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: <20230726002904.655377-1-yosryahmed@google.com>
 <20230726002904.655377-2-yosryahmed@google.com> <ZMCBzUH7qIdc3Y2X@P9FQF9L96D>
 <CAJD7tkZDpni+VM61i-jUgvn=TkZ5CySotTmUAFQPwMSjDfOEWQ@mail.gmail.com> <20230726152055.GC1365610@cmpxchg.org>
In-Reply-To: <20230726152055.GC1365610@cmpxchg.org>
From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Wed, 26 Jul 2023 08:23:28 -0700
Message-ID: <CAJD7tkb6SmCDLDSCciPfXGK7Z_zyr4vh_XKvhysL6pci88WjKw@mail.gmail.com>
Subject: Re: [PATCH v2] mm: memcg: use rstat for non-hierarchical stats
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Roman Gushchin <roman.gushchin@linux.dev>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <muchun.song@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-mm@kvack.org
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Wed, Jul 26, 2023 at 8:20=E2=80=AFAM Johannes Weiner <hannes@cmpxchg.org=
> wrote:
>
> 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@=
linux.dev> wrote:
> > >
> > > On Wed, Jul 26, 2023 at 12:29:04AM +0000, Yosry Ahmed wrote:
> > > > Currently, memcg uses rstat to maintain hierarchical stats. Counter=
s 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, t=
he
> > >                                                 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."

Rephrasing sounds good to me.

I will send a v3 with the correct commit log and collected Acks to
make Andrew's life easier.