linux-kernel - Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker

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Date:   Tue, 3 Jul 2018 12:54:31 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Kirill Tkhai <ktkhai@...tuozzo.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Philippe Ombredanne <pombredanne@...b.com>,
        stummala@...eaurora.org, gregkh@...uxfoundation.org,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Roman Gushchin <guro@...com>, mka@...omium.org,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Chris Wilson <chris@...is-wilson.co.uk>, longman@...hat.com,
        Minchan Kim <minchan@...nel.org>,
        Huang Ying <ying.huang@...el.com>,
        Mel Gorman <mgorman@...hsingularity.net>, jbacik@...com,
        Guenter Roeck <linux@...ck-us.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, lirongqing@...du.com,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker

On Tue, Jul 3, 2018 at 12:25 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Tue, Jul 03, 2018 at 12:19:35PM -0700, Shakeel Butt wrote:
> > On Tue, Jul 3, 2018 at 12:13 PM Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
> > > > Do we really have so very many !memcg-aware shrinkers?
> > > >
> > > > $ git grep -w register_shrinker |wc
> > > >      32     119    2221
> > > > $ git grep -w register_shrinker_prepared |wc
> > > >       4      13     268
> > > > (that's an overstatement; one of those is the declaration, one the definition,
> > > > and one an internal call, so we actually only have one caller of _prepared).
> > > >
> > > > So it looks to me like your average system has one shrinker per
> > > > filesystem, one per graphics card, one per raid5 device, and a few
> > > > miscellaneous.  I'd be shocked if anybody had more than 100 shrinkers
> > > > registered on their laptop.
> > > >
> > > > I think we should err on the side of simiplicity and just have one IDR for
> > > > every shrinker instead of playing games to solve a theoretical problem.
> > >
> > > It just a standard situation for the systems with many containers. Every mount
> > > introduce a new shrinker to the system, so it's easy to see a system with
> > > 100 or ever 1000 shrinkers. AFAIR, Shakeel said he also has the similar
> > > configurations.
> > >
> >
> > I can say on our production systems, a couple thousand shrinkers is normal.
>
> But how many are !memcg aware?  It sounds to me like almost all of the
> shrinkers come through the sget_userns() caller, so the other shrinkers
> are almost irrelevant.

I would say almost half. Sorry I do not have exact numbers. Basically
we use ext4 very extensively and majority of shrinkers are related to
ext4 (again I do not have exact numbers). One ext4 mount typically
registers three shrinkers, one memcg-aware (sget) and two non-memcg
aware (ext4_es_register_shrinker, ext4_xattr_create_cache).

Shakeel