[PATCH 0/2] Backport uclamp static key to 5.4.y stable

[PATCH 0/2] Backport uclamp static key to 5.4.y stable

[Qais Yousef]

Hi Greg/Sasha

The following 2 patches are backports of

	46609ce22703: ("sched/uclamp: Protect uclamp fast path code with static key")
	e65855a52b47: ("sched/uclamp: Fix a deadlock when enabling uclamp static key")

into 5.4.y stable tree. The conflict was trivial and due to:

	1. uclamp_rq_util_with() was renamed from util_util_with()
	2. 2 local variables were converted to unsigned long from unsigned int.

I did do compile test on aarch64 and x86_64 and both looked fine. Beside I ran
a quick and short mmtest to verify the functionality and got the following
results which is inline with what's expected.

                        5.4.y                  5.4.y-static-keys
Hmean     send-64       188.46 (   0.00%)      189.95 *   0.79%*
Hmean     send-128      375.65 (   0.00%)      379.75 *   1.09%*

7.32%     -0.33%  [kernel.kallsyms]        [k] try_to_wake_up
0.58%     -0.55%  [kernel.kallsyms]        [k] deactivate_task
0.50%     -0.45%  [kernel.kallsyms]        [k] activate_task


That said, it's Friday afternoon and I am off next week. If I did something
stupid and didn't find me, please accept my apologies in advance and will fix
it as soon as I am back online.

Thanks

--
Qais Yousef


Qais Yousef (2):
  sched/uclamp: Protect uclamp fast path code with static key
  sched/uclamp: Fix a deadlock when enabling uclamp static key

 kernel/sched/core.c              | 81 +++++++++++++++++++++++++++++++-
 kernel/sched/cpufreq_schedutil.c |  2 +-
 kernel/sched/sched.h             | 47 +++++++++++++++++-
 3 files changed, 126 insertions(+), 4 deletions(-)

-- 
2.17.1

[PATCH 1/2] sched/uclamp: Protect uclamp fast path code with static key

[Qais Yousef]

There is a report that when uclamp is enabled, a netperf UDP test
regresses compared to a kernel compiled without uclamp.

https://lore.kernel.org/lkml/20200529100806.GA3070@suse.de/

While investigating the root cause, there were no sign that the uclamp
code is doing anything particularly expensive but could suffer from bad
cache behavior under certain circumstances that are yet to be
understood.

https://lore.kernel.org/lkml/20200616110824.dgkkbyapn3io6wik@e107158-lin/

To reduce the pressure on the fast path anyway, add a static key that is
by default will skip executing uclamp logic in the
enqueue/dequeue_task() fast path until it's needed.

As soon as the user start using util clamp by:

	1. Changing uclamp value of a task with sched_setattr()
	2. Modifying the default sysctl_sched_util_clamp_{min, max}
	3. Modifying the default cpu.uclamp.{min, max} value in cgroup

We flip the static key now that the user has opted to use util clamp.
Effectively re-introducing uclamp logic in the enqueue/dequeue_task()
fast path. It stays on from that point forward until the next reboot.

This should help minimize the effect of util clamp on workloads that
don't need it but still allow distros to ship their kernels with uclamp
compiled in by default.

SCHED_WARN_ON() in uclamp_rq_dec_id() was removed since now we can end
up with unbalanced call to uclamp_rq_dec_id() if we flip the key while
a task is running in the rq. Since we know it is harmless we just
quietly return if we attempt a uclamp_rq_dec_id() when
rq->uclamp[].bucket[].tasks is 0.

In schedutil, we introduce a new uclamp_is_enabled() helper which takes
the static key into account to ensure RT boosting behavior is retained.

The following results demonstrates how this helps on 2 Sockets Xeon E5
2x10-Cores system.

                                   nouclamp                 uclamp      uclamp-static-key
Hmean     send-64         162.43 (   0.00%)      157.84 *  -2.82%*      163.39 *   0.59%*
Hmean     send-128        324.71 (   0.00%)      314.78 *  -3.06%*      326.18 *   0.45%*
Hmean     send-256        641.55 (   0.00%)      628.67 *  -2.01%*      648.12 *   1.02%*
Hmean     send-1024      2525.28 (   0.00%)     2448.26 *  -3.05%*     2543.73 *   0.73%*
Hmean     send-2048      4836.14 (   0.00%)     4712.08 *  -2.57%*     4867.69 *   0.65%*
Hmean     send-3312      7540.83 (   0.00%)     7425.45 *  -1.53%*     7621.06 *   1.06%*
Hmean     send-4096      9124.53 (   0.00%)     8948.82 *  -1.93%*     9276.25 *   1.66%*
Hmean     send-8192     15589.67 (   0.00%)    15486.35 *  -0.66%*    15819.98 *   1.48%*
Hmean     send-16384    26386.47 (   0.00%)    25752.25 *  -2.40%*    26773.74 *   1.47%*

The perf diff between nouclamp and uclamp-static-key when uclamp is
disabled in the fast path:

     8.73%     -1.55%  [kernel.kallsyms]        [k] try_to_wake_up
     0.07%     +0.04%  [kernel.kallsyms]        [k] deactivate_task
     0.13%     -0.02%  [kernel.kallsyms]        [k] activate_task

The diff between nouclamp and uclamp-static-key when uclamp is enabled
in the fast path:

     8.73%     -0.72%  [kernel.kallsyms]        [k] try_to_wake_up
     0.13%     +0.39%  [kernel.kallsyms]        [k] activate_task
     0.07%     +0.38%  [kernel.kallsyms]        [k] deactivate_task

Fixes: 69842cba9ace ("sched/uclamp: Add CPU's clamp buckets refcounting")
Reported-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://lkml.kernel.org/r/20200630112123.12076-3-qais.yousef@arm.com
(cherry picked from commit 46609ce227039fd192e0ecc7d940bed587fd2c78)
[ Fix minor conflict with kernel/sched.h because of function renamed
later ]
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
---
 kernel/sched/core.c              | 74 +++++++++++++++++++++++++++++++-
 kernel/sched/cpufreq_schedutil.c |  2 +-
 kernel/sched/sched.h             | 47 +++++++++++++++++++-
 3 files changed, 119 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b34b5c6e2524..a8ab68aa189a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -794,6 +794,26 @@ unsigned int sysctl_sched_uclamp_util_max = SCHED_CAPACITY_SCALE;
 /* All clamps are required to be less or equal than these values */
 static struct uclamp_se uclamp_default[UCLAMP_CNT];
 
+/*
+ * This static key is used to reduce the uclamp overhead in the fast path. It
+ * primarily disables the call to uclamp_rq_{inc, dec}() in
+ * enqueue/dequeue_task().
+ *
+ * This allows users to continue to enable uclamp in their kernel config with
+ * minimum uclamp overhead in the fast path.
+ *
+ * As soon as userspace modifies any of the uclamp knobs, the static key is
+ * enabled, since we have an actual users that make use of uclamp
+ * functionality.
+ *
+ * The knobs that would enable this static key are:
+ *
+ *   * A task modifying its uclamp value with sched_setattr().
+ *   * An admin modifying the sysctl_sched_uclamp_{min, max} via procfs.
+ *   * An admin modifying the cgroup cpu.uclamp.{min, max}
+ */
+DEFINE_STATIC_KEY_FALSE(sched_uclamp_used);
+
 /* Integer rounded range for each bucket */
 #define UCLAMP_BUCKET_DELTA DIV_ROUND_CLOSEST(SCHED_CAPACITY_SCALE, UCLAMP_BUCKETS)
 
@@ -990,10 +1010,38 @@ static inline void uclamp_rq_dec_id(struct rq *rq, struct task_struct *p,
 
 	lockdep_assert_held(&rq->lock);
 
+	/*
+	 * If sched_uclamp_used was enabled after task @p was enqueued,
+	 * we could end up with unbalanced call to uclamp_rq_dec_id().
+	 *
+	 * In this case the uc_se->active flag should be false since no uclamp
+	 * accounting was performed at enqueue time and we can just return
+	 * here.
+	 *
+	 * Need to be careful of the following enqeueue/dequeue ordering
+	 * problem too
+	 *
+	 *	enqueue(taskA)
+	 *	// sched_uclamp_used gets enabled
+	 *	enqueue(taskB)
+	 *	dequeue(taskA)
+	 *	// Must not decrement bukcet->tasks here
+	 *	dequeue(taskB)
+	 *
+	 * where we could end up with stale data in uc_se and
+	 * bucket[uc_se->bucket_id].
+	 *
+	 * The following check here eliminates the possibility of such race.
+	 */
+	if (unlikely(!uc_se->active))
+		return;
+
 	bucket = &uc_rq->bucket[uc_se->bucket_id];
+
 	SCHED_WARN_ON(!bucket->tasks);
 	if (likely(bucket->tasks))
 		bucket->tasks--;
+
 	uc_se->active = false;
 
 	/*
@@ -1021,6 +1069,15 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p)
 {
 	enum uclamp_id clamp_id;
 
+	/*
+	 * Avoid any overhead until uclamp is actually used by the userspace.
+	 *
+	 * The condition is constructed such that a NOP is generated when
+	 * sched_uclamp_used is disabled.
+	 */
+	if (!static_branch_unlikely(&sched_uclamp_used))
+		return;
+
 	if (unlikely(!p->sched_class->uclamp_enabled))
 		return;
 
@@ -1036,6 +1093,15 @@ static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p)
 {
 	enum uclamp_id clamp_id;
 
+	/*
+	 * Avoid any overhead until uclamp is actually used by the userspace.
+	 *
+	 * The condition is constructed such that a NOP is generated when
+	 * sched_uclamp_used is disabled.
+	 */
+	if (!static_branch_unlikely(&sched_uclamp_used))
+		return;
+
 	if (unlikely(!p->sched_class->uclamp_enabled))
 		return;
 
@@ -1145,8 +1211,10 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
 		update_root_tg = true;
 	}
 
-	if (update_root_tg)
+	if (update_root_tg) {
+		static_branch_enable(&sched_uclamp_used);
 		uclamp_update_root_tg();
+	}
 
 	/*
 	 * We update all RUNNABLE tasks only when task groups are in use.
@@ -1211,6 +1279,8 @@ static void __setscheduler_uclamp(struct task_struct *p,
 	if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)))
 		return;
 
+	static_branch_enable(&sched_uclamp_used);
+
 	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
 		uclamp_se_set(&p->uclamp_req[UCLAMP_MIN],
 			      attr->sched_util_min, true);
@@ -7294,6 +7364,8 @@ static ssize_t cpu_uclamp_write(struct kernfs_open_file *of, char *buf,
 	if (req.ret)
 		return req.ret;
 
+	static_branch_enable(&sched_uclamp_used);
+
 	mutex_lock(&uclamp_mutex);
 	rcu_read_lock();
 
diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index b6f56e7c8dd1..4cb80e6042c4 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -210,7 +210,7 @@ unsigned long schedutil_cpu_util(int cpu, unsigned long util_cfs,
 	unsigned long dl_util, util, irq;
 	struct rq *rq = cpu_rq(cpu);
 
-	if (!IS_BUILTIN(CONFIG_UCLAMP_TASK) &&
+	if (!uclamp_is_used() &&
 	    type == FREQUENCY_UTIL && rt_rq_is_runnable(&rq->rt)) {
 		return max;
 	}
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 570659f1c6e2..9f2a9e34a78d 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -841,6 +841,8 @@ struct uclamp_rq {
 	unsigned int value;
 	struct uclamp_bucket bucket[UCLAMP_BUCKETS];
 };
+
+DECLARE_STATIC_KEY_FALSE(sched_uclamp_used);
 #endif /* CONFIG_UCLAMP_TASK */
 
 /*
@@ -2319,12 +2321,35 @@ static inline void cpufreq_update_util(struct rq *rq, unsigned int flags) {}
 #ifdef CONFIG_UCLAMP_TASK
 unsigned int uclamp_eff_value(struct task_struct *p, enum uclamp_id clamp_id);
 
+/**
+ * uclamp_util_with - clamp @util with @rq and @p effective uclamp values.
+ * @rq:		The rq to clamp against. Must not be NULL.
+ * @util:	The util value to clamp.
+ * @p:		The task to clamp against. Can be NULL if you want to clamp
+ *		against @rq only.
+ *
+ * Clamps the passed @util to the max(@rq, @p) effective uclamp values.
+ *
+ * If sched_uclamp_used static key is disabled, then just return the util
+ * without any clamping since uclamp aggregation at the rq level in the fast
+ * path is disabled, rendering this operation a NOP.
+ *
+ * Use uclamp_eff_value() if you don't care about uclamp values at rq level. It
+ * will return the correct effective uclamp value of the task even if the
+ * static key is disabled.
+ */
 static __always_inline
 unsigned int uclamp_util_with(struct rq *rq, unsigned int util,
 			      struct task_struct *p)
 {
-	unsigned int min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value);
-	unsigned int max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);
+	unsigned int min_util;
+	unsigned int max_util;
+
+	if (!static_branch_likely(&sched_uclamp_used))
+		return util;
+
+	min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value);
+	max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);
 
 	if (p) {
 		min_util = max(min_util, uclamp_eff_value(p, UCLAMP_MIN));
@@ -2346,6 +2371,19 @@ static inline unsigned int uclamp_util(struct rq *rq, unsigned int util)
 {
 	return uclamp_util_with(rq, util, NULL);
 }
+
+/*
+ * When uclamp is compiled in, the aggregation at rq level is 'turned off'
+ * by default in the fast path and only gets turned on once userspace performs
+ * an operation that requires it.
+ *
+ * Returns true if userspace opted-in to use uclamp and aggregation at rq level
+ * hence is active.
+ */
+static inline bool uclamp_is_used(void)
+{
+	return static_branch_likely(&sched_uclamp_used);
+}
 #else /* CONFIG_UCLAMP_TASK */
 static inline unsigned int uclamp_util_with(struct rq *rq, unsigned int util,
 					    struct task_struct *p)
@@ -2356,6 +2394,11 @@ static inline unsigned int uclamp_util(struct rq *rq, unsigned int util)
 {
 	return util;
 }
+
+static inline bool uclamp_is_used(void)
+{
+	return false;
+}
 #endif /* CONFIG_UCLAMP_TASK */
 
 #ifdef arch_scale_freq_capacity
-- 
2.17.1

[PATCH 2/2] sched/uclamp: Fix a deadlock when enabling uclamp static key

[Qais Yousef]

The following splat was caught when setting uclamp value of a task:

  BUG: sleeping function called from invalid context at ./include/linux/percpu-rwsem.h:49

   cpus_read_lock+0x68/0x130
   static_key_enable+0x1c/0x38
   __sched_setscheduler+0x900/0xad8

Fix by ensuring we enable the key outside of the critical section in
__sched_setscheduler()

Fixes: 46609ce22703 ("sched/uclamp: Protect uclamp fast path code with static key")
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200716110347.19553-4-qais.yousef@arm.com
(cherry picked from commit e65855a52b479f98674998cb23b21ef5a8144b04)
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
---
 kernel/sched/core.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a8ab68aa189a..352239c411a4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1249,6 +1249,15 @@ static int uclamp_validate(struct task_struct *p,
 	if (upper_bound > SCHED_CAPACITY_SCALE)
 		return -EINVAL;
 
+	/*
+	 * We have valid uclamp attributes; make sure uclamp is enabled.
+	 *
+	 * We need to do that here, because enabling static branches is a
+	 * blocking operation which obviously cannot be done while holding
+	 * scheduler locks.
+	 */
+	static_branch_enable(&sched_uclamp_used);
+
 	return 0;
 }
 
@@ -1279,8 +1288,6 @@ static void __setscheduler_uclamp(struct task_struct *p,
 	if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)))
 		return;
 
-	static_branch_enable(&sched_uclamp_used);
-
 	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
 		uclamp_se_set(&p->uclamp_req[UCLAMP_MIN],
 			      attr->sched_util_min, true);
-- 
2.17.1

Re: [PATCH 0/2] Backport uclamp static key to 5.4.y stable

[Sasha Levin]

On Fri, Aug 28, 2020 at 01:56:08PM +0100, Qais Yousef wrote:
>Hi Greg/Sasha
>
>The following 2 patches are backports of
>
>	46609ce22703: ("sched/uclamp: Protect uclamp fast path code with static key")
>	e65855a52b47: ("sched/uclamp: Fix a deadlock when enabling uclamp static key")
>
>into 5.4.y stable tree. The conflict was trivial and due to:
>
>	1. uclamp_rq_util_with() was renamed from util_util_with()
>	2. 2 local variables were converted to unsigned long from unsigned int.
>
>I did do compile test on aarch64 and x86_64 and both looked fine. Beside I ran
>a quick and short mmtest to verify the functionality and got the following
>results which is inline with what's expected.

I've queued it up, thanks!

-- 
Thanks,
Sasha