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

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Date:   Sat, 19 Feb 2022 15:46:16 +0000
From:   Aaron Tomlin <atomlin@...mlin.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        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 Fri 2022-02-18 12:54 +0000, Aaron Tomlin wrote:
> On Thu 2022-02-17 17:32 +0100, Frederic Weisbecker wrote:
> > > If I understand correctly, in the context of nohz_full, since such work is
> > > deferred, it will only be handled in a scenario when the periodic/or
> > > scheduling-clock tick is enabled i.e. the timer was reprogrammed on exit
> > > from idle.
> > 
> > Oh I see, it's a deferrable delayed work...
> > Then I can see two other issues:
> > 
> > 1) Can an interrupt in idle modify the vmstat and thus trigger the need to
> >    flush it? I believe it's the case and then the problem goes beyond nohz_full
> >    because if the idle interrupt fired while the tick is stopped and didn't set
> >    TIF_RESCHED, we go back to sleep without calling quiet_vmstat().
> 
> Yes: e.g. a nohz_full CPU, in idle code, could indeed receive a reschedule
> IPI; re-enable local IRQs and generic idle code sees the TIF_NEED_RESCHED
> flag against the idle task. Additionally, the selected task could
> indirectly released a few pages [to satisfy a low-memory condition] and
> modify CPU-specific vmstat data i.e. vm_stat_diff[NR_FREE_PAGES].
> 
> 
> > 2) What if we are running task A in kernel mode while the tick is stopped
> >    (nohz_full). Task A modifies the vmstat and goes to userspace for a long
> >    while.
> > Your patch fixes case 1) but not case 2). The problem is that TIMER_DEFERRABLE
> > should really be about dynticks-idle only and not dynticks-full. I've always
> > been afraid about enforcing that rule though because that would break old
> > noise-free setups. But perhaps I should...
> 
> If I understand correctly, I agree. For the latter case, nothing can be
> done unfortunately since the scheduling-clock tick is stopped.

Hi Frederic,

As far as I understand, in the context of nohz_full, options are indeed
limited; albeit, if we can ensure CPU-specific vmstat data is folded on
return to idle [when required] then this should be good enough.


Kind regards,

-- 
Aaron Tomlin