Re: [PATCH] memcg: expose root cgroup's memory.stat

[Roman Gushchin]

On Fri, May 15, 2020 at 06:44:44AM -0700, Shakeel Butt wrote:
> On Fri, May 15, 2020 at 6:24 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> >
> > On Fri, May 15, 2020 at 10:29:55AM +0200, Michal Hocko wrote:
> > > On Sat 09-05-20 07:06:38, Shakeel Butt wrote:
> > > > On Fri, May 8, 2020 at 2:44 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> > > > >
> > > > > On Fri, May 08, 2020 at 10:06:30AM -0700, Shakeel Butt wrote:
> > > > > > One way to measure the efficiency of memory reclaim is to look at the
> > > > > > ratio (pgscan+pfrefill)/pgsteal. However at the moment these stats are
> > > > > > not updated consistently at the system level and the ratio of these are
> > > > > > not very meaningful. The pgsteal and pgscan are updated for only global
> > > > > > reclaim while pgrefill gets updated for global as well as cgroup
> > > > > > reclaim.
> > > > > >
> > > > > > Please note that this difference is only for system level vmstats. The
> > > > > > cgroup stats returned by memory.stat are actually consistent. The
> > > > > > cgroup's pgsteal contains number of reclaimed pages for global as well
> > > > > > as cgroup reclaim. So, one way to get the system level stats is to get
> > > > > > these stats from root's memory.stat, so, expose memory.stat for the root
> > > > > > cgroup.
> > > > > >
> > > > > >       from Johannes Weiner:
> > > > > >       There are subtle differences between /proc/vmstat and
> > > > > >       memory.stat, and cgroup-aware code that wants to watch the full
> > > > > >       hierarchy currently has to know about these intricacies and
> > > > > >       translate semantics back and forth.
> > >
> > > Can we have those subtle differences documented please?
> > >
> > > > > >
> > > > > >       Generally having the fully recursive memory.stat at the root
> > > > > >       level could help a broader range of usecases.
> > > > >
> > > > > The changelog begs the question why we don't just "fix" the
> > > > > system-level stats. It may be useful to include the conclusions from
> > > > > that discussion, and why there is value in keeping the stats this way.
> > > > >
> > > >
> > > > Right. Andrew, can you please add the following para to the changelog?
> > > >
> > > > Why not fix the stats by including both the global and cgroup reclaim
> > > > activity instead of exposing root cgroup's memory.stat? The reason is
> > > > the benefit of having metrics exposing the activity that happens
> > > > purely due to machine capacity rather than localized activity that
> > > > happens due to the limits throughout the cgroup tree. Additionally
> > > > there are userspace tools like sysstat(sar) which reads these stats to
> > > > inform about the system level reclaim activity. So, we should not
> > > > break such use-cases.
> > > >
> > > > > > Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > > > > > Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > > >
> > > > > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > >
> > > > Thanks a lot.
> > >
> > > I was quite surprised that the patch is so simple TBH. For some reason
> > > I've still had memories that we do not account for root memcg (likely
> > > because mem_cgroup_is_root(memcg) bail out in the try_charge. But stats
> > > are slightly different here.
> >
> > Yep, we skip the page_counter for root, but keep in mind that cgroup1
> > *does* have a root-level memory.stat, so (for the most part) we've
> > been keeping consumer stats for the root level the whole time.
> >
> > > counters because they are not really all the same. E.g.
> > > - mem_cgroup_charge_statistics accounts for each memcg
> >
> > Yep, that's heritage from cgroup1.
> >
> > > - memcg_charge_kernel_stack relies on pages being associated with a
> > >   memcg and that in turn relies on __memcg_kmem_charge_page which bails
> > >   out on root memcg
> >
> > You're right. It should only bypass the page_counter, but still set
> > page->mem_cgroup = root_mem_cgroup, just like user pages.

What about kernel threads? We consider them belonging to the root memory
cgroup. Should their memory consumption being considered in root-level stats?

I'm not sure we really want it, but I guess we need to document how
kernel threads are handled.

> >
> > This counter also doesn't get exported on cgroup1, so it would indeed
> > be a new bug. It needs to be fixed before this patch here.
> >
> > > - memcg_charge_slab (NR_SLAB*) skips over root memcg as well
> >
> > Same thing with these two.
> 
> Yes, we skip page_counter for root but not the stats. I will go over
> all the stats and make sure we are not skipping the stats for root.