linux-kernel - Re: [PATCH] mm/memcontrol.c: drains percpu charge caches in memory.reclaim

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Date:   Fri, 11 Nov 2022 11:08:44 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Lu Jialin <lujialin4@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm/memcontrol.c: drains percpu charge caches in
 memory.reclaim

On Thu, Nov 10, 2022 at 11:35:34AM -0800, Yosry Ahmed <yosryahmed@...gle.com> wrote:
> OTOH, it will reduce the page counters, so if userspace is relying on
> memory.current to gauge how much reclaim they want to do, it will make
> it "appear" like the usage dropped.

Assuming memory.current is used to drive the proactive reclaim, then
this patch makes some sense (and is slightly better than draining upon
every memory.current read(2)).

I just think the commit message should explain the real mechanics of
this.

> The difference in perceived usage coming from draining the stock IIUC
> has an upper bound of 63 * PAGE_SIZE (< 256 KB with 4KB pages), I
> wonder if this is really significant anyway.

times nr_cpus (if memcg had stocks all over the place).

Michal

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