Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230328221644.803272-1-yosryahmed@google.com>
 <20230328221644.803272-9-yosryahmed@google.com> <ZCU9ByZybEi5G5sl@dhcp22.suse.cz>
 <CAJD7tkZODvLZOfGaO3gjC2udKNg_G0mA2CT57djjJXrrHNEbbg@mail.gmail.com> <ZCU/zzgWSZb/ux6J@dhcp22.suse.cz>
In-Reply-To: <ZCU/zzgWSZb/ux6J@dhcp22.suse.cz>
From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Thu, 30 Mar 2023 00:54:49 -0700
Message-ID: <CAJD7tkaCK6e3a-T_5z3Ea3OFwiJjHoUfrEPGDW-wqduOr-5pVg@mail.gmail.com>
Subject: Re: [PATCH v2 8/9] vmscan: memcg: sleep when flushing stats during reclaim
To:     Michal Hocko <mhocko@suse.com>
Cc:     Tejun Heo <tj@kernel.org>, Josef Bacik <josef@toxicpanda.com>,
        Jens Axboe <axboe@kernel.dk>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <muchun.song@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>,
        Vasily Averin <vasily.averin@linux.dev>,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        bpf@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Thu, Mar 30, 2023 at 12:52=E2=80=AFAM Michal Hocko <mhocko@suse.com> wro=
te:
>
> On Thu 30-03-23 00:44:10, Yosry Ahmed wrote:
> > On Thu, Mar 30, 2023 at 12:40=E2=80=AFAM Michal Hocko <mhocko@suse.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 taki=
ng
> > > > 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@suse.com>

Thanks, will do for the respin.

> Thanks
>
> > > > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> > > > Acked-by: Shakeel Butt <shakeelb@google.com>
> > > > Acked-by: Johannes Weiner <hannes@cmpxchg.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 *pgd=
at, struct scan_control *sc)
> > > >        * Flush the memory cgroup stats, so that we read accurate pe=
r-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