linux-kernel - Re: [PATCH v2 3/3] mm/page_owner: Dump memcg information

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Date:   Wed, 2 Feb 2022 09:57:18 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Roman Gushchin <guro@...com>, Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Ira Weiny <ira.weiny@...el.com>,
        Rafael Aquini <aquini@...hat.com>
Subject: Re: [PATCH v2 3/3] mm/page_owner: Dump memcg information

On Tue 01-02-22 11:41:19, Waiman Long wrote:
> 
> On 2/1/22 05:49, Michal Hocko wrote:
[...]
> > Could you be more specific? Offlined memcgs are still part of the
> > hierarchy IIRC. So it shouldn't be much more than iterating the whole
> > cgroup tree and collect interesting data about dead cgroups.
> 
> What I mean is that without piggybacking on top of page_owner, we will to
> add a lot more code to collect and display those information which may have
> some overhead of its own.

Yes, there is nothing like a free lunch. Page owner is certainly a tool
that can be used. My main concern is that this tool doesn't really
scale on large machines with a lots of memory. It will provide a very
detailed information but I am not sure this is particularly helpful to
most admins (why should people process tons of allocation backtraces in
the first place). Wouldn't it be sufficient to have per dead memcg stats
to see where the memory sits?

Accumulated offline memcgs is something that bothers more people and I
am really wondering whether we can do more for those people to evaluate
the current state.
-- 
Michal Hocko
SUSE Labs