linux-kernel - Re: [PATCH v4 3/4] mm: memcg: let non-unified root stats flushes help unified flushes

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Message-ID: <ZPX6luPGqypp68+L@dhcp22.suse.cz>
Date:   Mon, 4 Sep 2023 17:41:10 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Yosry Ahmed <yosryahmed@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Ivan Babrou <ivan@...udflare.com>, Tejun Heo <tj@...nel.org>,
        Waiman Long <longman@...hat.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/4] mm: memcg: let non-unified root stats flushes
 help unified flushes

On Mon 04-09-23 17:29:14, Michal Koutny wrote:
> Hello.
> 
> On Mon, Sep 04, 2023 at 04:50:15PM +0200, Michal Hocko <mhocko@...e.com> wrote:
> > I have hard time to follow why we really want/need this. Does this cause
> > any observable changes to the behavior?
> 
> Behavior change depends on how much userspace triggers the root memcg
> flush, from nothing to effectively offloading flushing to userspace tasks.
> (Theory^^^)
> 
> It keeps stats_flush_threshold up to date representing global error
> estimate so that error-tolerant readers may save their time and it keeps
> the reasoning about the stats_flush_threshold effect simple.

So it also creates an undocumented but userspace visible behavior.
Something that userspace might start depending on, right?
-- 
Michal Hocko
SUSE Labs