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

[Konstantin Khlebnikov]

On 13.08.2018 20:53, Roman Gushchin wrote:
> On Mon, Aug 13, 2018 at 01:11:19PM -0400, Johannes Weiner wrote:
> > 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?
>
> Totally agree with the idea and was about to suggest something similar.
>
> > 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.
>
> I agree here.

I don't think that we could cover all cases with single interface.

This debugfs just gives simple entry point for debugging:
- paths for guessing user
- pointers for looking with gdb via kcore
- inode numbers for page-types - see second and third patch

For slab: this could show one of remaining slab. Anyway each of them pins css.

> It's hard to say in advance what numbers are useful, so let's export
> these numbers, but also make the format more extendable, so we can
> easily add new information later. Maybe, something like:
>
> cgroup {
>    path = ...
>    ino = ...
>    main css {
>      refcnt = ...
>      key = value
>      ...
>    }
>    memcg css {
>      refcnt = ...
>      ...
>    }
>    some other controller css {
>    }
>    ...
> }
>
> Also, because we do batch charges, printing numbers without draining stocks
> is not that useful. All stats are also per-cpu cached, what adds some
> inaccuracy.

Seems too verbose. And one-line key=value format is more grep/awk friendly.

Anyway such extended debugging could be implemented as gdb plugin.
While simple list of pointers gives enough information for dumping structures
with gdb alone without extra plugins.

> Thanks!