2)
> >>> +
> >>> +/*
> >>> + * Convert user-nice values [ -20 ... 0 ... 19 ]
> >>> + * to static latency [ 0..39 ],
> >>> + * and back.
> >>> + */
> >>> +#define NICE_TO_LATENCY(nice)  ((nice) + DEFAULT_LATENCY_PRIO)
> >>> +#define LATENCY_TO_NICE(prio)  ((prio) - DEFAULT_LATENCY_PRIO)
> >>>
> >>> #endif /* _LINUX_SCHED_PRIO_H */
> >>> diff --git a/init/init_task.c b/init/init_task.c
> >>> index 7dd71dd2d261..071deff8dbd1 100644
> >>> --- a/init/init_task.c
> >>> +++ b/init/init_task.c
> >>> @@ -78,7 +78,7 @@ struct task_struct init_task
> >>>        .prio           = MAX_PRIO - 20,
> >>>        .static_prio    = MAX_PRIO - 20,
> >>>        .normal_prio    = MAX_PRIO - 20,
> >>> -       .latency_nice   = DEFAULT_LATENCY_NICE,
> >>> +       .latency_prio   = DEFAULT_LATENCY_PRIO,
> >>>        .policy         = SCHED_NORMAL,
> >>>        .cpus_ptr       = &init_task.cpus_mask,
> >>>        .user_cpus_ptr  = NULL,
> >>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> >>> index 18c31a68eb18..b2b8cb6c08cd 100644
> >>> --- a/kernel/sched/core.c
> >>> +++ b/kernel/sched/core.c
> >>> @@ -1283,6 +1283,16 @@ static void set_load_weight(struct task_struct *p, bool update_load)
> >>>        }
> >>> }
> >>>
> >>> +static void set_latency_offset(struct task_struct *p)
> >>> +{
> >>> +       long weight = sched_latency_to_weight[p->latency_prio];
> >>> +       s64 offset;
> >>> +
> >>> +       offset = weight * get_sleep_latency(false);
> >>> +       offset = div_s64(offset, NICE_LATENCY_WEIGHT_MAX);
> >>> +       p->se.latency_offset = (long)offset;
> >>> +}
> >>> +
> >>> #ifdef CONFIG_UCLAMP_TASK
> >>> /*
> >>>  * Serializes updates of utilization clamp values
> >>> @@ -4592,7 +4602,9 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
> >>>                p->prio = p->normal_prio = p->static_prio;
> >>>                set_load_weight(p, false);
> >>>
> >>> -               p->latency_nice = DEFAULT_LATENCY_NICE;
> >>> +               p->latency_prio = NICE_TO_LATENCY(0);
> >>> +               set_latency_offset(p);
> >>> +
> >>>                /*
> >>>                 * We don't need the reset flag anymore after the fork. It has
> >>>                 * fulfilled its duty:
> >>> @@ -7358,8 +7370,10 @@ static void __setscheduler_params(struct task_struct *p,
> >>> static void __setscheduler_latency(struct task_struct *p,
> >>>                const struct sched_attr *attr)
> >>> {
> >>> -       if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE)
> >>> -               p->latency_nice = attr->sched_latency_nice;
> >>> +       if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE) {
> >>> +               p->latency_prio = NICE_TO_LATENCY(attr->sched_latency_nice);
> >>> +               set_latency_offset(p);
> >>> +       }
> >>> }
> >>>
> >>> /*
> >>> @@ -7544,7 +7558,7 @@ static int __sched_setscheduler(struct task_struct *p,
> >>>                if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
> >>>                        goto change;
> >>>                if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE &&
> >>> -                   attr->sched_latency_nice != p->latency_nice)
> >>> +                   attr->sched_latency_nice != LATENCY_TO_NICE(p->latency_prio))
> >>>                        goto change;
> >>>
> >>>                p->sched_reset_on_fork = reset_on_fork;
> >>> @@ -8085,7 +8099,7 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
> >>>        get_params(p, &kattr);
> >>>        kattr.sched_flags &= SCHED_FLAG_ALL;
> >>>
> >>> -       kattr.sched_latency_nice = p->latency_nice;
> >>> +       kattr.sched_latency_nice = LATENCY_TO_NICE(p->latency_prio);
> >>>
> >>> #ifdef CONFIG_UCLAMP_TASK
> >>>        /*
> >>> @@ -11294,6 +11308,20 @@ const u32 sched_prio_to_wmult[40] = {
> >>>  /*  15 */ 119304647, 148102320, 186737708, 238609294, 286331153,
> >>> };
> >>>
> >>> +/*
> >>> + * latency weight for wakeup preemption
> >>> + */
> >>> +const int sched_latency_to_weight[40] = {
> >>> + /* -20 */     -1024,     -973,     -922,      -870,      -819,
> >>> + /* -15 */      -768,     -717,     -666,      -614,      -563,
> >>> + /* -10 */      -512,     -461,     -410,      -358,      -307,
> >>> + /*  -5 */      -256,     -205,     -154,      -102,       -51,
> >>> + /*   0 */         0,       51,      102,       154,       205,
> >>> + /*   5 */       256,      307,      358,       410,       461,
> >>> + /*  10 */       512,      563,      614,       666,       717,
> >>> + /*  15 */       768,      819,      870,       922,       973,
> >>> +};
> >>> +
> >>
> >> The table is linear. You could approximate this as: weight = nice * 51
> >> since it is a linear scale and do the conversion in place.
> >>
> >> Or, since the only place you are using the latency_to_weight is in
> >> set_latency_offset(), can we drop the sched_latency_to_weight array
> >> and simplify as follows?
> >
> > It's also used in cgroup patch and keeps a coherency between
> > nice/weight an latency_nice/offset so I prefer
>
> I dont think it’s a valid comparison as nice/weight conversion are non linear and over there a table makes sense: weight = 1024 / 1.25 ^ nice
>
> > keeping current
> > implementation
>
> I could be missing something, but, since its a linear scale, why does cgroup need weight at all? Just store nice directly. Why would that not work?
>
> In the end the TG and SE has the latency offset in the struct, that is all you care about. All the conversion back and forth is unnecessary, as it is a linear scale and just increases LOC and takes more memory to store linear arrays.
>
> Again I could be missing something and I will try to play with your series and see if I can show you what I mean (or convince myself it’s needed).

I get what you mean but I think that having an array gives latitude to
adjust this internal offset mapping at a minimum cost of a const array

>
> >> static void set_latency_offset(struct task_struct *p)
> >> {
> >>  s64 offset = p->latency_prio * get_sleep_latency(false);
> >>  p->latency_prio = (long)div_s64(offset, 40);
> >> }
> >>
> >>> void call_trace_sched_update_nr_running(struct rq *rq, int count)
> >>> {
> >>>         trace_sched_update_nr_running_tp(rq, count);
> >>> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> >>> index 68be7a3e42a3..b3922184af91 100644
> >>> --- a/kernel/sched/debug.c
> >>> +++ b/kernel/sched/debug.c
> >>> @@ -1043,7 +1043,7 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
> >>> #endif
> >>>        P(policy);
> >>>        P(prio);
> >>> -       P(latency_nice);
> >>> +       P(latency_prio);
> >>>        if (task_has_dl_policy(p)) {
> >>>                P(dl.runtime);
> >>>                P(dl.deadline);
> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >>> index c8a697f8db88..0e80e65113bd 100644
> >>> --- a/kernel/sched/fair.c
> >>> +++ b/kernel/sched/fair.c
> >>> @@ -4858,6 +4858,8 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> >>>                update_idle_cfs_rq_clock_pelt(cfs_rq);
> >>> }
> >>>
> >>> +static long wakeup_latency_gran(struct sched_entity *curr, struct sched_entity *se);
> >>> +
> >>> /*
> >>>  * Preempt the current task with a newly woken task if needed:
> >>>  */
> >>> @@ -4866,7 +4868,7 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> >>> {
> >>>        unsigned long ideal_runtime, delta_exec;
> >>>        struct sched_entity *se;
> >>> -       s64 delta;
> >>> +       s64 delta, offset;
> >>>
> >>>        ideal_runtime = sched_slice(cfs_rq, curr);
> >>>        delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
> >>> @@ -4891,10 +4893,12 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> >>>        se = __pick_first_entity(cfs_rq);
> >>>        delta = curr->vruntime - se->vruntime;
> >>>
> >>> -       if (delta < 0)
> >>> +       offset = wakeup_latency_gran(curr, se);
> >>> +       if (delta < offset)
> >>>                return;
> >>
> >> Agreed.
> >>
> >>> -       if (delta > ideal_runtime)
> >>> +       if ((delta > ideal_runtime) ||
> >>> +           (delta > get_latency_max()))
> >>>                resched_curr(rq_of(cfs_rq));
> >>> }
> >>>
> >>> @@ -6019,6 +6023,35 @@ static int sched_idle_cpu(int cpu)
> >>> }
> >>> #endif
> >>>
> >>> +static void set_next_buddy(struct sched_entity *se);
> >>> +
> >>> +static void check_preempt_from_others(struct cfs_rq *cfs, struct sched_entity *se)
> >>> +{
> >>> +       struct sched_entity *next;
> >>> +
> >>> +       if (se->latency_offset >= 0)
> >>> +               return;
> >>> +
> >>> +       if (cfs->nr_running <= 1)
> >>> +               return;
> >>> +       /*
> >>> +        * When waking from another class, we don't need to check to preempt at
> >>> +        * wakeup and don't set next buddy as a candidate for being picked in
> >>> +        * priority.
> >>> +        * In case of simultaneous wakeup when current is another class, the
> >>> +        * latency sensitive tasks lost opportunity to preempt non sensitive
> >>> +        * tasks which woke up simultaneously.
> >>> +        */
> >>> +
> >>> +       if (cfs->next)
> >>> +               next = cfs->next;
> >>> +       else
> >>> +               next = __pick_first_entity(cfs);
> >>> +
> >>> +       if (next && wakeup_preempt_entity(next, se) == 1)
> >>> +               set_next_buddy(se);
> >>> +}
> >>> +
> >>> /*
> >>>  * The enqueue_task method is called before nr_running is
> >>>  * increased. Here we update the fair scheduling stats and
> >>> @@ -6105,14 +6138,15 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> >>>        if (!task_new)
> >>>                update_overutilized_status(rq);
> >>>
> >>> +       if (rq->curr->sched_class != &fair_sched_class)
> >>> +               check_preempt_from_others(cfs_rq_of(&p->se), &p->se);
> >>> +
> >>> enqueue_throttle:
> >>>        assert_list_leaf_cfs_rq(rq);
> >>>
> >>>        hrtick_update(rq);
> >>> }
> >>>
> >>> -static void set_next_buddy(struct sched_entity *se);
> >>> -
> >>> /*
> >>>  * The dequeue_task method is called before nr_running is
> >>>  * decreased. We remove the task from the rbtree and
> >>> @@ -7461,6 +7495,23 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> >>> }
> >>> #endif /* CONFIG_SMP */
> >>>
> >>> +static long wakeup_latency_gran(struct sched_entity *curr, struct sched_entity *se)
> >>> +{
> >>> +       long latency_offset = se->latency_offset;
> >>> +
> >>> +       /*
> >>> +        * A negative latency offset means that the sched_entity has latency
> >>> +        * requirement that needs to be evaluated versus other entity.
> >>> +        * Otherwise, use the latency weight to evaluate how much scheduling
> >>> +        * delay is acceptable by se.
> >>> +        */
> >>> +       if ((latency_offset < 0) || (curr->latency_offset < 0))
> >>> +               latency_offset -= curr->latency_offset;
> >>> +       latency_offset = min_t(long, latency_offset, get_latency_max());
> >>
> >> Over here can we make positive latency offsets also be evaluated
> >> "versus other entity"?
> >>
> >> It feels strange to have different rules for positive latency_offset
> >> when comparing curr and se. IMO we should also factor in latency
> >> requirements by comparing 2 positive nice values. It should be
> >> relative even for positive values, just like regular nice IMO and not
> >> have hidden meaning. If there is hidden meaning, it confuses the user
> >> and requires documentation that most users will not read. Especially
> >> because latency_nice shares the word "nice" with regular nice values.
> >
> > This has already been discussed in the previous revisions.
>
> Sorry to be late to the party.
>
> > This is not
> > a hidden behavior but the normal behavior.
> >
> > A negative latency nice, means that the task are not tolerant to
> > scheduling delay and it want to preempt current and run now. Or, if
> > the task is current, it doesn't want to be preempted and finish its
> > slice. In this case, we compare current and wake up task in case there
> > is 2 latency sensitive tasks that are fighting to run 1st.
> >
> > Whereas a positive latency nice means that the task is tolerant to
> > scheduling delay and you don't care preempting current as long as it's
> > in an acceptable vruntime range. Why would the latency nice of the
> > current task make the wakeup task less tolerant to the scheduling
> > delay ? As an example, If current is latency_nice 19 and the wakeup
> > task is latency nice 19 too, both are tolerant to scheduling delay and
> > the waking up task should preempt current only if there is an
> > unfairness problem. By comparing their positive latency nice values,
> > you are back to the normal behavior which defeats the purpose of the
> > feature.
>
> I see it as, if 2 tasks are latency tolerant, then they will have higher latency with respect to a third tasks that is latency in tolerant. But I am ok with your definition as well…
>
> Thanks!
>
>  - Joel
>
> >
> > Thanks
> > Vincent
> >
> >>
> >> Thanks,
> >>
> >> - Joel
> >> .
> >>
> >>> +
> >>> +       return latency_offset;
> >>> +}
> >>> +
> >>> static unsigned long wakeup_gran(struct sched_entity *se)
> >>> {
> >>>        unsigned long gran = sysctl_sched_wakeup_granularity;
> >>> @@ -7499,11 +7550,12 @@ static int
> >>> wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
> >>> {
> >>>        s64 gran, vdiff = curr->vruntime - se->vruntime;
> >>> +       s64 offset = wakeup_latency_gran(curr, se);
> >>>
> >>> -       if (vdiff <= 0)
> >>> +       if (vdiff < offset)
> >>>                return -1;
> >>>
> >>> -       gran = wakeup_gran(se);
> >>> +       gran = offset + wakeup_gran(se);
> >>>
> >>>        /*
> >>>         * At wake up, the vruntime of a task is capped to not be older than
> >>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> >>> index 842ce0094d9c..7292652731d0 100644
> >>> --- a/kernel/sched/sched.h
> >>> +++ b/kernel/sched/sched.h
> >>> @@ -125,6 +125,11 @@ extern int sched_rr_timeslice;
> >>>  */
> >>> #define NS_TO_JIFFIES(TIME)    ((unsigned long)(TIME) / (NSEC_PER_SEC / HZ))
> >>>
> >>> +/* Maximum nice latency weight used to scale the latency_offset */
> >>> +
> >>> +#define NICE_LATENCY_SHIFT     (SCHED_FIXEDPOINT_SHIFT)
> >>> +#define NICE_LATENCY_WEIGHT_MAX        (1L << NICE_LATENCY_SHIFT)
> >>> +
> >>> /*
> >>>  * Increase resolution of nice-level calculations for 64-bit architectures.
> >>>  * The extra resolution improves shares distribution and load balancing of
> >>> @@ -2115,6 +2120,7 @@ static_assert(WF_TTWU == SD_BALANCE_WAKE);
> >>>
> >>> extern const int               sched_prio_to_weight[40];
> >>> extern const u32               sched_prio_to_wmult[40];
> >>> +extern const int               sched_latency_to_weight[40];
> >>>
> >>> /*
> >>>  * {de,en}queue flags:
> >>> --
> >>> 2.17.1
> >>>

Re: [PATCH 5/9] sched/fair: Take into account latency priority at wakeup

[Joel Fernandes]

Hi Vincent,

On Tue, Nov 29, 2022 at 5:21 PM Vincent Guittot
<vincent.guittot@linaro.org> wrote:
[...]
> > >>> }
> > >>>
> > >>> /*
> > >>> @@ -7544,7 +7558,7 @@ static int __sched_setscheduler(struct task_struct *p,
> > >>>                if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
> > >>>                        goto change;
> > >>>                if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE &&
> > >>> -                   attr->sched_latency_nice != p->latency_nice)
> > >>> +                   attr->sched_latency_nice != LATENCY_TO_NICE(p->latency_prio))
> > >>>                        goto change;
> > >>>
> > >>>                p->sched_reset_on_fork = reset_on_fork;
> > >>> @@ -8085,7 +8099,7 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
> > >>>        get_params(p, &kattr);
> > >>>        kattr.sched_flags &= SCHED_FLAG_ALL;
> > >>>
> > >>> -       kattr.sched_latency_nice = p->latency_nice;
> > >>> +       kattr.sched_latency_nice = LATENCY_TO_NICE(p->latency_prio);
> > >>>
> > >>> #ifdef CONFIG_UCLAMP_TASK
> > >>>        /*
> > >>> @@ -11294,6 +11308,20 @@ const u32 sched_prio_to_wmult[40] = {
> > >>>  /*  15 */ 119304647, 148102320, 186737708, 238609294, 286331153,
> > >>> };
> > >>>
> > >>> +/*
> > >>> + * latency weight for wakeup preemption
> > >>> + */
> > >>> +const int sched_latency_to_weight[40] = {
> > >>> + /* -20 */     -1024,     -973,     -922,      -870,      -819,
> > >>> + /* -15 */      -768,     -717,     -666,      -614,      -563,
> > >>> + /* -10 */      -512,     -461,     -410,      -358,      -307,
> > >>> + /*  -5 */      -256,     -205,     -154,      -102,       -51,
> > >>> + /*   0 */         0,       51,      102,       154,       205,
> > >>> + /*   5 */       256,      307,      358,       410,       461,
> > >>> + /*  10 */       512,      563,      614,       666,       717,
> > >>> + /*  15 */       768,      819,      870,       922,       973,
> > >>> +};
> > >>> +
> > >>
> > >> The table is linear. You could approximate this as: weight = nice * 51
> > >> since it is a linear scale and do the conversion in place.
> > >>
> > >> Or, since the only place you are using the latency_to_weight is in
> > >> set_latency_offset(), can we drop the sched_latency_to_weight array
> > >> and simplify as follows?
> > >
> > > It's also used in cgroup patch and keeps a coherency between
> > > nice/weight an latency_nice/offset so I prefer
> >
> > I dont think it’s a valid comparison as nice/weight conversion are non linear and over there a table makes sense: weight = 1024 / 1.25 ^ nice
> >
> > > keeping current
> > > implementation
> >
> > I could be missing something, but, since its a linear scale, why does cgroup need weight at all? Just store nice directly. Why would that not work?
> >
> > In the end the TG and SE has the latency offset in the struct, that is all you care about. All the conversion back and forth is unnecessary, as it is a linear scale and just increases LOC and takes more memory to store linear arrays.
> >
> > Again I could be missing something and I will try to play with your series and see if I can show you what I mean (or convince myself it’s needed).
>
> I get what you mean but I think that having an array gives latitude to
> adjust this internal offset mapping at a minimum cost of a const array

Ok that makes sense. If you feel like there might be updates in the
future to this mapping array (like changing the constants as you
mentioned), then I am Ok with us keeping it.

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

I am excited about your series, the CFS latency issues have been
thorny. This feels like a step forward in the right direction. Cheers,

 - Joel

Re: [PATCH 5/9] sched/fair: Take into account latency priority at wakeup

[Vincent Guittot]

On Wed, 30 Nov 2022 at 04:10, Joel Fernandes <joel@joelfernandes.org> wrote:
>
> Hi Vincent,
>
> On Tue, Nov 29, 2022 at 5:21 PM Vincent Guittot
> <vincent.guittot@linaro.org> wrote:
> [...]
> > > >>> }
> > > >>>
> > > >>> /*
> > > >>> @@ -7544,7 +7558,7 @@ static int __sched_setscheduler(struct task_struct *p,
> > > >>>                if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
> > > >>>                        goto change;
> > > >>>                if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE &&
> > > >>> -                   attr->sched_latency_nice != p->latency_nice)
> > > >>> +                   attr->sched_latency_nice != LATENCY_TO_NICE(p->latency_prio))
> > > >>>                        goto change;
> > > >>>
> > > >>>                p->sched_reset_on_fork = reset_on_fork;
> > > >>> @@ -8085,7 +8099,7 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
> > > >>>        get_params(p, &kattr);
> > > >>>        kattr.sched_flags &= SCHED_FLAG_ALL;
> > > >>>
> > > >>> -       kattr.sched_latency_nice = p->latency_nice;
> > > >>> +       kattr.sched_latency_nice = LATENCY_TO_NICE(p->latency_prio);
> > > >>>
> > > >>> #ifdef CONFIG_UCLAMP_TASK
> > > >>>        /*
> > > >>> @@ -11294,6 +11308,20 @@ const u32 sched_prio_to_wmult[40] = {
> > > >>>  /*  15 */ 119304647, 148102320, 186737708, 238609294, 286331153,
> > > >>> };
> > > >>>
> > > >>> +/*
> > > >>> + * latency weight for wakeup preemption
> > > >>> + */
> > > >>> +const int sched_latency_to_weight[40] = {
> > > >>> + /* -20 */     -1024,     -973,     -922,      -870,      -819,
> > > >>> + /* -15 */      -768,     -717,     -666,      -614,      -563,
> > > >>> + /* -10 */      -512,     -461,     -410,      -358,      -307,
> > > >>> + /*  -5 */      -256,     -205,     -154,      -102,       -51,
> > > >>> + /*   0 */         0,       51,      102,       154,       205,
> > > >>> + /*   5 */       256,      307,      358,       410,       461,
> > > >>> + /*  10 */       512,      563,      614,       666,       717,
> > > >>> + /*  15 */       768,      819,      870,       922,       973,
> > > >>> +};
> > > >>> +
> > > >>
> > > >> The table is linear. You could approximate this as: weight = nice * 51
> > > >> since it is a linear scale and do the conversion in place.
> > > >>
> > > >> Or, since the only place you are using the latency_to_weight is in
> > > >> set_latency_offset(), can we drop the sched_latency_to_weight array
> > > >> and simplify as follows?
> > > >
> > > > It's also used in cgroup patch and keeps a coherency between
> > > > nice/weight an latency_nice/offset so I prefer
> > >
> > > I dont think it’s a valid comparison as nice/weight conversion are non linear and over there a table makes sense: weight = 1024 / 1.25 ^ nice
> > >
> > > > keeping current
> > > > implementation
> > >
> > > I could be missing something, but, since its a linear scale, why does cgroup need weight at all? Just store nice directly. Why would that not work?
> > >
> > > In the end the TG and SE has the latency offset in the struct, that is all you care about. All the conversion back and forth is unnecessary, as it is a linear scale and just increases LOC and takes more memory to store linear arrays.
> > >
> > > Again I could be missing something and I will try to play with your series and see if I can show you what I mean (or convince myself it’s needed).
> >
> > I get what you mean but I think that having an array gives latitude to
> > adjust this internal offset mapping at a minimum cost of a const array
>
> Ok that makes sense. If you feel like there might be updates in the
> future to this mapping array (like changing the constants as you
> mentioned), then I am Ok with us keeping it.
>
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>
> I am excited about your series, the CFS latency issues have been
> thorny. This feels like a step forward in the right direction. Cheers,

Thanks
Vincent

>
>  - Joel

Re: [PATCH v9 0/9] Add latency priority for CFS class

[K Prateek Nayak]

Hello Vincent,

Thank you for taking a look at the report.

On 11/28/2022 10:49 PM, Vincent Guittot wrote:
> Hi Prateek,
> 
> On Mon, 28 Nov 2022 at 12:52, K Prateek Nayak <kprateek.nayak@amd.com> wrote:
>>
>> Hello Vincent,
>>
>> Following are the test results on dual socket Zen3 machine (2 x 64C/128T)
>>
>> tl;dr
>>
>> o All benchmarks with DEFAULT_LATENCY_NICE value are comparable to tip.
>>   There is, however, a noticeable dip for unixbench-spawn test case.
>>
>> o With the 2 rbtree approach, I do not see much difference in the
>>   hackbench results with varying latency nice value. Tests on v5 did
>>   yield noticeable improvements for hackbench.
>>   (https://lore.kernel.org/lkml/cd48ebbb-9724-985f-28e3-e558dea07827@amd.com/)
> 
> The 2 rbtree approach is the one that was already used in v5. I just
> rerun hackbench tests with latest tip and v6.2-rc7 and I can see large
> performance improvement for pipe tests on my system (8 cores system).
> Could you try witha larger number of group ? like 64, 128 and 256
> groups

Ah! My bad. I've rerun hackbench with larger number of groups and I see a
clear win for pipes with latency nice 19. Hackbench with sockets too see a
small win.

o pipes

$ perf bench sched messaging -p -l 50000 -g <groups>

latency_nice:           0                       19                      -20
32-groups:         9.43 (0.00 pct)         6.42 (31.91 pct)        9.75 (-3.39 pct)
64-groups:        21.55 (0.00 pct)        12.97 (39.81 pct)       21.48 (0.32 pct)
128-groups:       41.15 (0.00 pct)        24.18 (41.23 pct)       46.69 (-13.46 pct)
256-groups:       78.87 (0.00 pct)        43.65 (44.65 pct)       78.84 (0.03 pct)
512-groups:      125.48 (0.00 pct)        78.91 (37.11 pct)      136.21 (-8.55 pct)
1024-groups:     292.81 (0.00 pct)       151.36 (48.30 pct)      323.57 (-10.50 pct)

o sockets

$ perf bench sched messaging  -l 100000 -g <groups>

latency_nice:           0                       19                      -20
32-groups:        27.23 (0.00 pct)        27.00 (0.84 pct)        26.92 (1.13 pct)
64-groups:        45.71 (0.00 pct)        44.58 (2.47 pct)        45.86 (-0.32 pct)
128-groups:       79.55 (0.00 pct)        78.22 (1.67 pct)        80.01 (-0.57 pct)
256-groups:      161.41 (0.00 pct)       164.04 (-1.62 pct)      169.57 (-5.05 pct)
512-groups:      326.41 (0.00 pct)       310.00 (5.02 pct)       342.17 (-4.82 pct)
1024-groups:     634.36 (0.00 pct)       633.59 (0.12 pct)       640.05 (-0.89 pct)

Note: All tests were done in NPS1 mode.

> 
>>
>> [..snip..]
>>
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> ~ Unixbench - DEFAULT_LATENCY_NICE ~
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> o NPS1
>>
>> Test                    Metric    Parallelism                   tip                   latency_nice
>> unixbench-dhry2reg      Hmean     unixbench-dhry2reg-1      48929419.48 (   0.00%)    49137039.06 (   0.42%)
>> unixbench-dhry2reg      Hmean     unixbench-dhry2reg-512  6275526953.25 (   0.00%)  6265580479.15 (  -0.16%)
>> unixbench-syscall       Amean     unixbench-syscall-1        2994319.73 (   0.00%)     3008596.83 *  -0.48%*
>> unixbench-syscall       Amean     unixbench-syscall-512      7349715.87 (   0.00%)     7420994.50 *  -0.97%*
>> unixbench-pipe          Hmean     unixbench-pipe-1           2830206.03 (   0.00%)     2854405.99 *   0.86%*
>> unixbench-pipe          Hmean     unixbench-pipe-512       326207828.01 (   0.00%)   328997804.52 *   0.86%*
>> unixbench-spawn         Hmean     unixbench-spawn-1             6394.21 (   0.00%)        6367.75 (  -0.41%)
>> unixbench-spawn         Hmean     unixbench-spawn-512          72700.64 (   0.00%)       71454.19 *  -1.71%*
>> unixbench-execl         Hmean     unixbench-execl-1             4723.61 (   0.00%)        4750.59 (   0.57%)
>> unixbench-execl         Hmean     unixbench-execl-512          11212.05 (   0.00%)       11262.13 (   0.45%)
>>
>> o NPS2
>>
>> Test                    Metric    Parallelism                   tip                   latency_nice
>> unixbench-dhry2reg      Hmean     unixbench-dhry2reg-1      49271512.85 (   0.00%)    49245260.43 (  -0.05%)
>> unixbench-dhry2reg      Hmean     unixbench-dhry2reg-512  6267992483.03 (   0.00%)  6264951100.67 (  -0.05%)
>> unixbench-syscall       Amean     unixbench-syscall-1        2995885.93 (   0.00%)     3005975.10 *  -0.34%*
>> unixbench-syscall       Amean     unixbench-syscall-512      7388865.77 (   0.00%)     7276275.63 *   1.52%*
>> unixbench-pipe          Hmean     unixbench-pipe-1           2828971.95 (   0.00%)     2856578.72 *   0.98%*
>> unixbench-pipe          Hmean     unixbench-pipe-512       326225385.37 (   0.00%)   328941270.81 *   0.83%*
>> unixbench-spawn         Hmean     unixbench-spawn-1             6958.71 (   0.00%)        6954.21 (  -0.06%)
>> unixbench-spawn         Hmean     unixbench-spawn-512          85443.56 (   0.00%)       70536.42 * -17.45%* (0.67% vs 0.93% - CoEff var)
> 
> I don't expect any perf improvement or regression when the latency
> nice is not changed

This regression can be ignored. Although the results from back to
back runs are very stable, I see the results vary when I rebuild
the unixbench binaries on my test setup.

			  tip	      latency_nice
unixbench-spawn-512	73489.0		78260.4		(kexec)
unixbench-spawn-512 	73332.7		77821.2		(reboot)
unixbench-spawn-512	86207.4		82281.2		(rebuilt + reboot)

I'll go back and look more into the spawn test because there is
something else at play there but other Unixbench results seem to
be stable looking at the rerun.

> 
>> unixbench-execl         Hmean     unixbench-execl-1             4767.99 (   0.00%)        4752.63 *  -0.32%*
>> unixbench-execl         Hmean     unixbench-execl-512          11250.72 (   0.00%)       11320.97 (   0.62%)
>>
>> o NPS4
>>
>> Test                    Metric    Parallelism                   tip                   latency_nice
>> unixbench-dhry2reg      Hmean     unixbench-dhry2reg-1      49041932.68 (   0.00%)    49156671.05 (   0.23%)
>> unixbench-dhry2reg      Hmean     unixbench-dhry2reg-512  6286981589.85 (   0.00%)  6285248711.40 (  -0.03%)
>> unixbench-syscall       Amean     unixbench-syscall-1        2992405.60 (   0.00%)     3008933.03 *  -0.55%*
>> unixbench-syscall       Amean     unixbench-syscall-512      7971789.70 (   0.00%)     7814622.23 *   1.97%*
>> unixbench-pipe          Hmean     unixbench-pipe-1           2822892.54 (   0.00%)     2852615.11 *   1.05%*
>> unixbench-pipe          Hmean     unixbench-pipe-512       326408309.83 (   0.00%)   329617202.56 *   0.98%*
>> unixbench-spawn         Hmean     unixbench-spawn-1             7685.31 (   0.00%)        7243.54 (  -5.75%)
>> unixbench-spawn         Hmean     unixbench-spawn-512          72245.56 (   0.00%)       77000.81 *   6.58%*
>> unixbench-execl         Hmean     unixbench-execl-1             4761.42 (   0.00%)        4733.12 *  -0.59%*
>> unixbench-execl         Hmean     unixbench-execl-512          11533.53 (   0.00%)       11660.17 (   1.10%)
>>
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> ~ Hackbench - Various Latency Nice Values ~
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> o 100000 loops
>>
>> - pipe (process)
>>
>> Test:                   LN: 0                   LN: 19                  LN: -20
>>  1-groups:         3.91 (0.00 pct)         3.91 (0.00 pct)         3.81 (2.55 pct)
>>  2-groups:         4.48 (0.00 pct)         4.52 (-0.89 pct)        4.53 (-1.11 pct)
>>  4-groups:         4.83 (0.00 pct)         4.83 (0.00 pct)         4.87 (-0.82 pct)
>>  8-groups:         5.09 (0.00 pct)         5.00 (1.76 pct)         5.07 (0.39 pct)
>> 16-groups:         6.92 (0.00 pct)         6.79 (1.87 pct)         6.96 (-0.57 pct)
>>
>> - pipe (thread)
>>
>>  1-groups:         4.13 (0.00 pct)         4.08 (1.21 pct)         4.11 (0.48 pct)
>>  2-groups:         4.78 (0.00 pct)         4.90 (-2.51 pct)        4.79 (-0.20 pct)
>>  4-groups:         5.12 (0.00 pct)         5.08 (0.78 pct)         5.16 (-0.78 pct)
>>  8-groups:         5.31 (0.00 pct)         5.28 (0.56 pct)         5.33 (-0.37 pct)
>> 16-groups:         7.34 (0.00 pct)         7.27 (0.95 pct)         7.33 (0.13 pct)
>>
>> - socket (process)
>>
>> Test:                   LN: 0                   LN: 19                  LN: -20
>>  1-groups:         6.61 (0.00 pct)         6.38 (3.47 pct)         6.54 (1.05 pct)
>>  2-groups:         6.59 (0.00 pct)         6.67 (-1.21 pct)        6.11 (7.28 pct)
>>  4-groups:         6.77 (0.00 pct)         6.78 (-0.14 pct)        6.79 (-0.29 pct)
>>  8-groups:         8.29 (0.00 pct)         8.39 (-1.20 pct)        8.36 (-0.84 pct)
>> 16-groups:        12.21 (0.00 pct)        12.03 (1.47 pct)        12.35 (-1.14 pct)
>>
>> - socket (thread)
>>
>> Test:                   LN: 0                   LN: 19                  LN: -20
>>  1-groups:         6.50 (0.00 pct)         5.99 (7.84 pct)         6.02 (7.38 pct)      ^
>>  2-groups:         6.07 (0.00 pct)         6.20 (-2.14 pct)        6.23 (-2.63 pct)
>>  4-groups:         6.61 (0.00 pct)         6.64 (-0.45 pct)        6.63 (-0.30 pct)
>>  8-groups:         8.87 (0.00 pct)         8.67 (2.25 pct)         8.78 (1.01 pct)
>> 16-groups:        12.63 (0.00 pct)        12.54 (0.71 pct)        12.59 (0.31 pct)
>>
>>> [..snip..]
>>>
>>
>> Apart from couple of anomalies, latency nice reduces wait time, especially
>> when the system is heavily loaded. If there is any data, or any specific
>> workload you would like me to run on the test system, please do let me know.
>> Meanwhile, I'll try to get some numbers for larger workloads like SpecJBB
>> that did see improvements with latency nice on v5.

Following are results for SpecJBB in NPS1 mode:

+----------------------------------------------+
|                |   Latency Nice    |         |
|     Metric     |-------------------|   tip   |
|                |    0    |    19   |         |
|----------------|-------------------|---------|
|    Max jOPS    | 100.00% | 102.19% | 101.02% |
| Criritcal jOPS | 100.00% | 122.41% | 100.41% |
+----------------------------------------------+

SpecJBB throughput for Max-jOPS is similar across the board
but Critical-jOPS throughput sees a good uplift again with
latency nice 19.

> 
> [..snip..]
>

If there is any specific workload you would like me to test,
please do let me know. I'll try to test more workloads I come
across with different latency nice values and update you
with the results on this thread.

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
--
Thanks and Regards,
Prateek