linux-kernel - Re: [PATCH v2 2/2] mm: memcg: normalize the value passed into memcg_rstat

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Date:   Tue, 3 Oct 2023 08:53:25 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>,
        Michal Koutný <mkoutny@...e.com>,
        linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm: memcg: normalize the value passed into memcg_rstat_updated()

On Tue, Oct 3, 2023 at 6:13 AM Johannes Weiner <hannes@...xchg.org> wrote:
>
> On Fri, Sep 22, 2023 at 05:57:40PM +0000, Yosry Ahmed wrote:
> > memcg_rstat_updated() uses the value of the state update to keep track
> > of the magnitude of pending updates, so that we only do a stats flush
> > when it's worth the work. Most values passed into memcg_rstat_updated()
> > are in pages, however, a few of them are actually in bytes or KBs.
> >
> > To put this into perspective, a 512 byte slab allocation today would
> > look the same as allocating 512 pages. This may result in premature
> > flushes, which means unnecessary work and latency.
>
> Yikes.
>
> I'm somewhat less concerned about the performance as I am about the
> variance in flushing cost that could be quite difficult to pinpoint.
> IMO this is a correctness fix and a code cleanup, not a performance
> thing.

Agreed, the code right now has a subtle mistake.

>
> > Normalize all the state values passed into memcg_rstat_updated() to
> > pages. Round up non-zero sub-page to 1 page, because
> > memcg_rstat_updated() ignores 0 page updates.
> >
> > Fixes: 5b3be698a872 ("memcg: better bounds on the memcg stats updates")
> > Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
>
> Acked-by: Johannes Weiner <hannes@...xchg.org>

Thanks for taking a look!