From mboxrd@z Thu Jan  1 00:00:00 1970
From: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
Subject: Re: [PATCH v2] mm: memcg: use rstat for non-hierarchical stats
Date: Wed, 26 Jul 2023 11:20:55 -0400
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To: Yosry Ahmed <yosryahmed-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Cc: Roman Gushchin <roman.gushchin-fxUVXftIFDnyG1zEObXtfA@public.gmane.org>, Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Muchun Song <muchun.song-fxUVXftIFDnyG1zEObXtfA@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org

On Tue, Jul 25, 2023 at 07:20:02PM -0700, Yosry Ahmed wrote:
> On Tue, Jul 25, 2023 at 7:15=E2=80=AFPM Roman Gushchin <roman.gushchin-fx=
UVXftIFDnyG1zEObXtfA@public.gmane.org> wrote:
> >
> > On Wed, Jul 26, 2023 at 12:29:04AM +0000, Yosry Ahmed wrote:
> > > Currently, memcg uses rstat to maintain hierarchical stats. Counters =
are
> > > maintained for hierarchical stats at each memcg. Rstat tracks which
> > > cgroups have updates on which cpus to keep those counters fresh on the
> > > read-side.
> > >
> > > For non-hierarchical stats, we do not maintain counters. Instead, the
> >                                                 global?
>=20
> Do you mean "we do not maintain global counters"? I think "global" is
> confusing, because it can be thought of as all cpus or as including
> the subtree (as opposed to local for non-hierarchical stats).

"global" seems fine to me, I don't think it's ambiguous in the direct
comparison with per-cpu counts.

Alternatively, rephrase the whole thing? Something like:

"Non-hierarchical stats are currently not covered by rstat. Their
per-cpu counters are summed up on every read, which is expensive."