linux-kernel - Re: [PATCH] memcg: sync flush only if periodic flush is delayed

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Date:   Sun, 6 Mar 2022 19:06:37 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Michal Koutný <mkoutny@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Ivan Babrou <ivan@...udflare.com>,
        Frank Hofmann <fhofmann@...udflare.com>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Daniel Dao <dqminh@...udflare.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] memcg: sync flush only if periodic flush is delayed

On Sun, Mar 6, 2022 at 6:44 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Fri,  4 Mar 2022 18:40:40 +0000 Shakeel Butt <shakeelb@...gle.com> wrote:
>
> > Daniel Dao has reported [1] a regression on workloads that may trigger
> > a lot of refaults (anon and file). The underlying issue is that flushing
> > rstat is expensive. Although rstat flush are batched with (nr_cpus *
> > MEMCG_BATCH) stat updates, it seems like there are workloads which
> > genuinely do stat updates larger than batch value within short amount of
> > time. Since the rstat flush can happen in the performance critical
> > codepaths like page faults, such workload can suffer greatly.
> >
> > This patch fixes this regression by making the rstat flushing
> > conditional in the performance critical codepaths. More specifically,
> > the kernel relies on the async periodic rstat flusher to flush the stats
> > and only if the periodic flusher is delayed by more than twice the
> > amount of its normal time window then the kernel allows rstat flushing
> > from the performance critical codepaths.
> >
> > Now the question: what are the side-effects of this change? The worst
> > that can happen is the refault codepath will see 4sec old lruvec stats
> > and may cause false (or missed) activations of the refaulted page which
> > may under-or-overestimate the workingset size. Though that is not very
> > concerning as the kernel can already miss or do false activations.
> >
> > There are two more codepaths whose flushing behavior is not changed by
> > this patch and we may need to come to them in future. One is the
> > writeback stats used by dirty throttling and second is the deactivation
> > heuristic in the reclaim. For now keeping an eye on them and if there is
> > report of regression due to these codepaths, we will reevaluate then.
> >
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> >
> > ...
> >
> > @@ -648,10 +652,16 @@ void mem_cgroup_flush_stats(void)
> >               __mem_cgroup_flush_stats();
> >  }
> >
> > +void mem_cgroup_flush_stats_delayed(void)
> > +{
> > +     if (rstat_flush_time && time_after64(jiffies_64, flush_next_time))
>
> rstat_flush_time isn't defined for me and my googling indicates this is
> the first time the symbol has been used in the history of the world.
> I'm stumped.
>

Oh sorry about that. I thought I renamed all instances of
"rstat_flush_time" to "flush_next_time" before sending out the email.
Please just remove "rstat_flush_time &&" from the if-condition.

thanks,
Shakeel