linux-kernel - Re: [RFC PATCH] memcg: use root_mem

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Message-ID: <YwRjyx6wFLk8WTDe@dhcp22.suse.cz>
Date:   Tue, 23 Aug 2022 07:21:15 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Zhaoyang Huang <huangzhaoyang@...il.com>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, Tejun Heo <tj@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        "zhaoyang.huang" <zhaoyang.huang@...soc.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>, Ke Wang <ke.wang@...soc.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [RFC PATCH] memcg: use root_mem_cgroup when css is inherited

On Tue 23-08-22 10:31:57, Zhaoyang Huang wrote:
> On Mon, Aug 22, 2022 at 7:31 PM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Fri 19-08-22 07:10:26, Tejun Heo wrote:
> > > On Fri, Aug 19, 2022 at 10:08:59AM -0700, Shakeel Butt wrote:
> > > > On Fri, Aug 19, 2022 at 9:29 AM Tejun Heo <tj@...nel.org> wrote:
> > > > >
> > > > > On Fri, Aug 19, 2022 at 07:29:22PM +0800, zhaoyang.huang wrote:
> > > > > > From: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> > > > > >
> > > > > > It is observed in android system where per-app cgroup is demanded by freezer
> > > > > > subsys and part of groups require memory control. The hierarchy could be simplized
> > > > > > as bellowing where memory charged on group B abserved while we only want have
> > > > > > group E's memory be controlled and B's descendants compete freely for memory.
> > > > > > This should be the consequences of unified hierarchy.
> > > > > > Under this scenario, less efficient memory reclaim is observed when comparing
> > > > > > with no memory control. It is believed that multi LRU scanning introduces some
> > > > > > of the overhead. Furthermore, page thrashing is also heavier than global LRU
> > > > > > which could be the consequences of partial failure of WORKINGSET mechanism as
> > > > > > LRU is too short to protect the active pages.
> > > > > >
> > > > > > A(subtree_control = memory) - B(subtree_control = NULL) - C()
> > > > > >                                                       \ D()
> > > > > >                           - E(subtree_control = memory) - F()
> > > > > >                                                         \ G()
> > > > > >
> > > > > > Signed-off-by: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> > > > >
> > > > > Just in case it wasn't clear.
> > > > >
> > > > > Nacked-by: Tejun Heo <tj@...nel.org>
> > > > >
> > > > > Thanks.
> > > > >
> > > >
> > > > Was there a previous discussion on this? The commit message is unreadable.
> > >
> > > http://lkml.kernel.org/r/1660298966-11493-1-git-send-email-zhaoyang.huang@unisoc.com
> >
> > Even that discussion doesn't really explain the real underlying problem.
> > There are statements about inefficiency and trashing without any further
> > details or clarifications.
> I would like to quote the comments from google side for more details
> which can also be observed from different vendors.
> "Also be advised that when you enable memcg v2 you will be using
> per-app memcg configuration which implies noticeable overhead because
> every app will have its own group. For example pagefault path will
> regress by about 15%. And obviously there will be some memory overhead
> as well. That's the reason we don't enable them in Android by
> default."

This should be reported and investigated. Because per-application memcg
vs. memcg in general shouldn't make much of a difference from the
performance side. I can see a potential performance impact for no-memcg
vs. memcg case but even then 15% is quite a lot.

> > My very vague understanding is that the Android system would like to
> > freeze specific applications and for that it requires each application
> > to live in its own cgroup. This clashes with a requirement to age and
> > reclaim memory on a different granularity (aka no per process reclaim).
> > So in fact something that cgroup v1 would achieve by having 2
> > hierarchies, one for the freezer which would have a dedicated cgroup for
> > each application and the other for the memory controller where tasks are
> > grouped by a different criteria. This would rule out that a global (or
> > any external memory pressure) reclaim would age LRUs that contain a mix
> > bag of application pages rather than iterate over per-application LRUs.
> > Is that understanding correct?
> Correct, this is just our confusion. Besides, we believe that charge
> the pages to implicit memory enabled parent control group doesn't make
> sense as the memory cannot be managed at all.

I do not get that part. The parent can manange and control the memory
usage so how come it cannot be managed at all?
-- 
Michal Hocko
SUSE Labs