linux-kernel - Re: [PATCH] mm: memcg: provide accurate stats for userspace reads

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Message-ID: <CALvZod46Cz_=5UgiyAKM+VgKyk=KJCqDqXu91=9uHy7-2wk53g@mail.gmail.com>
Date:   Fri, 11 Aug 2023 19:29:14 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Muchun Song <muchun.song@...ux.dev>, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: memcg: provide accurate stats for userspace reads

On Fri, Aug 11, 2023 at 7:12 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
[...]
>
> I am worried that writing to a stat for flushing then reading will
> increase the staleness window which we are trying to reduce here.
> Would it be acceptable to add a separate interface to explicitly read
> flushed stats without having to write first? If the distinction
> disappears in the future we can just short-circuit both interfaces.

What is the acceptable staleness time window for your case? It is hard
to imagine that a write+read will always be worse than just a read.
Even the proposed patch can have an unintended and larger than
expected staleness window due to some processing on
return-to-userspace or some scheduling delay.