linux-kernel - Re: [PATCH] memcg: async flush memcg stats from perf sensitive codepaths

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Date:   Tue, 1 Mar 2022 18:57:03 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Ivan Babrou <ivan@...udflare.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Dao <dqminh@...udflare.com>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] memcg: async flush memcg stats from perf sensitive
 codepaths

Making decisions based on up to 2 s old information.

On Tue, Mar 01, 2022 at 09:21:12AM -0800, Shakeel Butt <shakeelb@...gle.com> wrote:
> Without flushing the worst that can happen in the refault path is
> false (or missed) activations of the refaulted page.

Yeah, this may under- or overestimate workingset size (when it's
changing), the result is likely only less efficient reclaim.

> For reclaim code, some heuristics (like deactivating active LRU or
> cache-trim) may act on old information.

Here, I'd be more careful whether such a delay cannot introduce some
unstable behavior (permanent oscillation in the worst case).

> Now, coming to your question, yes, we can remove the flushing from
> these performance critical codepaths as the stats at most will be 2
> second old due to periodic flush. 

Another aspect is that people will notice and report such a narrowly
located performance regression more easily than reduced/less predictable
reclaim behavior. (IMO the former is better, OTOH, it can also be
interpreted that noone notices (is able to notice).)

Michal