Re: [PATCH] sched/util_est: fix util_est_dequeue() for throttled cfs rq

[Patrick Bellasi]

On 14-Jun 12:33, Vincent Guittot wrote:
> When a cfs_rq is throttled, parent cfs_rq->nr_running is decreased and
> everything happens at cfs_rq level. Currently util_est stays unchanged
> in such case and it keeps accounting the utilization of throttled tasks.
> This can somewhat make sense as we don't dequeue tasks but only throttled
> cfs_rq.

I think the idea here was that, if tasks are throttled, this should
manifest in a reduction of their utilization... and thus the estimated
utilization should still represent the amount of bandwidth required by
that tasks. Although one could argue that, while a TG is throttled we
would like to be able to drop the frequency if possible.

This has not been implemented that way so far because the
attach/detach of TGs will require to walk them to account for all
child tasks's util_est or, otherwise, to aggregate util_est across TGs.

> If a task of another group is enqueued/dequeued and root cfs_rq becomes
> idle during the dequeue, util_est will be cleared whereas it was
> accounting util_est of throttled tasks before.

Yep :/

> So the behavior of util_est
> is not always the same regarding throttled tasks and depends of side
> activity. Furthermore, util_est will not be updated when the cfs_rq is
> unthrottled

right... that happens because (un)throttling does not involve (en/de)queue.

> as everything happens at cfs rq level. Main results is that
> util_est will stay null whereas we now have running tasks. We have to wait
> for the next dequeue/enqueue of the previously throttled tasks to get an
> up to date util_est.
> 
> Remove the assumption that cfs_rq's estimated utilization of a CPU is 0
> if there is no running task so the util_est of a task remains until the
> latter is dequeued even if its cfs_rq has been throttled.

Right...

> Fixes: 7f65ea42eb00 ("sched/fair: Add util_est on top of PELT")
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 16 ++++------------
>  1 file changed, 4 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e497c05..d3121fc 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3982,18 +3982,10 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
>  	if (!sched_feat(UTIL_EST))
>  		return;
>  
> -	/*
> -	 * Update root cfs_rq's estimated utilization
> -	 *
> -	 * If *p is the last task then the root cfs_rq's estimated utilization
> -	 * of a CPU is 0 by definition.
> -	 */
> -	ue.enqueued = 0;

... AFAIR, this reset what there since one of the first posts as an
"optimization". But actually I was not considering the scenario you
describe.

> -	if (cfs_rq->nr_running) {
> -		ue.enqueued  = cfs_rq->avg.util_est.enqueued;
> -		ue.enqueued -= min_t(unsigned int, ue.enqueued,
> -				     (_task_util_est(p) | UTIL_AVG_UNCHANGED));
> -	}
> +	/* Update root cfs_rq's estimated utilization */
> +	ue.enqueued  = cfs_rq->avg.util_est.enqueued;
> +	ue.enqueued -= min_t(unsigned int, ue.enqueued,
> +			     (_task_util_est(p) | UTIL_AVG_UNCHANGED));

So, this should still be bound-safe thanks to the min() for the
subtraction.


>  	WRITE_ONCE(cfs_rq->avg.util_est.enqueued, ue.enqueued);
>  
>  	/*
> -- 
> 2.7.4
> 

LGTM:

Reviewed-by: Patrick Bellasi <patrick.bellasi@xxxxxxx>

-- 
#include <best/regards.h>

Patrick Bellasi