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

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Date:   Fri, 23 Jun 2023 11:59:09 -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] Sched/fair: Block nohz tick_stop when cfs bandwidth in use

Phil Auld <pauld@...hat.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@...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>
>> > > ---
>> > >  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)