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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CALvZod7SdgXv0Dmm3q5H=EH4dzg8pXZenMUaDObfoRv5EX-Pkw@mail.gmail.com>
Date:   Fri, 15 May 2020 06:44:44 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Michal Hocko <mhocko@...nel.org>, Mel Gorman <mgorman@...e.de>,
        Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yafang Shao <laoar.shao@...il.com>,
        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 Fri, May 15, 2020 at 6:24 AM Johannes Weiner <hannes@...xchg.org> 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@...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.
> >
> > 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@...gle.com>
> > > > > Suggested-by: Johannes Weiner <hannes@...xchg.org>
> > > >
> > > > Acked-by: Johannes Weiner <hannes@...xchg.org>
> > >
> > > 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.
>
> 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.