linux-kernel - Re: [PATCH v2] mm/list_lru: Fix possible race in memcg_reparent_list_lru

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Message-ID: <YkVcbElWjomA7ofF@dhcp22.suse.cz>
Date:   Thu, 31 Mar 2022 09:46:52 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Waiman Long <longman@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v2] mm/list_lru: Fix possible race in
 memcg_reparent_list_lru_node()

On Thu 31-03-22 06:39:56, Shakeel Butt wrote:
> On Wed, Mar 30, 2022 at 07:48:45PM -0700, Roman Gushchin wrote:
> > 
> > 
> [...]
> > 
> > 
> > But honestly, I’d drop the original optimization together with
> > the fix, if only there is no _real world_ data on the problem and
> > the improvement. It seems like it has started as a nice simple
> > improvement, but the race makes it complex and probably not worth
> > the added complexity and fragility.
> 
> I agree with dropping the original optimization as it is not really
> fixing an observed issue which may justify adding some complexity.

Completely agreed. The patch as it is proposed is not really acceptable
IMHO and I have to say I am worried that this is not the first time we
are in a situation when a follow up fixes or unrelated patches are
growing in complexity to fit on top of a performance optimizations which
do not refer to any actual numbers.
-- 
Michal Hocko
SUSE Labs