linux-kernel - Re: [PATCH mm-unstable RFC 3/5] memcg: calculate root usage from global state

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Message-ID: <CALvZod5WNiiV2NmikaGMPd62hiGtKWZrO0f2amk2HE8quBxkDQ@mail.gmail.com>
Date:   Thu, 20 Apr 2023 11:57:06 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mm-unstable RFC 3/5] memcg: calculate root usage from
 global state

On Mon, Apr 3, 2023 at 3:03 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> Currently, we approximate the root usage by adding the memcg stats for
> anon, file, and conditionally swap (for memsw). To read the memcg stats
> we need to invoke an rstat flush. rstat flushes can be expensive, they
> scale with the number of cpus and cgroups on the system.
>
> mem_cgroup_usage() is called by memcg_events()->mem_cgroup_threshold()
> with irqs disabled, so such an expensive operation with irqs disabled
> can cause problems.
>
> Instead, approximate the root usage from global state. This is not 100%
> accurate, but the root usage has always been ill-defined anyway.
>
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>

Acked-by: Shakeel Butt <shakeelb@...gle.com>