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

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Message-ID: <qejogc2nxbcekhujsh56zlyqssgxolf7vboxwyr7dk7zjznhzy@yt7bqkxefjyp>
Date:   Tue, 3 Oct 2023 20:22:48 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>, 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 Fri, Sep 22, 2023 at 05:57:40PM +0000, Yosry Ahmed <yosryahmed@...gle.com> 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.
> 
> Normalize all the state values passed into memcg_rstat_updated() to
> pages.

I've dreamed about such normalization since error estimates were
introduced :-)

(As touched in the previous patch) it makes me wonder whether it makes
sense to add up state and event counters (apples and oranges).

Shouldn't with this approach events: a) have a separate counter, b)
wight with zero and rely on time-based flushing only?

Thanks,
Michal

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