[PATCH v3] sched/fair: limit sched slice duration

[PATCH v3] sched/fair: limit sched slice duration

[Vincent Guittot]

In presence of a lot of small weight tasks like sched_idle tasks, normal
or high weight tasks can see their ideal runtime (sched_slice) to increase
to hundreds ms whereas it normally stays below sysctl_sched_latency.

2 normal tasks running on a CPU will have a max sched_slice of 12ms
(half of the sched_period). This means that they will make progress
every sysctl_sched_latency period.

If we now add 1000 idle tasks on the CPU, the sched_period becomes
3006 ms and the ideal runtime of the normal tasks becomes 609 ms.
It will even become 1500ms if the idle tasks belongs to an idle cgroup.
This means that the scheduler will look for picking another waiting task
after 609ms running time (1500ms respectively). The idle tasks change
significantly the way the 2 normal tasks interleave their running time
slot whereas they should have a small impact.

Such long sched_slice can delay significantly the release of resources
as the tasks can wait hundreds of ms before the next running slot just
because of idle tasks queued on the rq.

Cap the ideal_runtime to sysctl_sched_latency to make sure that tasks will
regularly make progress and will not be significantly impacted by
idle/background tasks queued on the rq.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---

Change since v2:
- Cap ideal_runtime from the beg as suggested by Peter

 kernel/sched/fair.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5ffec4370602..c309d57efb2c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4584,7 +4584,13 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 	struct sched_entity *se;
 	s64 delta;
 
-	ideal_runtime = sched_slice(cfs_rq, curr);
+	/*
+	 * When many tasks blow up the sched_period; it is possible that
+	 * sched_slice() reports unusually large results (when many tasks are
+	 * very light for example). Therefore impose a maximum.
+	 */
+	ideal_runtime = min_t(u64, sched_slice(cfs_rq, curr), sysctl_sched_latency);
+
 	delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
 	if (delta_exec > ideal_runtime) {
 		resched_curr(rq_of(cfs_rq));
-- 
2.17.1

Re: [PATCH v3] sched/fair: limit sched slice duration

[Dietmar Eggemann]

On 03/10/2022 14:21, Vincent Guittot wrote:
> In presence of a lot of small weight tasks like sched_idle tasks, normal
> or high weight tasks can see their ideal runtime (sched_slice) to increase
> to hundreds ms whereas it normally stays below sysctl_sched_latency.
> 
> 2 normal tasks running on a CPU will have a max sched_slice of 12ms
> (half of the sched_period). This means that they will make progress
> every sysctl_sched_latency period.
> 
> If we now add 1000 idle tasks on the CPU, the sched_period becomes
> 3006 ms and the ideal runtime of the normal tasks becomes 609 ms.
> It will even become 1500ms if the idle tasks belongs to an idle cgroup.
> This means that the scheduler will look for picking another waiting task
> after 609ms running time (1500ms respectively). The idle tasks change
> significantly the way the 2 normal tasks interleave their running time
> slot whereas they should have a small impact.
> 
> Such long sched_slice can delay significantly the release of resources
> as the tasks can wait hundreds of ms before the next running slot just
> because of idle tasks queued on the rq.
> 
> Cap the ideal_runtime to sysctl_sched_latency to make sure that tasks will
> regularly make progress and will not be significantly impacted by
> idle/background tasks queued on the rq.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
> 
> Change since v2:
> - Cap ideal_runtime from the beg as suggested by Peter
> 
>  kernel/sched/fair.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 5ffec4370602..c309d57efb2c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4584,7 +4584,13 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>  	struct sched_entity *se;
>  	s64 delta;
>  
> -	ideal_runtime = sched_slice(cfs_rq, curr);
> +	/*
> +	 * When many tasks blow up the sched_period; it is possible that
> +	 * sched_slice() reports unusually large results (when many tasks are
> +	 * very light for example). Therefore impose a maximum.
> +	 */
> +	ideal_runtime = min_t(u64, sched_slice(cfs_rq, curr), sysctl_sched_latency);
> +
>  	delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
>  	if (delta_exec > ideal_runtime) {
>  		resched_curr(rq_of(cfs_rq));

Tested on 6 CPU system (sysctl_sched_latency=18ms,
sysctl_sched_min_granularity=2.25ms)

I start to see `slice > period` when I run:

(a) > ~50 idle tasks in '/' for an arbitrary nice=0 task

(b) > ~50 nice=0 tasks in '/A' w/ cpu.shares = max for se of '/A'

Essentially in moments in which cfs_rq->nr_running > sched_nr_latency
and se_weight is relatively high compared to cfs_rq_weight.

Tested-By: Dietmar Eggemann <dietmar.eggemann@arm.com>