linux-kernel - Re: [PATCH v2] mm: memcg: use rstat for non-hierarchical stats

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Message-ID: <20230726152055.GC1365610@cmpxchg.org>
Date:   Wed, 26 Jul 2023 11:20:55 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: memcg: use rstat for non-hierarchical stats

On Tue, Jul 25, 2023 at 07:20:02PM -0700, Yosry Ahmed wrote:
> On Tue, Jul 25, 2023 at 7:15 PM Roman Gushchin <roman.gushchin@...ux.dev> 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?
> 
> 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."