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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Feb 24, 2022 at 09:27:14AM -0300, Marcelo Tosatti wrote:
> On Sat, Feb 19, 2022 at 03:46:16PM +0000, Aaron Tomlin wrote:
> > 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? 
> 
> Yes. Page allocation and page freeing for example.
> 
>    6   3730  ../mm/page_alloc.c <<rmqueue>>
>              __mod_zone_freepage_state(zone, -(1 << order),
>    4   1096  ../mm/page_alloc.c <<__free_one_page>
>              __mod_zone_freepage_state(zone, -(1 << order),
> 
> > > >    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 vmstat_updateas 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.
> 
> I suppose the desired behaviour, with the deferred timer for vmstat_sync, is:
> 
> "Allow the per-CPU vmstats to be out of sync, but for a maximum of 
> sysctl_stat_interval".
> 
> But Aaron, vmstat_shepherd should be ensuring that per-CPU vmstat_update
> work are queued, if the per-CPU vmstat are out of sync.
> 
> 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.

Maybe just switching to a non-deferrable timer does not increase the
frequency of vmstat_update calls so much ? It should happen once per
second anyway.

Then the "vmstats out of sync but for a maximum of sysctl_stat_interval"
would be respected, rather than existance of non-deferrable timers.