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Date:   Wed, 2 Feb 2022 17:38:07 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Roman Gushchin <guro@fb.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>,
        Steven Rostedt <rostedt@goodmis.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        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 02-02-22 07:54:48, Roman Gushchin wrote:
> 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?

Do we have anything more specific than nr_dying_descendants? I was
thinking about an interface which would provide paths and stats for dead
memcgs. But I have to confess I haven't really spent much time thinking
about how much work that would be. I am by no means against adding memcg
information to the page owner. I just think there must be a better way
to present resource consumption by dead memcgs.

-- 
Michal Hocko
SUSE Labs