linux-kernel - Re: [PATCH 3/3] mm: memcg: use non-unified stats flushing for userspace reads

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Message-ID: <ZOW2PZN8Sgqq6uR2@dhcp22.suse.cz>
Date:   Wed, 23 Aug 2023 09:33:17 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     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>,
        linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] mm: memcg: use non-unified stats flushing for
 userspace reads

On Tue 22-08-23 08:30:05, Yosry Ahmed wrote:
> On Tue, Aug 22, 2023 at 2:06 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Mon 21-08-23 20:54:58, Yosry Ahmed wrote:
[...]
> So to answer your question, I don't think a random user can really
> affect the system in a significant way by constantly flushing. In
> fact, in the test script (which I am now attaching, in case you're
> interested), there are hundreds of threads that are reading stats of
> different cgroups every 1s, and I don't see any negative effects on
> in-kernel flushers in this case (reclaimers).

I suspect you have missed my point. Maybe I am just misunderstanding
the code but it seems to me that the lock dropping inside
cgroup_rstat_flush_locked effectivelly allows unbounded number of
contenders which is really dangerous when it is triggerable from the
userspace. The number of spinners at a moment is always bound by the
number CPUs but depending on timing many potential spinners might be
cond_rescheded and the worst time latency to complete can be really
high. Makes more sense?
-- 
Michal Hocko
SUSE Labs