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Date:   Tue, 11 Jul 2023 09:10:24 -0400
From:   Phil Auld <pauld@redhat.com>
To:     Benjamin Segall <bsegall@google.com>
Cc:     linux-kernel@vger.kernel.org, Juri Lelli <juri.lelli@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Frederic Weisbecker <frederic@kernel.org>,
        Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH v5 2/2] Sched/fair: Block nohz tick_stop when cfs
 bandwidth in use
Message-ID: <20230711131024.GA150804@lorien.usersys.redhat.com>
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On Mon, Jul 10, 2023 at 04:54:58PM -0700 Benjamin Segall wrote:
> Phil Auld <pauld@redhat.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. Currently, 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. We use cfs_b->hierarchical_quota to
> > determine if the task requires the tick.
> >
> > Add check in pick_next_task_fair() as well since that is where
> > we have a handle on the task that is actually going to be running.
> >
> > Add check in sched_can_stop_tick() to cover some edge cases such 
> > as nr_running going from 2->1 and the 1 remains the running task.
> >
> > Add sched_feat HZ_BW (off by default) to control the tick_stop
> > behavior.
> >
> > Signed-off-by: Phil Auld <pauld@redhat.com>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Vincent Guittot <vincent.guittot@linaro.org>
> > Cc: Juri Lelli <juri.lelli@redhat.com>
> > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> > Cc: Valentin Schneider <vschneid@redhat.com>
> > Cc: Ben Segall <bsegall@google.com>
> > Cc: Frederic Weisbecker <frederic@kernel.org>
> > ---
> >  kernel/sched/core.c     | 12 ++++++++++
> >  kernel/sched/fair.c     | 49 +++++++++++++++++++++++++++++++++++++++++
> >  kernel/sched/features.h |  2 ++
> >  kernel/sched/sched.h    |  1 +
> >  4 files changed, 64 insertions(+)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 1b214e10c25d..4b8534abdf4f 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1229,6 +1229,18 @@ 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.
> > +	 * This check prevents clearing the bit if a newly enqueued task here is
> > +	 * dequeued by migrating while the constrained task continues to run.
> > +	 * E.g. going from 2->1 without going through pick_next_task().
> > +	 */
> > +	if (sched_feat(HZ_BW) && rq->nr_running == 1 && task_on_rq_queued(rq->curr)) {
> > +		if (cfs_task_bw_constrained(rq->curr))
> > +			return false;
> > +	}
> > +
> 
> I think we still need the fair_sched_class check with the bit being on
> cfs_rq/tg rather than task.
> 

Is there a way a non-fair_sched_class task will be in a cfs_rq with
cfs_rq->runtime_enabled and/or cfs_b->hierarchical_quota set to non
RUNTIME_INF?  I suppose if they are stale and it's had its class changed?

That makes the condition pretty ugly but I can add that back if needed.


Thanks,
Phil


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