linux-kernel - Re: [RFC PATCH 0/8] memcg: Enable fine-grained per process memory control

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Date:   Mon, 7 Sep 2020 12:47:45 +0100
From:   Chris Down <chris@...isdown.name>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Waiman Long <longman@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [RFC PATCH 0/8] memcg: Enable fine-grained per process memory
 control

Johannes Weiner writes:
>That all being said, the semantics of the new 'high' limit in cgroup2
>have allowed us to move reclaim/limit enforcement out of the
>allocation context and into the userspace return path.
>
>See the call to mem_cgroup_handle_over_high() from
>tracehook_notify_resume(), and the comments in try_charge() around
>set_notify_resume().
>
>This already solves the free->alloc ordering problem by allowing the
>allocation to exceed the limit temporarily until at least all locks
>are dropped, we know we can sleep etc., before performing enforcement.
>
>That means we may not need the timed sleeps anymore for that purpose,
>and could bring back directed waits for freeing-events again.
>
>What do you think? Any hazards around indefinite sleeps in that resume
>path? It's called before __rseq_handle_notify_resume and the
>arch-specific resume callback (which appears to be a no-op currently).
>
>Chris, Michal, what are your thoughts? It would certainly be simpler
>conceptually on the memcg side.

I'm not against that, although I personally don't feel very strongly about it 
either way, since the current behaviour clearly works in practice.