* Re: [PATCH v4] sched,fair: Skip newidle_balance if a wakeup is pending
2021-04-22 17:02 [PATCH v4] sched,fair: Skip newidle_balance if a wakeup is pending Rik van Riel
@ 2021-04-22 17:09 ` Vincent Guittot
2021-05-04 9:21 ` Peter Zijlstra
2021-04-24 9:30 ` Mel Gorman
2021-05-12 10:28 ` [tip: sched/core] " tip-bot2 for Rik van Riel
2 siblings, 1 reply; 5+ messages in thread
From: Vincent Guittot @ 2021-04-22 17:09 UTC (permalink / raw)
To: Rik van Riel
Cc: linux-kernel, Kernel Team, Valentin Schneider, Peter Zijlstra,
Ingo Molnar, Dietmar Eggemann, Mel Gorman
On Thu, 22 Apr 2021 at 19:02, Rik van Riel <riel@surriel.com> wrote:
>
> The try_to_wake_up function has an optimization where it can queue
> a task for wakeup on its previous CPU, if the task is still in the
> middle of going to sleep inside schedule().
>
> Once schedule() re-enables IRQs, the task will be woken up with an
> IPI, and placed back on the runqueue.
>
> If we have such a wakeup pending, there is no need to search other
> CPUs for runnable tasks. Just skip (or bail out early from) newidle
> balancing, and run the just woken up task.
>
> For a memcache like workload test, this reduces total CPU use by
> about 2%, proportionally split between user and system time,
> and p99 and p95 application response time by 10% on average.
> The schedstats run_delay number shows a similar improvement.
>
> Signed-off-by: Rik van Riel <riel@surriel.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
> kernel/sched/fair.c | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 69680158963f..6a18688a37f8 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10594,6 +10594,14 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> u64 curr_cost = 0;
>
> update_misfit_status(NULL, this_rq);
> +
> + /*
> + * There is a task waiting to run. No need to search for one.
> + * Return 0; the task will be enqueued when switching to idle.
> + */
> + if (this_rq->ttwu_pending)
> + return 0;
> +
> /*
> * We must set idle_stamp _before_ calling idle_balance(), such that we
> * measure the duration of idle_balance() as idle time.
> @@ -10661,7 +10669,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> * Stop searching for tasks to pull if there are
> * now runnable tasks on this rq.
> */
> - if (pulled_task || this_rq->nr_running > 0)
> + if (pulled_task || this_rq->nr_running > 0 ||
> + this_rq->ttwu_pending)
> break;
> }
> rcu_read_unlock();
> --
> 2.25.4
>
>
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH v4] sched,fair: Skip newidle_balance if a wakeup is pending
2021-04-22 17:09 ` Vincent Guittot
@ 2021-05-04 9:21 ` Peter Zijlstra
0 siblings, 0 replies; 5+ messages in thread
From: Peter Zijlstra @ 2021-05-04 9:21 UTC (permalink / raw)
To: Vincent Guittot
Cc: Rik van Riel, linux-kernel, Kernel Team, Valentin Schneider,
Ingo Molnar, Dietmar Eggemann, Mel Gorman
On Thu, Apr 22, 2021 at 07:09:46PM +0200, Vincent Guittot wrote:
> On Thu, 22 Apr 2021 at 19:02, Rik van Riel <riel@surriel.com> wrote:
> >
> > The try_to_wake_up function has an optimization where it can queue
> > a task for wakeup on its previous CPU, if the task is still in the
> > middle of going to sleep inside schedule().
> >
> > Once schedule() re-enables IRQs, the task will be woken up with an
> > IPI, and placed back on the runqueue.
> >
> > If we have such a wakeup pending, there is no need to search other
> > CPUs for runnable tasks. Just skip (or bail out early from) newidle
> > balancing, and run the just woken up task.
> >
> > For a memcache like workload test, this reduces total CPU use by
> > about 2%, proportionally split between user and system time,
> > and p99 and p95 application response time by 10% on average.
> > The schedstats run_delay number shows a similar improvement.
> >
> > Signed-off-by: Rik van Riel <riel@surriel.com>
>
> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Thanks!
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH v4] sched,fair: Skip newidle_balance if a wakeup is pending
2021-04-22 17:02 [PATCH v4] sched,fair: Skip newidle_balance if a wakeup is pending Rik van Riel
2021-04-22 17:09 ` Vincent Guittot
@ 2021-04-24 9:30 ` Mel Gorman
2021-05-12 10:28 ` [tip: sched/core] " tip-bot2 for Rik van Riel
2 siblings, 0 replies; 5+ messages in thread
From: Mel Gorman @ 2021-04-24 9:30 UTC (permalink / raw)
To: Rik van Riel
Cc: linux-kernel, kernel-team, Vincent Guittot, Valentin Schneider,
Peter Zijlstra, Ingo Molnar, Dietmar Eggemann
On Thu, Apr 22, 2021 at 01:02:36PM -0400, Rik van Riel wrote:
> The try_to_wake_up function has an optimization where it can queue
> a task for wakeup on its previous CPU, if the task is still in the
> middle of going to sleep inside schedule().
>
> Once schedule() re-enables IRQs, the task will be woken up with an
> IPI, and placed back on the runqueue.
>
> If we have such a wakeup pending, there is no need to search other
> CPUs for runnable tasks. Just skip (or bail out early from) newidle
> balancing, and run the just woken up task.
>
> For a memcache like workload test, this reduces total CPU use by
> about 2%, proportionally split between user and system time,
> and p99 and p95 application response time by 10% on average.
> The schedstats run_delay number shows a similar improvement.
>
> Signed-off-by: Rik van Riel <riel@surriel.com>
Acked-by: Mel Gorman <mgorman@suse.de>
--
Mel Gorman
SUSE Labs
^ permalink raw reply [flat|nested] 5+ messages in thread
* [tip: sched/core] sched,fair: Skip newidle_balance if a wakeup is pending
2021-04-22 17:02 [PATCH v4] sched,fair: Skip newidle_balance if a wakeup is pending Rik van Riel
2021-04-22 17:09 ` Vincent Guittot
2021-04-24 9:30 ` Mel Gorman
@ 2021-05-12 10:28 ` tip-bot2 for Rik van Riel
2 siblings, 0 replies; 5+ messages in thread
From: tip-bot2 for Rik van Riel @ 2021-05-12 10:28 UTC (permalink / raw)
To: linux-tip-commits
Cc: Rik van Riel, Peter Zijlstra (Intel), Vincent Guittot, Mel Gorman,
x86, linux-kernel
The following commit has been merged into the sched/core branch of tip:
Commit-ID: e5e678e4fea26d73444f4427cbbaeab4fa79ecee
Gitweb: https://git.kernel.org/tip/e5e678e4fea26d73444f4427cbbaeab4fa79ecee
Author: Rik van Riel <riel@surriel.com>
AuthorDate: Thu, 22 Apr 2021 13:02:36 -04:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 12 May 2021 11:43:23 +02:00
sched,fair: Skip newidle_balance if a wakeup is pending
The try_to_wake_up function has an optimization where it can queue
a task for wakeup on its previous CPU, if the task is still in the
middle of going to sleep inside schedule().
Once schedule() re-enables IRQs, the task will be woken up with an
IPI, and placed back on the runqueue.
If we have such a wakeup pending, there is no need to search other
CPUs for runnable tasks. Just skip (or bail out early from) newidle
balancing, and run the just woken up task.
For a memcache like workload test, this reduces total CPU use by
about 2%, proportionally split between user and system time,
and p99 and p95 application response time by 10% on average.
The schedstats run_delay number shows a similar improvement.
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Link: https://lkml.kernel.org/r/20210422130236.0bb353df@imladris.surriel.com
---
kernel/sched/fair.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3248e24..d10c6cc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10592,6 +10592,14 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
u64 curr_cost = 0;
update_misfit_status(NULL, this_rq);
+
+ /*
+ * There is a task waiting to run. No need to search for one.
+ * Return 0; the task will be enqueued when switching to idle.
+ */
+ if (this_rq->ttwu_pending)
+ return 0;
+
/*
* We must set idle_stamp _before_ calling idle_balance(), such that we
* measure the duration of idle_balance() as idle time.
@@ -10657,7 +10665,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
* Stop searching for tasks to pull if there are
* now runnable tasks on this rq.
*/
- if (pulled_task || this_rq->nr_running > 0)
+ if (pulled_task || this_rq->nr_running > 0 ||
+ this_rq->ttwu_pending)
break;
}
rcu_read_unlock();
^ permalink raw reply related [flat|nested] 5+ messages in thread