linux-kernel - Re: [PATCH v5 2/2] Sched/fair: Block nohz tick

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Date:   Tue, 11 Jul 2023 15:07:12 -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>,
        Frederic Weisbecker <frederic@...nel.org>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v5 2/2] Sched/fair: Block nohz tick_stop when cfs
 bandwidth in use

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

> On Tue, Jul 11, 2023 at 09:10:24AM -0400 Phil Auld wrote:
>> On Mon, Jul 10, 2023 at 04:54:58PM -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. 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@...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>
>> > > Cc: Frederic Weisbecker <frederic@...nel.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.
>> 
>
> Sigh, yeah. I took that out when I had the bit in the task. I'll put it
> back in...
>

Yeah, cfs_rq (and rt_rq) are set unconditionally, and a cgroup can have
a mix of fair and RT tasks (whether or not that's a good idea from a
sysadmin perspective).