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

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Message-ID: <20220217142615.xqtiydixvnumyvei@ava.usersys.com>
Date:   Thu, 17 Feb 2022 14:26:15 +0000
From:   Aaron Tomlin <atomlin@...mlin.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Aaron Tomlin <atomlin@...hat.com>,
        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 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.


Kind regards,

-- 
Aaron Tomlin