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

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Message-ID: <20190124152122.GG50184@devbig004.ftw2.facebook.com>
Date:   Thu, 24 Jan 2019 07:21:22 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Chris Down <chris@...isdown.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.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

Hello, Michal.

On Thu, Jan 24, 2019 at 09:22:52AM +0100, Michal Hocko wrote:
> I do not think we can do that for two reasons. It breaks the existing
> semantic userspace might depend on and more importantly this is not a
> correct behavior IMO.

This is a valid concern but I'll come back to this later.

> You have to realize that stats are hierarchical because that is how we
> account. Events represent a way to inform that something has happened at
> the specific level of the tree though. If you do not setup low/high/max

This isn't true.  e.g. cgroup.events's populated event is
hierarchical.  Everything in cgroup should be hierarchical by default.

> limit then you simply cannot expect to be informed those get hit because
> they cannot by definition. Or put it other way, if you are waiting for
> those events you really want to know the (sub)tree they happened and if
> you propagate the event up the hierarchy you have hard time to tell that
> (you would basically have to exclude all but the lowest one and that is
> an awkward semantic at best.

I don't think it's a good idea to argue this for each piece of
information.  Again, everything should be hierarchical unless there
are clear and strong reasons against; otherwise, we end up with random
mix of hierarchical and flat behaviors, something that we want to
avoid the most - remember .use_hierarchy?.

> Maybe we want to document this better but I do not see we are going to
> change the behavior.

I beg you to reconsider.  This was a clear oversight and the cgroup2
usage is still relatively limited.  We sure can add local-specific
counters if needed but must not mix local and hierarchical counters
without a clear way to tell what's what.

Thanks.

-- 
tejun