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From:   Shakeel Butt <shakeelb@google.com>
Date:   Fri, 11 Aug 2023 19:08:19 -0700
Message-ID: <CALvZod55S3XeK-MquTq0mDuipq8j0vFymQeX_XnPb_HuPK+oGQ@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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Hi all,

(sorry for late response as I was away)

On Fri, Aug 11, 2023 at 1:40=E2=80=AFPM Yosry Ahmed <yosryahmed@google.com>=
 wrote:
>
[...]
> > > >
> > > > Last note, for /proc/vmstat we have /proc/sys/vm/stat_refresh to tr=
igger
> > > > an explicit refresh. For those users who really need more accurate
> > > > numbers we might consider interface like that. Or allow to write to=
 stat
> > > > file and do that in the write handler.
> > >
> > > This wouldn't be my first option, but if that's the only way to get
> > > accurate stats I'll take it.
> >
> > To be honest, this would be my preferable option because of 2 reasons.
> > a) we do not want to guarantee to much on the precision front because
> > that would just makes maintainability much more harder with different
> > people having a different opinion of how much precision is enough and b=
)
> > it makes the more rare (need precise) case the special case rather than
> > the default.
>
> How about we go with the proposed approach in this patch (or the mutex
> approach as it's much cleaner), and if someone complains about slow
> reads we revert the change and introduce the refresh API? We might
> just get away with making all reads accurate and avoid the hassle of
> updating some userspace readers to do write-then-read. We don't know
> for sure that something will regress.
>
> What do you think?

Actually I am with Michal on this one. As I see multiple regression
reports for reading the stats, I am inclined towards rate limiting the
sync stats flushing from user readable interfaces (through
mem_cgroup_flush_stats_ratelimited()) and providing a separate
interface as suggested by Michal to explicitly flush the stats for
users ok with the cost. Since we flush the stats every 2 seconds, most
of the users should be fine and the users who care about accuracy can
pay for it.