linux-kernel - Re: [PATCH RFC 1/3] cgroup: list all subsystem states in debugfs files

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Message-ID: <20180813171119.GA24658@cmpxchg.org>
Date:   Mon, 13 Aug 2018 13:11:19 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Roman Gushchin <guro@...com>
Subject: Re: [PATCH RFC 1/3] cgroup: list all subsystem states in debugfs
 files

On Mon, Aug 13, 2018 at 06:48:42AM -0700, Tejun Heo wrote:
> Hello, Konstantin.
> 
> On Mon, Aug 13, 2018 at 09:58:05AM +0300, Konstantin Khlebnikov wrote:
> > After removing cgroup subsystem state could leak or live in background
> > forever because it is pinned by some reference. For example memory cgroup
> > could be pinned by pages in cache or tmpfs.
> > 
> > This patch adds common debugfs interface for listing basic state for each
> > controller. Controller could define callback for dumping own attributes.
> > 
> > In file /sys/kernel/debug/cgroup/<controller> each line shows state in
> > format: <common_attr>=<value>... [-- <controller_attr>=<value>... ]
> 
> Seems pretty useful to me.  Roman, Johannes, what do you guys think?

Generally I like the idea of having more introspection into offlined
cgroups, but I wonder if having only memory= and swap= could be a
little too terse to track down what exactly is pinning the groups.

Roman has more experience debugging these pileups, but it seems to me
that unless we add a breakdown off memory, and maybe make slabinfo
available for these groups, that in practice this might not provide
that much more insight than per-cgroup stat counters of dead children.