linux-kernel - Re: [PATCH 2/2] mm: Consider subtrees in memory.events

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Message-ID: <20190128143033.GN50184@devbig004.ftw2.facebook.com>
Date:   Mon, 28 Jan 2019 06:30:33 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Chris Down <chris@...isdown.name>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, Roman Gushchin <guro@...com>,
        Dennis Zhou <dennis@...nel.org>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org, kernel-team@...com
Subject: Re: [PATCH 2/2] mm: Consider subtrees in memory.events

On Wed, Jan 23, 2019 at 05:31:44PM -0500, Chris Down wrote:
> memory.stat and other files already consider subtrees in their output,
> and we should too in order to not present an inconsistent interface.
> 
> The current situation is fairly confusing, because people interacting
> with cgroups expect hierarchical behaviour in the vein of memory.stat,
> cgroup.events, and other files. For example, this causes confusion when
> debugging reclaim events under low, as currently these always read "0"
> at non-leaf memcg nodes, which frequently causes people to misdiagnose
> breach behaviour. The same confusion applies to other counters in this
> file when debugging issues.
> 
> Aggregation is done at write time instead of at read-time since these
> counters aren't hot (unlike memory.stat which is per-page, so it does it
> at read time), and it makes sense to bundle this with the file
> notifications.
...
> Signed-off-by: Chris Down <chris@...isdown.name>
> Acked-by: Johannes Weiner <hannes@...xchg.org>

Acked-by: Tejun Heo <tj@...nel.org>

Michal has a valid counterpoint that this is a change in userland
visible behavior but to me this patch seems to be more of a bug fix
than anything else in that it's addressing an obvious inconsistency in
the interface.

Thanks.

-- 
tejun