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

[Frederic Weisbecker <frederic@kernel.org>]

On Thu, Feb 17, 2022 at 02:26:15PM +0000, Aaron Tomlin wrote:
> On Thu 2022-02-17 13:47 +0100, Frederic Weisbecker wrote:
> > So, to make sure I understand, the issue is that with nohz_full, we may
> > well enter into the idle loop with the tick already stopped. We may also
> > exit from idle without restarting the tick (again only with nohz_full). And
> > so this can cause the vmstat to not be flushed upon idle entry. Right?
> 
> Hi Frederic,
> 
> Yes - this is exactly it.
> 
> > > A customer provided some evidence which indicates that the idle tick was
> > > stopped; albeit, CPU-specific vmstat counters still remained populated.
> > > Thus one can only assume quiet_vmstat() was not invoked on return to the
> > > idle loop.
> > > 
> > > Unfortunately, I suspect this divergence might erroneously prevent a
> > > reclaim attempt by kswapd. If the number of zone specific free pages are
> > > below their per-cpu drift value then zone_page_state_snapshot() is used to
> > > compute a more accurate view of the aforementioned statistic.
> > > Thus any task blocked on the NUMA node specific pfmemalloc_wait queue will
> > > be unable to make significant progress via direct reclaim unless it is
> > > killed after being woken up by kswapd (see throttle_direct_reclaim()).
> > > That being said, eventually reclaim should give up if the conditions are
> > > correct, no?
> 
> > Now if quiet_vmstat() isn't called, the vmstat_work should fix this later,
> > right? Or does that happen too late perhaps?
> 
> 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().

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...