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Date:   Fri, 23 Jun 2023 15:59:47 -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>
Subject: Re: [PATCH] Sched/fair: Block nohz tick_stop when cfs bandwidth in
 use
Message-ID: <20230623195947.GC766130@lorien.usersys.redhat.com>
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On Fri, Jun 23, 2023 at 11:59:09AM -0700 Benjamin Segall wrote:
> Phil Auld <pauld@redhat.com> writes:
> 
> > On Thu, Jun 22, 2023 at 05:37:30PM -0400 Phil Auld wrote:
> >> On Thu, Jun 22, 2023 at 01:49:52PM -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. 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@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>
> >> > > ---
> >> > >  kernel/sched/fair.c     | 33 ++++++++++++++++++++++++++++++++-
> >> > >  kernel/sched/features.h |  2 ++
> >> > >  2 files changed, 34 insertions(+), 1 deletion(-)
> >> > >
> >> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> > > index 373ff5f55884..880eadfac330 100644
> >> > > --- a/kernel/sched/fair.c
> >> > > +++ b/kernel/sched/fair.c
> >> > > @@ -6139,6 +6139,33 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
> >> > >  	rcu_read_unlock();
> >> > >  }
> >> > >  
> >> > > +#ifdef CONFIG_NO_HZ_FULL
> >> > > +/* called from pick_next_task_fair() */
> >> > > +static void sched_fair_update_stop_tick(struct rq *rq, struct task_struct *p)
> >> > > +{
> >> > > +	struct cfs_rq *cfs_rq = task_cfs_rq(p);
> >> > > +	int cpu = cpu_of(rq);
> >> > > +
> >> > > +	if (!sched_feat(HZ_BW) || !cfs_bandwidth_used())
> >> > > +		return;
> >> > > +
> >> > > +	if (!tick_nohz_full_cpu(cpu))
> >> > > +		return;
> >> > > +
> >> > > +	if (rq->nr_running != 1 || !sched_can_stop_tick(rq))
> >> > > +		return;
> >> > > +
> >> > > +	/*
> >> > > +	 *  We know there is only one task runnable and we've just picked it. The
> >> > > +	 *  normal enqueue path will have cleared TICK_DEP_BIT_SCHED if we will
> >> > > +	 *  be otherwise able to stop the tick. Just need to check if we are using
> >> > > +	 *  bandwidth control.
> >> > > +	 */
> >> > > +	if (cfs_rq->runtime_enabled)
> >> > > +		tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_SCHED);
> >> > > +}
> >> > > +#endif
> >> > 
> >> > So from a CFS_BANDWIDTH pov runtime_enabled && nr_running == 1 seems
> >> > fine. But working around sched_can_stop_tick instead of with it seems
> >> > sketchy in general, and in an edge case like "migrate a task onto the
> >> > cpu and then off again" you'd get sched_update_tick_dependency resetting
> >> > the TICK_DEP_BIT and then not call PNT (ie a task wakes up onto this cpu
> >> > without preempting, and then another cpu goes idle and pulls it, causing
> >> > this cpu to go into nohz_full).
> >> > 
> >> 
> >> The information to make these tests is not available in sched_can_stop_tick.
> >> I did start there. When that is called, and we are likely to go nohz_full,
> >> curr is null so it's hard to find the right cfs_rq to make that
> >> runtime_enabled test against.  We could, maybe, plumb the task being enqueued
> >> in but it would not be valid for the dequeue path and would be a bit messy.
> >>
> >
> > Sorry, mispoke... rq->curr == rq-idle not null. But still we don't have
> > access to the task and its cfs_rq which will have runtime_enabled set.
> >
> 
> That is unfortunate. I suppose then you'd wind up needing both this
> extra bit in PNT to handle the switch into nr_running == 1 territory,
> and a "HZ_BW && nr_running == 1 && curr is fair && curr->on_rq &&
> curr->cfs_rq->runtime_enabled" check in sched_can_stop_tick to catch
> edge cases. (I think that would be sufficient, if an annoyingly long set
> of conditionals)
>

Right. That's more or less what the version I'm testing now does.

Thanks again.


Cheers,
Phil

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