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Date:   Wed, 2 Feb 2022 07:54:48 -0800
From:   Roman Gushchin <guro@fb.com>
To:     Michal Hocko <mhocko@suse.com>
CC:     Waiman Long <longman@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Petr Mladek <pmladek@suse.com>,
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        <linux-mm@kvack.org>, Ira Weiny <ira.weiny@intel.com>,
        Rafael Aquini <aquini@redhat.com>
Subject: Re: [PATCH v2 3/3] mm/page_owner: Dump memcg information
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On Wed, Feb 02, 2022 at 09:57:18AM +0100, Michal Hocko wrote:
> 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.

Cgroup v2 has corresponding counters for years. Or do you mean something different?