linux-kernel - Re: [PATCH] mm: memcontrol: skip propagate percpu vmstat values to current memcg

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Message-ID: <20210120074956.GB9371@dhcp22.suse.cz>
Date:   Wed, 20 Jan 2021 08:49:56 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Muchun Song <songmuchun@...edance.com>, vdavydov.dev@...il.com,
        akpm@...ux-foundation.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: memcontrol: skip propagate percpu vmstat values to
 current memcg

On Tue 19-01-21 11:58:42, Johannes Weiner wrote:
> On Tue, Jan 19, 2021 at 01:27:44PM +0800, Muchun Song wrote:
> > The current memcg will be freed soon, so updating it's vmstat and
> > vmevent values is pointless. Just skip updating it.
> > 
> > Signed-off-by: Muchun Song <songmuchun@...edance.com>
> 
> Oof, that's pretty subtle! Somebody trying to refactor that code for
> other purposes may not immediately notice that optimization and add
> potentially tedious bugs.

Absolutely agreed!

> How much does this save? Cgroup creation and deletion isn't really
> considered a hot path. It takes global locks and such...

This is not the first time when an (micro)optimization is posted without
any data showing benefit or otherwise appealing justification. Sigh.

-- 
Michal Hocko
SUSE Labs