linux-kernel - Re: [PATCH] mm: memcontrol: don't batch updates of local VM stats and events

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Message-ID: <CAHk-=wgaLQjZ8AZj76_cwvk_wLPJjr+Dc=Qvac_vHY2RruuBww@mail.gmail.com>
Date:   Tue, 28 May 2019 10:37:15 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel test robot <rong.a.chen@...el.com>, LKP <lkp@...org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...nel.org>, Roman Gushchin <guro@...com>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH] mm: memcontrol: don't batch updates of local VM stats and events

On Tue, May 28, 2019 at 9:00 AM Shakeel Butt <shakeelb@...gle.com> wrote:
>
> I was suspecting the following for-loop+atomic-add for the regression.

If I read the kernel test robot reports correctly, Johannes' fix patch
does fix the regression (well - mostly. The original reported
regression was 26%, and with Johannes' fix patch it was 3% - so still
a slight performance regression, but not nearly as bad).

> Why the above atomic-add is the culprit?

I think the problem with that one is that it's cross-cpu statistics,
so you end up with lots of cacheline bounces on the local counts when
you have lots of load.

But yes, the recursive updates still do show a small regression,
probably because there's still some overhead from the looping up in
the hierarchy. You still get *those* cacheline bounces, but now they
are limited to the upper hierarchies that only get updated at batch
time.

Johannes? Am I reading this right?

                   Linus