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References: <20230403220337.443510-1-yosryahmed@google.com> <20230403220337.443510-4-yosryahmed@google.com>
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From:   Shakeel Butt <shakeelb@google.com>
Date:   Thu, 20 Apr 2023 11:57:06 -0700
Message-ID: <CALvZod5WNiiV2NmikaGMPd62hiGtKWZrO0f2amk2HE8quBxkDQ@mail.gmail.com>
Subject: Re: [PATCH mm-unstable RFC 3/5] memcg: calculate root usage from
 global state
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <songmuchun@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, linux-mm@kvack.org
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On Mon, Apr 3, 2023 at 3:03=E2=80=AFPM Yosry Ahmed <yosryahmed@google.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@google.com>

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