linux-kernel - Re: [PATCH v2] sched/fair : fix reordering of enqueue_task

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Date:   Thu, 05 Mar 2020 10:39:31 -0800
From:   bsegall@...gle.com
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, linux-kernel@...r.kernel.org, pauld@...hat.com,
        parth@...ux.ibm.com, valentin.schneider@....com, hdanton@...a.com,
        zhout@...aldi.net
Subject: Re: [PATCH v2] sched/fair : fix reordering of enqueue_task_fair

Vincent Guittot <vincent.guittot@...aro.org> writes:

> Even when a cgroup is throttled, the group se of a child cgroup can still
> be enqueued and its gse->on_rq stays true. When a task is enqueued on such
> child, we still have to update the load_avg and increase
> h_nr_running of the throttled cfs. Nevertheless, the 1st
> for_each_sched_entity loop is skipped because of gse->on_rq == true and the
> 2nd loop because the cfs is throttled whereas we have to update both
> load_avg with the old h_nr_running and increase h_nr_running in such case.
> Note that the update of load_avg will effectively happen only once in order
> to sync up to the throttled time. Next call for updating load_avg will stop
> early because the clock stays unchanged.
>
> Fixes: 6d4d22468dae ("sched/fair: Reorder enqueue/dequeue_task_fair path")
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fcc968669aea..5b232d261842 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5431,16 +5431,16 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  	for_each_sched_entity(se) {
>  		cfs_rq = cfs_rq_of(se);
>  
> -		/* end evaluation on encountering a throttled cfs_rq */
> -		if (cfs_rq_throttled(cfs_rq))
> -			goto enqueue_throttle;
> -
>  		update_load_avg(cfs_rq, se, UPDATE_TG);
>  		se_update_runnable(se);
>  		update_cfs_group(se);
>  
>  		cfs_rq->h_nr_running++;
>  		cfs_rq->idle_h_nr_running += idle_h_nr_running;
> +
> +		/* end evaluation on encountering a throttled cfs_rq */
> +		if (cfs_rq_throttled(cfs_rq))
> +			goto enqueue_throttle;
>  	}
>  
>  enqueue_throttle:


I think there's an equivalent issue on dequeue as well, though that's
much rarer to trigger (but still possible). I think the same fix works
there too?