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

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Date:   Sat, 9 May 2020 22:43:53 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>, Mel Gorman <mgorman@...e.de>,
        Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memcg: expose root cgroup's memory.stat

On Sat, May 9, 2020 at 10:06 PM Shakeel Butt <shakeelb@...gle.com> wrote:
>
> On Fri, May 8, 2020 at 2:44 PM Johannes Weiner <hannes@...xchg.org> 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.
> > >
> > >       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.
>

Acked-by: Yafang Shao <laoar.shao@...il.com>

> > > Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
> > > Suggested-by: Johannes Weiner <hannes@...xchg.org>
> >
> > Acked-by: Johannes Weiner <hannes@...xchg.org>
>
> Thanks a lot.



-- 
Thanks
Yafang