[PATCH] sched/deadline: Reduce rq lock contention in dl_add_task_root_domain()

[PATCH] sched/deadline: Reduce rq lock contention in dl_add_task_root_domain()

[Dietmar Eggemann]

dl_add_task_root_domain() is called during sched domain rebuild:

  rebuild_sched_domains_locked()
    partition_and_rebuild_sched_domains()
      rebuild_root_domains()
         for all top_cpuset descendants:
           update_tasks_root_domain()
             for all tasks of cpuset:
               dl_add_task_root_domain()

Change it so that only the task pi lock is taken to check if the task
has a SCHED_DEADLINE (DL) policy. In case that p is a DL task take the
rq lock as well to be able to safely de-reference root domain's DL
bandwidth structure.

Most of the tasks will have another policy (namely SCHED_NORMAL) and
can now bail without taking the rq lock.

One thing to note here: Even in case that there aren't any DL user
tasks, a slow frequency switching system with cpufreq gov schedutil has
a DL task (sugov) per frequency domain running which participates in DL
bandwidth management.

Reviewed-by: Quentin Perret <qperret@google.com>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---

The use case in which this makes a noticeable difference is Android's
'CPU pause' power management feature.

It uses CPU hotplug control to clear a CPU from active state to force
all threads which are not per-cpu kthreads away from this CPU.

Making DL bandwidth management faster during sched domain rebuild helps
to reduce the time to pause/un-pause a CPU.

 kernel/sched/deadline.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 5421782fe897..c7b1a63a053b 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2409,9 +2409,13 @@ void dl_add_task_root_domain(struct task_struct *p)
 	struct rq *rq;
 	struct dl_bw *dl_b;
 
-	rq = task_rq_lock(p, &rf);
-	if (!dl_task(p))
-		goto unlock;
+	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
+	if (!dl_task(p)) {
+		raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags);
+		return;
+	}
+
+	rq = __task_rq_lock(p, &rf);
 
 	dl_b = &rq->rd->dl_bw;
 	raw_spin_lock(&dl_b->lock);
@@ -2420,7 +2424,6 @@ void dl_add_task_root_domain(struct task_struct *p)
 
 	raw_spin_unlock(&dl_b->lock);
 
-unlock:
 	task_rq_unlock(rq, p, &rf);
 }
 
-- 
2.25.1

Re: [PATCH] sched/deadline: Reduce rq lock contention in dl_add_task_root_domain()

[Valentin Schneider]

On 19/01/21 09:35, Dietmar Eggemann wrote:
> dl_add_task_root_domain() is called during sched domain rebuild:
>
>   rebuild_sched_domains_locked()
>     partition_and_rebuild_sched_domains()
>       rebuild_root_domains()
>          for all top_cpuset descendants:
>            update_tasks_root_domain()
>              for all tasks of cpuset:
>                dl_add_task_root_domain()
>
> Change it so that only the task pi lock is taken to check if the task
> has a SCHED_DEADLINE (DL) policy. In case that p is a DL task take the
> rq lock as well to be able to safely de-reference root domain's DL
> bandwidth structure.
>

A task's policy is stable under ->pi_lock, so LGTM.

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>

> Most of the tasks will have another policy (namely SCHED_NORMAL) and
> can now bail without taking the rq lock.
>
> One thing to note here: Even in case that there aren't any DL user
> tasks, a slow frequency switching system with cpufreq gov schedutil has
> a DL task (sugov) per frequency domain running which participates in DL
> bandwidth management.
>
> Reviewed-by: Quentin Perret <qperret@google.com>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

Re: [PATCH] sched/deadline: Reduce rq lock contention in dl_add_task_root_domain()

[Daniel Bristot de Oliveira]

On 1/19/21 9:35 AM, Dietmar Eggemann wrote:
> dl_add_task_root_domain() is called during sched domain rebuild:
> 
>   rebuild_sched_domains_locked()
>     partition_and_rebuild_sched_domains()
>       rebuild_root_domains()
>          for all top_cpuset descendants:
>            update_tasks_root_domain()
>              for all tasks of cpuset:
>                dl_add_task_root_domain()
> 
> Change it so that only the task pi lock is taken to check if the task
> has a SCHED_DEADLINE (DL) policy. In case that p is a DL task take the
> rq lock as well to be able to safely de-reference root domain's DL
> bandwidth structure.
> 
> Most of the tasks will have another policy (namely SCHED_NORMAL) and
> can now bail without taking the rq lock.
> 
> One thing to note here: Even in case that there aren't any DL user
> tasks, a slow frequency switching system with cpufreq gov schedutil has
> a DL task (sugov) per frequency domain running which participates in DL
> bandwidth management.
> 
> Reviewed-by: Quentin Perret <qperret@google.com>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

Reviewed-by: Daniel Bristot de Oliveira <bristot@redhat.com>

Thanks!
-- Daniel

Re: [PATCH] sched/deadline: Reduce rq lock contention in dl_add_task_root_domain()

[Juri Lelli]

Hi,

On 19/01/21 09:35, Dietmar Eggemann wrote:
> dl_add_task_root_domain() is called during sched domain rebuild:
> 
>   rebuild_sched_domains_locked()
>     partition_and_rebuild_sched_domains()
>       rebuild_root_domains()
>          for all top_cpuset descendants:
>            update_tasks_root_domain()
>              for all tasks of cpuset:
>                dl_add_task_root_domain()
> 
> Change it so that only the task pi lock is taken to check if the task
> has a SCHED_DEADLINE (DL) policy. In case that p is a DL task take the
> rq lock as well to be able to safely de-reference root domain's DL
> bandwidth structure.
> 
> Most of the tasks will have another policy (namely SCHED_NORMAL) and
> can now bail without taking the rq lock.
> 
> One thing to note here: Even in case that there aren't any DL user
> tasks, a slow frequency switching system with cpufreq gov schedutil has
> a DL task (sugov) per frequency domain running which participates in DL
> bandwidth management.
> 
> Reviewed-by: Quentin Perret <qperret@google.com>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

Looks good to me, thanks!

Acked-by: Juri Lelli <juri.lelli@redhat.com>

Best,
Juri

Re: [PATCH] sched/deadline: Reduce rq lock contention in dl_add_task_root_domain()

[Peter Zijlstra]

On Tue, Jan 19, 2021 at 09:35:42AM +0100, Dietmar Eggemann wrote:
> dl_add_task_root_domain() is called during sched domain rebuild:
> 
>   rebuild_sched_domains_locked()
>     partition_and_rebuild_sched_domains()
>       rebuild_root_domains()
>          for all top_cpuset descendants:
>            update_tasks_root_domain()
>              for all tasks of cpuset:
>                dl_add_task_root_domain()
> 
> Change it so that only the task pi lock is taken to check if the task
> has a SCHED_DEADLINE (DL) policy. In case that p is a DL task take the
> rq lock as well to be able to safely de-reference root domain's DL
> bandwidth structure.
> 
> Most of the tasks will have another policy (namely SCHED_NORMAL) and
> can now bail without taking the rq lock.
> 
> One thing to note here: Even in case that there aren't any DL user
> tasks, a slow frequency switching system with cpufreq gov schedutil has
> a DL task (sugov) per frequency domain running which participates in DL
> bandwidth management.
> 
> Reviewed-by: Quentin Perret <qperret@google.com>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

Thanks!

[tip: sched/core] sched/deadline: Reduce rq lock contention in dl_add_task_root_domain()

[tip-bot2 for Dietmar Eggemann]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     3096b6fe494b7b4e45d20cb77aa6b715a3efe344
Gitweb:        https://git.kernel.org/tip/3096b6fe494b7b4e45d20cb77aa6b715a3efe344
Author:        Dietmar Eggemann <dietmar.eggemann@arm.com>
AuthorDate:    Tue, 19 Jan 2021 09:35:42 +01:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 10 Feb 2021 14:44:48 +01:00

sched/deadline: Reduce rq lock contention in dl_add_task_root_domain()

dl_add_task_root_domain() is called during sched domain rebuild:

  rebuild_sched_domains_locked()
    partition_and_rebuild_sched_domains()
      rebuild_root_domains()
         for all top_cpuset descendants:
           update_tasks_root_domain()
             for all tasks of cpuset:
               dl_add_task_root_domain()

Change it so that only the task pi lock is taken to check if the task
has a SCHED_DEADLINE (DL) policy. In case that p is a DL task take the
rq lock as well to be able to safely de-reference root domain's DL
bandwidth structure.

Most of the tasks will have another policy (namely SCHED_NORMAL) and
can now bail without taking the rq lock.

One thing to note here: Even in case that there aren't any DL user
tasks, a slow frequency switching system with cpufreq gov schedutil has
a DL task (sugov) per frequency domain running which participates in DL
bandwidth management.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Quentin Perret <qperret@google.com>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20210119083542.19856-1-dietmar.eggemann@arm.com
---
 kernel/sched/deadline.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 1508d12..6f37796 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2388,9 +2388,13 @@ void dl_add_task_root_domain(struct task_struct *p)
 	struct rq *rq;
 	struct dl_bw *dl_b;
 
-	rq = task_rq_lock(p, &rf);
-	if (!dl_task(p))
-		goto unlock;
+	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
+	if (!dl_task(p)) {
+		raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags);
+		return;
+	}
+
+	rq = __task_rq_lock(p, &rf);
 
 	dl_b = &rq->rd->dl_bw;
 	raw_spin_lock(&dl_b->lock);
@@ -2399,7 +2403,6 @@ void dl_add_task_root_domain(struct task_struct *p)
 
 	raw_spin_unlock(&dl_b->lock);
 
-unlock:
 	task_rq_unlock(rq, p, &rf);
 }
 

[tip: sched/core] sched/deadline: Reduce rq lock contention in dl_add_task_root_domain()

[tip-bot2 for Dietmar Eggemann]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     de40f33e788b0c016bfde512ace2f76339ef7ddb
Gitweb:        https://git.kernel.org/tip/de40f33e788b0c016bfde512ace2f76339ef7ddb
Author:        Dietmar Eggemann <dietmar.eggemann@arm.com>
AuthorDate:    Tue, 19 Jan 2021 09:35:42 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Wed, 17 Feb 2021 14:12:42 +01:00

sched/deadline: Reduce rq lock contention in dl_add_task_root_domain()

dl_add_task_root_domain() is called during sched domain rebuild:

  rebuild_sched_domains_locked()
    partition_and_rebuild_sched_domains()
      rebuild_root_domains()
         for all top_cpuset descendants:
           update_tasks_root_domain()
             for all tasks of cpuset:
               dl_add_task_root_domain()

Change it so that only the task pi lock is taken to check if the task
has a SCHED_DEADLINE (DL) policy. In case that p is a DL task take the
rq lock as well to be able to safely de-reference root domain's DL
bandwidth structure.

Most of the tasks will have another policy (namely SCHED_NORMAL) and
can now bail without taking the rq lock.

One thing to note here: Even in case that there aren't any DL user
tasks, a slow frequency switching system with cpufreq gov schedutil has
a DL task (sugov) per frequency domain running which participates in DL
bandwidth management.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Quentin Perret <qperret@google.com>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20210119083542.19856-1-dietmar.eggemann@arm.com
---
 kernel/sched/deadline.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 1508d12..6f37796 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2388,9 +2388,13 @@ void dl_add_task_root_domain(struct task_struct *p)
 	struct rq *rq;
 	struct dl_bw *dl_b;
 
-	rq = task_rq_lock(p, &rf);
-	if (!dl_task(p))
-		goto unlock;
+	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
+	if (!dl_task(p)) {
+		raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags);
+		return;
+	}
+
+	rq = __task_rq_lock(p, &rf);
 
 	dl_b = &rq->rd->dl_bw;
 	raw_spin_lock(&dl_b->lock);
@@ -2399,7 +2403,6 @@ void dl_add_task_root_domain(struct task_struct *p)
 
 	raw_spin_unlock(&dl_b->lock);
 
-unlock:
 	task_rq_unlock(rq, p, &rf);
 }