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

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Date:   Wed, 28 Jun 2023 20:53:42 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Benjamin Segall <bsegall@...gle.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

On Wed, Jun 28, 2023 at 02:42:16PM -0700 Benjamin Segall wrote:
> 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.
> 

Are you saying a child cfs_rq with a parent that has runtime_enabled could
itself not have runtime_enabled?   I may be missing something but I don't
see how that works.

account_cfs_rq_runtime() for example just looks at the immediate cfs_rq of
curr and bails if it does not have runtime_enabled. How could that task get
throttled if it exceeds some parent's limit?

Confused :)

Cheers,
Phil


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