[PATCH v7 0/3] sched/uclamp: new sysctl for default RT boost value

[PATCH v7 0/3] sched/uclamp: new sysctl for default RT boost value

[Qais Yousef]

Changes in v7:

	* Rebase on top of tip/sched/core
	* Hold task_rq_lock() instead of using RCU.
	* Better document that changes to p->uclamp_ require task_rq_lock()
	* Remove smp_{wr}mp()
	* Hold the the tasklist_lock with smp_mp__after_spinlock()
	* Add patch 3 which addresses a splat I've seen while testing.
	  static_branch_enable() in __setscheduler_uclamp() was causing it.
	  Remove the call outside of the critical section to fix it.


*** v6 cover-letter ***

This series introduces a new sysctl_sched_uclamp_util_min_rt_default to control
at runtime the default boost value of RT tasks.

Full rationale is in patch 1 commit message.

v6 has changed the approach taken in v5 [1] and earlier by moving away from the
lazy update approach that touched the fast path to a synchronous one that is
performed when the write to the procfs entry is done.

for_each_process_thread() is used to update all existing RT tasks now. And to
handle the race with a concurrent fork() we introduce sched_post_fork() in
_do_fork() to ensure a concurrently forked RT tasks gets the right update.

To ensure the race condition is handled correctly, I wrote this small (simple!)
test program:

	https://github.com/qais-yousef/uclamp_test.git

And ran it on 4core x86 system and 8core big.LITTLE juno-r2 system.

From juno-r2 run, 10 iterations each run:

Without sched_post_fork()

	# ./run.sh
	pid 3105 has 336 but default should be 337
	pid 13162 has 336 but default should be 337
	pid 23256 has 338 but default should be 339
	All forked RT tasks had the correct uclamp.min
	pid 10638 has 334 but default should be 335
	All forked RT tasks had the correct uclamp.min
	pid 30683 has 335 but default should be 336
	pid 8247 has 336 but default should be 337
	pid 18170 has 1024 but default should be 334
	pid 28274 has 336 but default should be 337

With sched_post_fork()

	# ./run.sh
	All forked RT tasks had the correct uclamp.min
	All forked RT tasks had the correct uclamp.min
	All forked RT tasks had the correct uclamp.min
	All forked RT tasks had the correct uclamp.min
	All forked RT tasks had the correct uclamp.min
	All forked RT tasks had the correct uclamp.min
	All forked RT tasks had the correct uclamp.min
	All forked RT tasks had the correct uclamp.min
	All forked RT tasks had the correct uclamp.min
	All forked RT tasks had the correct uclamp.min

Thanks

--
Qais Yousef

[1] https://lore.kernel.org/lkml/20200511154053.7822-1-qais.yousef@arm.com/

CC: Jonathan Corbet <corbet@lwn.net>
CC: Juri Lelli <juri.lelli@redhat.com>
CC: Vincent Guittot <vincent.guittot@linaro.org>
CC: Dietmar Eggemann <dietmar.eggemann@arm.com>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ben Segall <bsegall@google.com>
CC: Mel Gorman <mgorman@suse.de>
CC: Luis Chamberlain <mcgrof@kernel.org>
CC: Kees Cook <keescook@chromium.org>
CC: Iurii Zaikin <yzaikin@google.com>
CC: Quentin Perret <qperret@google.com>
CC: Valentin Schneider <valentin.schneider@arm.com>
CC: Patrick Bellasi <patrick.bellasi@matbug.net>
CC: Pavan Kondeti <pkondeti@codeaurora.org>
CC: linux-doc@vger.kernel.org
CC: linux-kernel@vger.kernel.org
CC: linux-fsdevel@vger.kernel.org


Qais Yousef (3):
  sched/uclamp: Add a new sysctl to control RT default boost value
  Documentation/sysctl: Document uclamp sysctl knobs
  sched/uclamp: Fix a deadlock when enabling uclamp static key

 Documentation/admin-guide/sysctl/kernel.rst |  54 +++++++
 include/linux/sched.h                       |  10 +-
 include/linux/sched/sysctl.h                |   1 +
 include/linux/sched/task.h                  |   1 +
 kernel/fork.c                               |   1 +
 kernel/sched/core.c                         | 149 ++++++++++++++++++--
 kernel/sysctl.c                             |   7 +
 7 files changed, 208 insertions(+), 15 deletions(-)

-- 
2.17.1

[PATCH v7 1/3] sched/uclamp: Add a new sysctl to control RT default boost value

[Qais Yousef]

RT tasks by default run at the highest capacity/performance level. When
uclamp is selected this default behavior is retained by enforcing the
requested uclamp.min (p->uclamp_req[UCLAMP_MIN]) of the RT tasks to be
uclamp_none(UCLAMP_MAX), which is SCHED_CAPACITY_SCALE; the maximum
value.

This is also referred to as 'the default boost value of RT tasks'.

See commit 1a00d999971c ("sched/uclamp: Set default clamps for RT tasks").

On battery powered devices, it is desired to control this default
(currently hardcoded) behavior at runtime to reduce energy consumed by
RT tasks.

For example, a mobile device manufacturer where big.LITTLE architecture
is dominant, the performance of the little cores varies across SoCs, and
on high end ones the big cores could be too power hungry.

Given the diversity of SoCs, the new knob allows manufactures to tune
the best performance/power for RT tasks for the particular hardware they
run on.

They could opt to further tune the value when the user selects
a different power saving mode or when the device is actively charging.

The runtime aspect of it further helps in creating a single kernel image
that can be run on multiple devices that require different tuning.

Keep in mind that a lot of RT tasks in the system are created by the
kernel. On Android for instance I can see over 50 RT tasks, only
a handful of which created by the Android framework.

To control the default behavior globally by system admins and device
integrator, introduce the new sysctl_sched_uclamp_util_min_rt_default
to change the default boost value of the RT tasks.

I anticipate this to be mostly in the form of modifying the init script
of a particular device.

To avoid polluting the fast path with unnecessary code, the approach
taken is to synchronously do the update by traversing all the existing
tasks in the system. This could race with a concurrent fork(), which is
dealt with by introducing sched_post_fork() function which will ensure
the racy fork will get the right update applied.

Tested on Juno-r2 in combination with the RT capacity awareness [1].
By default an RT task will go to the highest capacity CPU and run at the
maximum frequency, which is particularly energy inefficient on high end
mobile devices because the biggest core[s] are 'huge' and power hungry.

With this patch the RT task can be controlled to run anywhere by
default, and doesn't cause the frequency to be maximum all the time.
Yet any task that really needs to be boosted can easily escape this
default behavior by modifying its requested uclamp.min value
(p->uclamp_req[UCLAMP_MIN]) via sched_setattr() syscall.

[1] 804d402fb6f6: ("sched/rt: Make RT capacity-aware")

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
CC: Jonathan Corbet <corbet@lwn.net>
CC: Juri Lelli <juri.lelli@redhat.com>
CC: Vincent Guittot <vincent.guittot@linaro.org>
CC: Dietmar Eggemann <dietmar.eggemann@arm.com>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ben Segall <bsegall@google.com>
CC: Mel Gorman <mgorman@suse.de>
CC: Luis Chamberlain <mcgrof@kernel.org>
CC: Kees Cook <keescook@chromium.org>
CC: Iurii Zaikin <yzaikin@google.com>
CC: Quentin Perret <qperret@google.com>
CC: Valentin Schneider <valentin.schneider@arm.com>
CC: Patrick Bellasi <patrick.bellasi@matbug.net>
CC: Pavan Kondeti <pkondeti@codeaurora.org>
CC: linux-doc@vger.kernel.org
CC: linux-kernel@vger.kernel.org
CC: linux-fsdevel@vger.kernel.org
---
 include/linux/sched.h        |  10 ++-
 include/linux/sched/sysctl.h |   1 +
 include/linux/sched/task.h   |   1 +
 kernel/fork.c                |   1 +
 kernel/sched/core.c          | 119 +++++++++++++++++++++++++++++++++--
 kernel/sysctl.c              |   7 +++
 6 files changed, 131 insertions(+), 8 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 683372943093..86d90bc40e0b 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -686,9 +686,15 @@ struct task_struct {
 	struct sched_dl_entity		dl;
 
 #ifdef CONFIG_UCLAMP_TASK
-	/* Clamp values requested for a scheduling entity */
+	/*
+	 * Clamp values requested for a scheduling entity.
+	 * Must be updated with task_rq_lock() held.
+	 */
 	struct uclamp_se		uclamp_req[UCLAMP_CNT];
-	/* Effective clamp values used for a scheduling entity */
+	/*
+	 * Effective clamp values used for a scheduling entity.
+	 * Must be updated with task_rq_lock() held.
+	 */
 	struct uclamp_se		uclamp[UCLAMP_CNT];
 #endif
 
diff --git a/include/linux/sched/sysctl.h b/include/linux/sched/sysctl.h
index 24be30a40814..3c31ba88aca5 100644
--- a/include/linux/sched/sysctl.h
+++ b/include/linux/sched/sysctl.h
@@ -67,6 +67,7 @@ extern unsigned int sysctl_sched_dl_period_min;
 #ifdef CONFIG_UCLAMP_TASK
 extern unsigned int sysctl_sched_uclamp_util_min;
 extern unsigned int sysctl_sched_uclamp_util_max;
+extern unsigned int sysctl_sched_uclamp_util_min_rt_default;
 #endif
 
 #ifdef CONFIG_CFS_BANDWIDTH
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 38359071236a..e7ddab095baf 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -55,6 +55,7 @@ extern asmlinkage void schedule_tail(struct task_struct *prev);
 extern void init_idle(struct task_struct *idle, int cpu);
 
 extern int sched_fork(unsigned long clone_flags, struct task_struct *p);
+extern void sched_post_fork(struct task_struct *p);
 extern void sched_dead(struct task_struct *p);
 
 void __noreturn do_task_dead(void);
diff --git a/kernel/fork.c b/kernel/fork.c
index efc5493203ae..e75c2e41f3d1 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2304,6 +2304,7 @@ static __latent_entropy struct task_struct *copy_process(
 	write_unlock_irq(&tasklist_lock);
 
 	proc_fork_connector(p);
+	sched_post_fork(p);
 	cgroup_post_fork(p, args);
 	perf_event_fork(p);
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 31c07e6c00b1..e1578c3ad40c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -793,6 +793,23 @@ unsigned int sysctl_sched_uclamp_util_min = SCHED_CAPACITY_SCALE;
 /* Max allowed maximum utilization */
 unsigned int sysctl_sched_uclamp_util_max = SCHED_CAPACITY_SCALE;
 
+/*
+ * By default RT tasks run at the maximum performance point/capacity of the
+ * system. Uclamp enforces this by always setting UCLAMP_MIN of RT tasks to
+ * SCHED_CAPACITY_SCALE.
+ *
+ * This knob allows admins to change the default behavior when uclamp is being
+ * used. In battery powered devices, particularly, running at the maximum
+ * capacity and frequency will increase energy consumption and shorten the
+ * battery life.
+ *
+ * This knob only affects RT tasks that their uclamp_se->user_defined == false.
+ *
+ * This knob will not override the system default sched_util_clamp_min defined
+ * above.
+ */
+unsigned int sysctl_sched_uclamp_util_min_rt_default = SCHED_CAPACITY_SCALE;
+
 /* All clamps are required to be less or equal than these values */
 static struct uclamp_se uclamp_default[UCLAMP_CNT];
 
@@ -895,6 +912,64 @@ unsigned int uclamp_rq_max_value(struct rq *rq, enum uclamp_id clamp_id,
 	return uclamp_idle_value(rq, clamp_id, clamp_value);
 }
 
+static void __uclamp_sync_util_min_rt_default_locked(struct task_struct *p)
+{
+	unsigned int default_util_min;
+	struct uclamp_se *uc_se;
+
+	lockdep_assert_held(&p->pi_lock);
+
+	uc_se = &p->uclamp_req[UCLAMP_MIN];
+
+	/* Only sync if user didn't override the default */
+	if (uc_se->user_defined)
+		return;
+
+	default_util_min = sysctl_sched_uclamp_util_min_rt_default;
+	uclamp_se_set(uc_se, default_util_min, false);
+}
+
+static void __uclamp_sync_util_min_rt_default(struct task_struct *p)
+{
+	struct rq_flags rf;
+	struct rq *rq;
+
+	if (!rt_task(p))
+		return;
+
+	/* Protect updates to p->uclamp_* */
+	rq = task_rq_lock(p, &rf);
+	__uclamp_sync_util_min_rt_default_locked(p);
+	task_rq_unlock(rq, p, &rf);
+}
+
+static void uclamp_sync_util_min_rt_default(void)
+{
+	struct task_struct *g, *p;
+
+	/*
+	 * copy_process()			sysctl_uclamp
+	 *					  uclamp_min_rt = X;
+	 *   write_lock(&tasklist_lock)		  read_lock(&tasklist_lock)
+	 *   // link thread			  smp_mb__after_spinlock()
+	 *   write_unlock(&tasklist_lock)	  read_unlock(&tasklist_lock);
+	 *   sched_post_fork()			  for_each_process_thread()
+	 *     __uclamp_sync_rt()		    __uclamp_sync_rt()
+	 *
+	 * Ensures that either sched_post_fork() will observe the new
+	 * uclamp_min_rt or for_each_process_thread() will observe the new
+	 * task.
+	 */
+	read_lock(&tasklist_lock);
+	smp_mb__after_spinlock();
+	read_unlock(&tasklist_lock);
+
+	rcu_read_lock();
+	for_each_process_thread(g, p)
+		__uclamp_sync_util_min_rt_default(p);
+	rcu_read_unlock();
+}
+
 static inline struct uclamp_se
 uclamp_tg_restrict(struct task_struct *p, enum uclamp_id clamp_id)
 {
@@ -1193,12 +1268,13 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
 				void *buffer, size_t *lenp, loff_t *ppos)
 {
 	bool update_root_tg = false;
-	int old_min, old_max;
+	int old_min, old_max, old_min_rt;
 	int result;
 
 	mutex_lock(&uclamp_mutex);
 	old_min = sysctl_sched_uclamp_util_min;
 	old_max = sysctl_sched_uclamp_util_max;
+	old_min_rt = sysctl_sched_uclamp_util_min_rt_default;
 
 	result = proc_dointvec(table, write, buffer, lenp, ppos);
 	if (result)
@@ -1207,7 +1283,9 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
 		goto done;
 
 	if (sysctl_sched_uclamp_util_min > sysctl_sched_uclamp_util_max ||
-	    sysctl_sched_uclamp_util_max > SCHED_CAPACITY_SCALE) {
+	    sysctl_sched_uclamp_util_max > SCHED_CAPACITY_SCALE	||
+	    sysctl_sched_uclamp_util_min_rt_default > SCHED_CAPACITY_SCALE) {
+
 		result = -EINVAL;
 		goto undo;
 	}
@@ -1228,6 +1306,11 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
 		uclamp_update_root_tg();
 	}
 
+	if (old_min_rt != sysctl_sched_uclamp_util_min_rt_default) {
+		static_branch_enable(&sched_uclamp_used);
+		uclamp_sync_util_min_rt_default();
+	}
+
 	/*
 	 * We update all RUNNABLE tasks only when task groups are in use.
 	 * Otherwise, keep it simple and do just a lazy update at each next
@@ -1239,6 +1322,7 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
 undo:
 	sysctl_sched_uclamp_util_min = old_min;
 	sysctl_sched_uclamp_util_max = old_max;
+	sysctl_sched_uclamp_util_min_rt_default = old_min_rt;
 done:
 	mutex_unlock(&uclamp_mutex);
 
@@ -1275,17 +1359,20 @@ static void __setscheduler_uclamp(struct task_struct *p,
 	 */
 	for_each_clamp_id(clamp_id) {
 		struct uclamp_se *uc_se = &p->uclamp_req[clamp_id];
-		unsigned int clamp_value = uclamp_none(clamp_id);
 
 		/* Keep using defined clamps across class changes */
 		if (uc_se->user_defined)
 			continue;
 
-		/* By default, RT tasks always get 100% boost */
+		/*
+		 * RT by default have a 100% boost value that could be modified
+		 * at runtime.
+		 */
 		if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
-			clamp_value = uclamp_none(UCLAMP_MAX);
+			__uclamp_sync_util_min_rt_default_locked(p);
+		else
+			uclamp_se_set(uc_se, uclamp_none(clamp_id), false);
 
-		uclamp_se_set(uc_se, clamp_value, false);
 	}
 
 	if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)))
@@ -1308,6 +1395,10 @@ static void uclamp_fork(struct task_struct *p)
 {
 	enum uclamp_id clamp_id;
 
+	/*
+	 * We don't need to hold task_rq_lock() when updating p->uclamp_* here
+	 * as the task is still at its early fork stages.
+	 */
 	for_each_clamp_id(clamp_id)
 		p->uclamp[clamp_id].active = false;
 
@@ -1320,6 +1411,11 @@ static void uclamp_fork(struct task_struct *p)
 	}
 }
 
+static void uclamp_post_fork(struct task_struct *p)
+{
+	__uclamp_sync_util_min_rt_default(p);
+}
+
 static void __init init_uclamp_rq(struct rq *rq)
 {
 	enum uclamp_id clamp_id;
@@ -1372,6 +1468,7 @@ static inline int uclamp_validate(struct task_struct *p,
 static void __setscheduler_uclamp(struct task_struct *p,
 				  const struct sched_attr *attr) { }
 static inline void uclamp_fork(struct task_struct *p) { }
+static inline void uclamp_post_fork(struct task_struct *p) { }
 static inline void init_uclamp(void) { }
 #endif /* CONFIG_UCLAMP_TASK */
 
@@ -3074,6 +3171,11 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
 	return 0;
 }
 
+void sched_post_fork(struct task_struct *p)
+{
+	uclamp_post_fork(p);
+}
+
 unsigned long to_ratio(u64 period, u64 runtime)
 {
 	if (runtime == RUNTIME_INF)
@@ -5597,6 +5699,11 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
 		kattr.sched_nice = task_nice(p);
 
 #ifdef CONFIG_UCLAMP_TASK
+	/*
+	 * This could race with another potential updater, but this is fine
+	 * because it'll correctly read the old or the new value. We don't need
+	 * to guarantee who wins the race as long as it doesn't return garbage.
+	 */
 	kattr.sched_util_min = p->uclamp_req[UCLAMP_MIN].value;
 	kattr.sched_util_max = p->uclamp_req[UCLAMP_MAX].value;
 #endif
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 4aea67d3d552..1b4d2dc270a5 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1815,6 +1815,13 @@ static struct ctl_table kern_table[] = {
 		.mode		= 0644,
 		.proc_handler	= sysctl_sched_uclamp_handler,
 	},
+	{
+		.procname	= "sched_util_clamp_min_rt_default",
+		.data		= &sysctl_sched_uclamp_util_min_rt_default,
+		.maxlen		= sizeof(unsigned int),
+		.mode		= 0644,
+		.proc_handler	= sysctl_sched_uclamp_handler,
+	},
 #endif
 #ifdef CONFIG_SCHED_AUTOGROUP
 	{
-- 
2.17.1

Re: [PATCH v7 1/3] sched/uclamp: Add a new sysctl to control RT default boost value

[Peter Zijlstra]

On Thu, Jul 16, 2020 at 12:03:45PM +0100, Qais Yousef wrote:

Would you mind terribly if I rename things like so?

I tried and failed to come up with a shorter name in general, these
functions names are somewhat unwieldy. I considered s/_default//.

---
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -914,7 +914,7 @@ unsigned int uclamp_rq_max_value(struct
 	return uclamp_idle_value(rq, clamp_id, clamp_value);
 }
 
-static void __uclamp_sync_util_min_rt_default_locked(struct task_struct *p)
+static void __uclamp_update_util_min_rt_default(struct task_struct *p)
 {
 	unsigned int default_util_min;
 	struct uclamp_se *uc_se;
@@ -931,7 +931,7 @@ static void __uclamp_sync_util_min_rt_de
 	uclamp_se_set(uc_se, default_util_min, false);
 }
 
-static void __uclamp_sync_util_min_rt_default(struct task_struct *p)
+static void uclamp_update_util_min_rt_default(struct task_struct *p)
 {
 	struct rq_flags rf;
 	struct rq *rq;
@@ -941,7 +941,7 @@ static void __uclamp_sync_util_min_rt_de
 
 	/* Protect updates to p->uclamp_* */
 	rq = task_rq_lock(p, &rf);
-	__uclamp_sync_util_min_rt_default_locked(p);
+	__uclamp_update_util_min_rt_default(p);
 	task_rq_unlock(rq, p, &rf);
 }
 
@@ -968,7 +968,7 @@ static void uclamp_sync_util_min_rt_defa
 
 	rcu_read_lock();
 	for_each_process_thread(g, p)
-		__uclamp_sync_util_min_rt_default(p);
+		uclamp_update_util_min_rt_default(p);
 	rcu_read_unlock();
 }
 
@@ -1360,7 +1360,7 @@ static void __setscheduler_uclamp(struct
 		 * at runtime.
 		 */
 		if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
-			__uclamp_sync_util_min_rt_default_locked(p);
+			__uclamp_update_util_min_rt_default(p);
 		else
 			uclamp_se_set(uc_se, uclamp_none(clamp_id), false);
 
@@ -1404,7 +1404,7 @@ static void uclamp_fork(struct task_stru
 
 static void uclamp_post_fork(struct task_struct *p)
 {
-	__uclamp_sync_util_min_rt_default(p);
+	uclamp_update_util_min_rt_default(p);
 }
 
 static void __init init_uclamp_rq(struct rq *rq)

Re: [PATCH v7 1/3] sched/uclamp: Add a new sysctl to control RT default boost value

[Qais Yousef]

On 07/24/20 10:54, Peter Zijlstra wrote:
> On Thu, Jul 16, 2020 at 12:03:45PM +0100, Qais Yousef wrote:
> 
> Would you mind terribly if I rename things like so?

Nope, the new name is good for me too.

> 
> I tried and failed to come up with a shorter name in general, these
> functions names are somewhat unwieldy. I considered s/_default//.

Can do. Me thinking that maybe we need to sprinkle more comments then. But if
I felt the need for more comments, I can always post another patch on top :-)

If you'd like a shorter name, a slightly shorter one would be

	update_uclamp_min_rt()

Thanks

--
Qais Yousef

> 
> ---
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -914,7 +914,7 @@ unsigned int uclamp_rq_max_value(struct
>  	return uclamp_idle_value(rq, clamp_id, clamp_value);
>  }
>  
> -static void __uclamp_sync_util_min_rt_default_locked(struct task_struct *p)
> +static void __uclamp_update_util_min_rt_default(struct task_struct *p)
>  {
>  	unsigned int default_util_min;
>  	struct uclamp_se *uc_se;
> @@ -931,7 +931,7 @@ static void __uclamp_sync_util_min_rt_de
>  	uclamp_se_set(uc_se, default_util_min, false);
>  }
>  
> -static void __uclamp_sync_util_min_rt_default(struct task_struct *p)
> +static void uclamp_update_util_min_rt_default(struct task_struct *p)
>  {
>  	struct rq_flags rf;
>  	struct rq *rq;
> @@ -941,7 +941,7 @@ static void __uclamp_sync_util_min_rt_de
>  
>  	/* Protect updates to p->uclamp_* */
>  	rq = task_rq_lock(p, &rf);
> -	__uclamp_sync_util_min_rt_default_locked(p);
> +	__uclamp_update_util_min_rt_default(p);
>  	task_rq_unlock(rq, p, &rf);
>  }
>  
> @@ -968,7 +968,7 @@ static void uclamp_sync_util_min_rt_defa
>  
>  	rcu_read_lock();
>  	for_each_process_thread(g, p)
> -		__uclamp_sync_util_min_rt_default(p);
> +		uclamp_update_util_min_rt_default(p);
>  	rcu_read_unlock();
>  }
>  
> @@ -1360,7 +1360,7 @@ static void __setscheduler_uclamp(struct
>  		 * at runtime.
>  		 */
>  		if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> -			__uclamp_sync_util_min_rt_default_locked(p);
> +			__uclamp_update_util_min_rt_default(p);
>  		else
>  			uclamp_se_set(uc_se, uclamp_none(clamp_id), false);
>  
> @@ -1404,7 +1404,7 @@ static void uclamp_fork(struct task_stru
>  
>  static void uclamp_post_fork(struct task_struct *p)
>  {
> -	__uclamp_sync_util_min_rt_default(p);
> +	uclamp_update_util_min_rt_default(p);
>  }
>  
>  static void __init init_uclamp_rq(struct rq *rq)

[PATCH v7 2/3] Documentation/sysctl: Document uclamp sysctl knobs

[Qais Yousef]

Uclamp exposes 3 sysctl knobs:

	* sched_util_clamp_min
	* sched_util_clamp_max
	* sched_util_clamp_min_rt_default

Document them in sysctl/kernel.rst.

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
CC: Jonathan Corbet <corbet@lwn.net>
CC: Juri Lelli <juri.lelli@redhat.com>
CC: Vincent Guittot <vincent.guittot@linaro.org>
CC: Dietmar Eggemann <dietmar.eggemann@arm.com>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ben Segall <bsegall@google.com>
CC: Mel Gorman <mgorman@suse.de>
CC: Luis Chamberlain <mcgrof@kernel.org>
CC: Kees Cook <keescook@chromium.org>
CC: Iurii Zaikin <yzaikin@google.com>
CC: Quentin Perret <qperret@google.com>
CC: Valentin Schneider <valentin.schneider@arm.com>
CC: Patrick Bellasi <patrick.bellasi@matbug.net>
CC: Pavan Kondeti <pkondeti@codeaurora.org>
CC: linux-doc@vger.kernel.org
CC: linux-kernel@vger.kernel.org
CC: linux-fsdevel@vger.kernel.org
---
 Documentation/admin-guide/sysctl/kernel.rst | 54 +++++++++++++++++++++
 1 file changed, 54 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 83acf5025488..55bf6b4de4ec 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1062,6 +1062,60 @@ Enables/disables scheduler statistics. Enabling this feature
 incurs a small amount of overhead in the scheduler but is
 useful for debugging and performance tuning.
 
+sched_util_clamp_min:
+=====================
+
+Max allowed *minimum* utilization.
+
+Default value is 1024, which is the maximum possible value.
+
+It means that any requested uclamp.min value cannot be greater than
+sched_util_clamp_min, i.e., it is restricted to the range
+[0:sched_util_clamp_min].
+
+sched_util_clamp_max:
+=====================
+
+Max allowed *maximum* utilization.
+
+Default value is 1024, which is the maximum possible value.
+
+It means that any requested uclamp.max value cannot be greater than
+sched_util_clamp_max, i.e., it is restricted to the range
+[0:sched_util_clamp_max].
+
+sched_util_clamp_min_rt_default:
+================================
+
+By default Linux is tuned for performance. Which means that RT tasks always run
+at the highest frequency and most capable (highest capacity) CPU (in
+heterogeneous systems).
+
+Uclamp achieves this by setting the requested uclamp.min of all RT tasks to
+1024 by default, which effectively boosts the tasks to run at the highest
+frequency and biases them to run on the biggest CPU.
+
+This knob allows admins to change the default behavior when uclamp is being
+used. In battery powered devices particularly, running at the maximum
+capacity and frequency will increase energy consumption and shorten the battery
+life.
+
+This knob is only effective for RT tasks which the user hasn't modified their
+requested uclamp.min value via sched_setattr() syscall.
+
+This knob will not escape the range constraint imposed by sched_util_clamp_min
+defined above.
+
+For example if
+
+	sched_util_clamp_min_rt_default = 800
+	sched_util_clamp_min = 600
+
+Then the boost will be clamped to 600 because 800 is outside of the permissible
+range of [0:600]. This could happen for instance if a powersave mode will
+restrict all boosts temporarily by modifying sched_util_clamp_min. As soon as
+this restriction is lifted, the requested sched_util_clamp_min_rt_default
+will take effect.
 
 seccomp
 =======
-- 
2.17.1