linux-kernel - Re: [PATCH v2 8/9] vmscan: memcg: sleep when flushing stats during reclaim

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Message-ID: <CAJD7tkaCK6e3a-T_5z3Ea3OFwiJjHoUfrEPGDW-wqduOr-5pVg@mail.gmail.com>
Date:   Thu, 30 Mar 2023 00:54:49 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Michal Hocko <mhocko@...e.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>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        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 v2 8/9] vmscan: memcg: sleep when flushing stats during reclaim

On Thu, Mar 30, 2023 at 12:52 AM Michal Hocko <mhocko@...e.com> wrote:
>
> On Thu 30-03-23 00:44:10, Yosry Ahmed wrote:
> > On Thu, Mar 30, 2023 at 12:40 AM Michal Hocko <mhocko@...e.com> wrote:
> > >
> > > On Tue 28-03-23 22:16:43, Yosry Ahmed 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.
> > >
> > > Why is this preferred? Memory reclaim is surely a slow path but what is
> > > the advantage of calling mem_cgroup_flush_stats here?
> >
> > The purpose of this series is to limit calls to atomic flushing as
> > much as possible, as flushing can become really expensive on systems
> > with high cpu counts and a lot of cgroups, and performing such an
> > expensive operation atomically causes problems -- so we'd rather avoid
> > doing it atomically where possible.
>
> Please add that to the changelog. While the intention might be obvious
> now (although cover is not explicit about it either) it can cause some
> head scratching in the future when somebody looks at this commit without
> a broader context (e.g. previous ML discussions).
>
> with that
> Acked-by: Michal Hocko <mhocko@...e.com>

Thanks, will do for the respin.

> Thanks
>
> > > > Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> > > > Acked-by: Shakeel Butt <shakeelb@...gle.com>
> > > > Acked-by: Johannes Weiner <hannes@...xchg.org>
> > > > ---
> > > >  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();
> > > >
> > > >       /*
> > > >        * Determine the scan balance between anon and file LRUs.
> > > > --
> > > > 2.40.0.348.gf938b09366-goog
> > >
> > > --
> > > Michal Hocko
> > > SUSE Labs
>
> --
> Michal Hocko
> SUSE Labs