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

[Aaron Tomlin <atomlin@redhat.com>]

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.


Kind regards,

-- 
Aaron Tomlin