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From: "T.J. Mercier" <tjmercier@google.com>
Date: Fri, 2 Feb 2024 14:29:17 -0800
Message-ID: <CABdmKX3_jCjZdOQeinKCKBS3m4XS8heE9WMDU-z1oFpCcPc5fg@mail.gmail.com>
Subject: Re: [PATCH v2] mm: memcg: Use larger batches for proactive reclaim
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@kernel.org>, 
	Roman Gushchin <roman.gushchin@linux.dev>, Shakeel Butt <shakeelb@google.com>, 
	Muchun Song <muchun.song@linux.dev>, Andrew Morton <akpm@linux-foundation.org>, 
	Efly Young <yangyifei03@kuaishou.com>, android-mm@google.com, yuzhao@google.com, 
	mkoutny@suse.com, cgroups@vger.kernel.org, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org
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On Fri, Feb 2, 2024 at 2:14=E2=80=AFPM Yosry Ahmed <yosryahmed@google.com> =
wrote:
>
> On Fri, Feb 2, 2024 at 2:10=E2=80=AFPM T.J. Mercier <tjmercier@google.com=
> wrote:
> >
> > Before 388536ac291 ("mm:vmscan: fix inaccurate reclaim during proactive
> > reclaim") we passed the number of pages for the reclaim request directl=
y
> > to try_to_free_mem_cgroup_pages, which could lead to significant
> > overreclaim. After 0388536ac291 the number of pages was limited to a
> > maximum 32 (SWAP_CLUSTER_MAX) to reduce the amount of overreclaim.
> > However such a small batch size caused a regression in reclaim
> > performance due to many more reclaim start/stop cycles inside
> > memory_reclaim.
> >
> > Reclaim tries to balance nr_to_reclaim fidelity with fairness across
> > nodes and cgroups over which the pages are spread. As such, the bigger
> > the request, the bigger the absolute overreclaim error. Historic
> > in-kernel users of reclaim have used fixed, small sized requests to
> > approach an appropriate reclaim rate over time. When we reclaim a user
> > request of arbitrary size, use decaying batch sizes to manage error whi=
le
> > maintaining reasonable throughput.
> >
> > root - full reclaim       pages/sec   time (sec)
> > pre-0388536ac291      :    68047        10.46
> > post-0388536ac291     :    13742        inf
> > (reclaim-reclaimed)/4 :    67352        10.51
> >
> > /uid_0 - 1G reclaim       pages/sec   time (sec)  overreclaim (MiB)
> > pre-0388536ac291      :    258822       1.12            107.8
> > post-0388536ac291     :    105174       2.49            3.5
> > (reclaim-reclaimed)/4 :    233396       1.12            -7.4
> >
> > /uid_0 - full reclaim     pages/sec   time (sec)
> > pre-0388536ac291      :    72334        7.09
> > post-0388536ac291     :    38105        14.45
> > (reclaim-reclaimed)/4 :    72914        6.96
> >
> > Fixes: 0388536ac291 ("mm:vmscan: fix inaccurate reclaim during proactiv=
e reclaim")
> > Signed-off-by: T.J. Mercier <tjmercier@google.com>
>
> LGTM with a nit below:
> Reviewed-by: Yosry Ahmed <yosryahmed@google.com>

Thanks

> >
> > ---
> > v2: Simplify the request size calculation per Johannes Weiner and Micha=
l Koutn=C3=BD
> >
> >  mm/memcontrol.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 46d8d02114cf..e6f921555e07 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -6965,6 +6965,9 @@ static ssize_t memory_reclaim(struct kernfs_open_=
file *of, char *buf,
> >         while (nr_reclaimed < nr_to_reclaim) {
> >                 unsigned long reclaimed;
> >
> > +               /* Will converge on zero, but reclaim enforces a minimu=
m */
> > +               unsigned long batch_size =3D (nr_to_reclaim - nr_reclai=
med) / 4;
> > +
> >                 if (signal_pending(current))
> >                         return -EINTR;
> >
> > @@ -6977,7 +6980,7 @@ static ssize_t memory_reclaim(struct kernfs_open_=
file *of, char *buf,
> >                         lru_add_drain_all();
> >
> >                 reclaimed =3D try_to_free_mem_cgroup_pages(memcg,
> > -                                       min(nr_to_reclaim - nr_reclaime=
d, SWAP_CLUSTER_MAX),
> > +                                       batch_size,
> >                                         GFP_KERNEL, reclaim_options);
>
> I think the above two lines should now fit into one.

It goes out to 81 characters. I wasn't brave enough, even though the
80 char limit is no more. :)

This takes it out to 100 but gets rid of batch_size if folks are ok with it=
:

                reclaimed =3D try_to_free_mem_cgroup_pages(memcg,
-                                       min(nr_to_reclaim -
nr_reclaimed, SWAP_CLUSTER_MAX),
+                                       /* Will converge on zero, but
reclaim enforces a minimum */
+                                       (nr_to_reclaim - nr_reclaimed) / 4,
                                        GFP_KERNEL, reclaim_options);