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From:   Shakeel Butt <shakeelb@google.com>
Date:   Fri, 11 Aug 2023 19:29:14 -0700
Message-ID: <CALvZod46Cz_=5UgiyAKM+VgKyk=KJCqDqXu91=9uHy7-2wk53g@mail.gmail.com>
Subject: Re: [PATCH] mm: memcg: provide accurate stats for userspace reads
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Michal Hocko <mhocko@suse.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        Muchun Song <muchun.song@linux.dev>, cgroups@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
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On Fri, Aug 11, 2023 at 7:12=E2=80=AFPM Yosry Ahmed <yosryahmed@google.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.