linux-kernel - Re: [RFC PATCH] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

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Date:   Thu, 24 Feb 2022 10:14:58 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Aaron Tomlin <atomlin@...mlin.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Christoph Lameter <cl@...ux.com>, tglx@...utronix.de,
        mingo@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Phil Auld <pauld@...hat.com>
Subject: Re: [RFC PATCH] tick/sched: Ensure quiet_vmstat() is called when the
 idle tick was stopped too

On Thu, Feb 24, 2022 at 01:00:14PM +0000, Aaron Tomlin wrote:
> On Thu 2022-02-24 09:27 -0300, Marcelo Tosatti wrote:
> > But Aaron, vmstat_shepherd should be ensuring that per-CPU vmstat_update
> > work are queued, if the per-CPU vmstat are out of sync.
> 
> Hi Marcelo,
> 
> Yes, I agree; albeit, as far as I understand, in the context of a nohz_full
> CPU that has its scheduling-clock tick stopped, we cannot rely on any
> deferred work.
> 
> The purpose of my patch was to prevent a nohz_full CPU from entering idle
> state when CPU-specific vmstat data is non-zero.
> 
> > And:
> > 
> > static void
> > trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
> > {
> >         if (!is_timers_nohz_active())
> >                 return;
> > 
> >         /*
> >          * TODO: This wants some optimizing similar to the code below, but we
> >          * will do that when we switch from push to pull for deferrable timers.
> >          */
> >         if (timer->flags & TIMER_DEFERRABLE) {
> >                 if (tick_nohz_full_cpu(base->cpu))
> >                         wake_up_nohz_cpu(base->cpu);
> >                 return;
> >         }
> > 
> >  * @TIMER_DEFERRABLE: A deferrable timer will work normally when the
> >  * system is busy, but will not cause a CPU to come out of idle just
> >  * to service it; instead, the timer will be serviced when the CPU
> >  * eventually wakes up with a subsequent non-deferrable timer.
> > 
> > You'd want that vmstat_update to execute regardless of whether there are 
> > armed non-deferrable timers.
> > 
> > Should fix both 1 and 2 AFAICS.
> > 
> 
> If I understand correctly, you are suggesting to switch to a non-deferred
> timer for such work when the scheduling-clock tick is stopped? Indeed, it
> would address both scenarios yet I'm not sure we'd want that due to the
> performance impact which might be more than negligible.

Aaron,

If the per-CPU vmstat_update is limited to happen once per second, that
shouldnt be a significant performance impact?