linux-kernel - Re: [PATCH v2] Sched/fair: Block nohz tick

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Message-ID: <xm26jzvn8ds7.fsf@google.com>
Date:   Wed, 28 Jun 2023 14:42:16 -0700
From:   Benjamin Segall <bsegall@...gle.com>
To:     Phil Auld <pauld@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Juri Lelli <juri.lelli@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <vschneid@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v2] Sched/fair: Block nohz tick_stop when cfs bandwidth
 in use

Phil Auld <pauld@...hat.com> writes:

> CFS bandwidth limits and NOHZ full don't play well together.  Tasks
> can easily run well past their quotas before a remote tick does
> accounting.  This leads to long, multi-period stalls before such
> tasks can run again. Currentlyi, when presented with these conflicting
> requirements the scheduler is favoring nohz_full and letting the tick
> be stopped. However, nohz tick stopping is already best-effort, there
> are a number of conditions that can prevent it, whereas cfs runtime
> bandwidth is expected to be enforced.
>
> Make the scheduler favor bandwidth over stopping the tick by setting
> TICK_DEP_BIT_SCHED when the only running task is a cfs task with
> runtime limit enabled.
>
> Add sched_feat HZ_BW (off by default) to control this behavior.
>
> Signed-off-by: Phil Auld <pauld@...hat.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Valentin Schneider <vschneid@...hat.com>
> Cc: Ben Segall <bsegall@...gle.com>
> ---
>
> v2:  Ben pointed out that the bit could get cleared in the dequeue path
> if we migrate a newly enqueued task without preempting curr. Added a 
> check for that edge case to sched_can_stop_tick. Removed the call to 
> sched_can_stop_tick from sched_fair_update_stop_tick since it was 
> redundant.
>
>  kernel/sched/core.c     | 12 +++++++++++
>  kernel/sched/fair.c     | 45 +++++++++++++++++++++++++++++++++++++++++
>  kernel/sched/features.h |  2 ++
>  3 files changed, 59 insertions(+)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a68d1276bab0..646f60bfc7e7 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1194,6 +1194,8 @@ static void nohz_csd_func(void *info)
>  #endif /* CONFIG_NO_HZ_COMMON */
>  
>  #ifdef CONFIG_NO_HZ_FULL
> +extern bool sched_cfs_bandwidth_active(struct cfs_rq *cfs_rq);
> +
>  bool sched_can_stop_tick(struct rq *rq)
>  {
>  	int fifo_nr_running;
> @@ -1229,6 +1231,16 @@ bool sched_can_stop_tick(struct rq *rq)
>  	if (rq->nr_running > 1)
>  		return false;
>  
> +	/*
> +	 * If there is one task and it has CFS runtime bandwidth constraints
> +	 * and it's on the cpu now we don't want to stop the tick.
> +	 */
> +	if (sched_feat(HZ_BW) && rq->nr_running == 1 && rq->curr
> +	    && rq->curr->sched_class == &fair_sched_class && task_on_rq_queued(rq->curr)) {
> +		if (sched_cfs_bandwidth_active(task_cfs_rq(rq->curr)))

Actually, something I should have noticed earlier is that this should
probably be hierarchical, right? You need to check every ancestor
cfs_rq, not just the immediate parent. And at that point it probably
makes sense to have sched_cfs_bandwidth_active take a task_struct.