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From:   Shakeel Butt <shakeelb@google.com>
Date:   Fri, 11 Aug 2023 19:48:14 -0700
Message-ID: <CALvZod5fH9xu_+6x85K38f63GfKGWD1LqtD2R4d09xmDtLB7ew@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 7:36=E2=80=AFPM Yosry Ahmed <yosryahmed@google.com>=
 wrote:
>
> On Fri, Aug 11, 2023 at 7:29=E2=80=AFPM Shakeel Butt <shakeelb@google.com=
> wrote:
> >
> > 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.
>
> Maybe I am worrying too much, we can just go for writing to
> memory.stat for explicit stats refresh.
>
> Do we still want to go with the mutex approach Michal suggested for
> do_flush_stats() to support either waiting for ongoing flushes
> (mutex_lock) or skipping (mutex_trylock)?

I would say keep that as a separate patch.