Re: [PATCH] sched/pelt: avoid underestimate of task utilization

[Lukasz Luba <lukasz.luba@arm.com>]

On 11/22/23 14:01, Vincent Guittot wrote:
> It has been reported that thread's util_est can significantly decrease as
> a result of sharing the CPU with other threads. The use case can be easily
> reproduced with a periodic task TA that runs 1ms and sleeps 100us.
> When the task is alone on the CPU, its max utilization and its util_est is
> around 888. If another similar task starts to run on the same CPU, TA will
> have to share the CPU runtime and its maximum utilization will decrease
> around half the CPU capacity (512) then TA's util_est will follow this new
> maximum trend which is only the result of sharing the CPU with others
> tasks. Such situation can be detected with runnable_avg wich is close or
> equal to util_avg when TA is alone but increases above util_avg when TA
> shares the CPU with other threads and wait on the runqueue.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
> 
> This patch implements what I mentioned in [1]. I have been able to
> reproduce such pattern with rt-app.
> 
> [1] https://lore.kernel.org/lkml/CAKfTPtDd-HhF-YiNTtL9i5k0PfJbF819Yxu4YquzfXgwi7voyw@mail.gmail.com/#t

Thanks Vincet for looking at it! I didn't have to to come
back to this issue.

> 
>   kernel/sched/fair.c | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 07f555857698..eeb505d28905 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4774,6 +4774,11 @@ static inline unsigned long task_util(struct task_struct *p)
>   	return READ_ONCE(p->se.avg.util_avg);
>   }
>   
> +static inline unsigned long task_runnable(struct task_struct *p)
> +{
> +	return READ_ONCE(p->se.avg.runnable_avg);
> +}
> +
>   static inline unsigned long _task_util_est(struct task_struct *p)
>   {
>   	struct util_est ue = READ_ONCE(p->se.avg.util_est);
> @@ -4892,6 +4897,14 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
>   	if (task_util(p) > arch_scale_cpu_capacity(cpu_of(rq_of(cfs_rq))))
>   		return;
>   
> +	/*
> +	 * To avoid underestimate of task utilization, skip updates of ewma if
> +	 * we cannot grant that thread got all CPU time it wanted.
> +	 */
> +	if ((ue.enqueued + UTIL_EST_MARGIN) < task_runnable(p))
> +		goto done;
> +
> +
>   	/*
>   	 * Update Task's estimated utilization
>   	 *

That looks reasonable to do. I cannot test it right now on my pixel6.
It should do the trick to the task util that we need in bigger apps
than rt-app as well.

Reviewed-by: Lukasz luba <lukasz.luba@arm.com>