linux-kernel - Re: [PATCH v2] mm, memcg: introduce memory.events.local

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Date:   Mon, 20 May 2019 13:05:28 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Vladimir Davydov <vdavydov.dev@...il.com>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Chris Down <chris@...isdown.name>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm, memcg: introduce memory.events.local

On Fri, May 17, 2019 at 05:18:18PM -0700, Shakeel Butt wrote:
> The memory controller in cgroup v2 exposes memory.events file for each
> memcg which shows the number of times events like low, high, max, oom
> and oom_kill have happened for the whole tree rooted at that memcg.
> Users can also poll or register notification to monitor the changes in
> that file. Any event at any level of the tree rooted at memcg will
> notify all the listeners along the path till root_mem_cgroup. There are
> existing users which depend on this behavior.
> 
> However there are users which are only interested in the events
> happening at a specific level of the memcg tree and not in the events in
> the underlying tree rooted at that memcg. One such use-case is a
> centralized resource monitor which can dynamically adjust the limits of
> the jobs running on a system. The jobs can create their sub-hierarchy
> for their own sub-tasks. The centralized monitor is only interested in
> the events at the top level memcgs of the jobs as it can then act and
> adjust the limits of the jobs. Using the current memory.events for such
> centralized monitor is very inconvenient. The monitor will keep
> receiving events which it is not interested and to find if the received
> event is interesting, it has to read memory.event files of the next
> level and compare it with the top level one. So, let's introduce
> memory.events.local to the memcg which shows and notify for the events
> at the memcg level.
> 
> Now, does memory.stat and memory.pressure need their local versions.
> IMHO no due to the no internal process contraint of the cgroup v2. The
> memory.stat file of the top level memcg of a job shows the stats and
> vmevents of the whole tree. The local stats or vmevents of the top level
> memcg will only change if there is a process running in that memcg but
> v2 does not allow that. Similarly for memory.pressure there will not be
> any process in the internal nodes and thus no chance of local pressure.
> 
> Signed-off-by: Shakeel Butt <shakeelb@...gle.com>

This looks reasonable to me. Thanks for working out a clear use case
and also addressing how it compares to the stats and pressure files.

Acked-by: Johannes Weiner <hannes@...xchg.org>