linux-kernel - Re: [PATCH 1/7] mm: memcontrol: fix cpuhotplug statistics flushing

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Message-ID: <20210203022853.GG1812008@carbon.dhcp.thefacebook.com>
Date:   Tue, 2 Feb 2021 18:28:53 -0800
From:   Roman Gushchin <guro@...com>
To:     Johannes Weiner <hannes@...xchg.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Michal Hocko <mhocko@...e.com>,
        <linux-mm@...ck.org>, <cgroups@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH 1/7] mm: memcontrol: fix cpuhotplug statistics flushing

On Tue, Feb 02, 2021 at 03:07:47PM -0800, Roman Gushchin wrote:
> On Tue, Feb 02, 2021 at 01:47:40PM -0500, Johannes Weiner wrote:
> > The memcg hotunplug callback erroneously flushes counts on the local
> > CPU, not the counts of the CPU going away; those counts will be lost.
> > 
> > Flush the CPU that is actually going away.
> > 
> > Also simplify the code a bit by using mod_memcg_state() and
> > count_memcg_events() instead of open-coding the upward flush - this is
> > comparable to how vmstat.c handles hotunplug flushing.
> 
> To the whole series: it's really nice to have an accurate stats at
> non-leaf levels. Just as an illustration: if there are 32 CPUs and
> 1000 sub-cgroups (which is an absolutely realistic number, because
> often there are many dying generations of each cgroup), the error
> margin is 3.9GB. It makes all numbers pretty much random and all
> possible tests extremely flaky.

Btw, I was just looking into kmem kselftests failures/flakiness,
which is caused by exactly this problem: without waiting for the
finish of dying cgroups reclaim, we can't make any reliable assumptions
about what to expect from memcg stats.

So looking forward to have this patchset merged!