From: Frederic Weisbecker <frederic@kernel.org>
To: Aaron Tomlin <atomlin@atomlin.com>
Cc: Aaron Tomlin <atomlin@redhat.com>,
Marcelo Tosatti <mtosatti@redhat.com>,
Christoph Lameter <cl@linux.com>,
tglx@linutronix.de, mingo@kernel.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Phil Auld <pauld@redhat.com>
Subject: Re: [RFC PATCH] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too
Date: Thu, 17 Feb 2022 17:32:05 +0100 [thread overview]
Message-ID: <20220217163205.GA748087@lothringen> (raw)
In-Reply-To: <20220217142615.xqtiydixvnumyvei@ava.usersys.com>
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...
next prev parent reply other threads:[~2022-02-17 16:32 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-03 21:43 [RFC PATCH] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too Aaron Tomlin
2022-02-03 22:22 ` Phil Auld
2022-02-16 14:34 ` Aaron Tomlin
2022-02-16 21:20 ` Phil Auld
2022-02-17 12:57 ` Frederic Weisbecker
2022-02-17 14:45 ` Aaron Tomlin
2022-02-17 12:47 ` Frederic Weisbecker
2022-02-17 14:26 ` Aaron Tomlin
2022-02-17 16:32 ` Frederic Weisbecker [this message]
2022-02-18 12:54 ` Aaron Tomlin
2022-02-19 15:46 ` Aaron Tomlin
2022-02-24 12:27 ` Marcelo Tosatti
2022-02-24 12:30 ` Marcelo Tosatti
2022-02-24 13:01 ` Aaron Tomlin
2022-02-24 12:37 ` Marcelo Tosatti
2022-02-24 13:00 ` Aaron Tomlin
2022-02-24 13:14 ` Marcelo Tosatti
2022-02-24 13:28 ` Aaron Tomlin
2022-02-24 13:40 ` Marcelo Tosatti
2022-02-24 13:44 ` Aaron Tomlin
2022-03-31 14:33 ` Aaron Tomlin
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20220217163205.GA748087@lothringen \
--to=frederic@kernel.org \
--cc=atomlin@atomlin.com \
--cc=atomlin@redhat.com \
--cc=cl@linux.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mingo@kernel.org \
--cc=mtosatti@redhat.com \
--cc=pauld@redhat.com \
--cc=tglx@linutronix.de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).