linux-kernel - Re: [PATCH v1 8/9] vmscan: memcg: sleep when flushing stats during reclaim

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Message-ID: <CALvZod7jhZCHXc1ShxHwhepAbhrdordFw4Wci8cSqtOxyQE6Gw@mail.gmail.com>
Date:   Tue, 28 Mar 2023 12:29:38 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Tejun Heo <tj@...nel.org>, Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Koutný <mkoutny@...e.com>,
        Vasily Averin <vasily.averin@...ux.dev>,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH v1 8/9] vmscan: memcg: sleep when flushing stats during reclaim

On Tue, Mar 28, 2023 at 12:02 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Tue, Mar 28, 2023 at 8:19 AM Shakeel Butt <shakeelb@...gle.com> wrote:
> >
> > On Mon, Mar 27, 2023 at 11:16 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> > >
> > > Memory reclaim is a sleepable context. Allow sleeping when flushing
> > > memcg stats to avoid unnecessarily performing a lot of work without
> > > sleeping. This can slow down reclaim code if flushing stats is taking
> > > too long, but there is already multiple cond_resched()'s in reclaim
> > > code.
> > >
> > > Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> >
> > Acked-by: Shakeel Butt <shakeelb@...gle.com>
> >
> > > ---
> > >  mm/vmscan.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index a9511ccb936f..9c1c5e8b24b8 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -2845,7 +2845,7 @@ static void prepare_scan_count(pg_data_t *pgdat, struct scan_control *sc)
> > >          * Flush the memory cgroup stats, so that we read accurate per-memcg
> > >          * lruvec stats for heuristics.
> > >          */
> > > -       mem_cgroup_flush_stats_atomic();
> > > +       mem_cgroup_flush_stats();
> >
> > I wonder if we should just replace this with
> > mem_cgroup_flush_stats_ratelimited().
>
> Thanks for taking a look!
>
> I was hesitant about doing this because the flush call is inside the
> retry loop, and it seems like we want to get fresh stats on each
> retry. It seems very likely that we end up not flushing between
> retries with mem_cgroup_flush_stats_ratelimited().
>
> Maybe change it if we observe problems with non-atomic flushing?

Yeah, let's leave it for the future if we see the issue.