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Date:   Tue, 3 Jul 2018 10:58:08 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
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Subject: Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker
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On Tue, Jul 03, 2018 at 06:46:57PM +0300, Kirill Tkhai wrote:
> shrinker_idr now contains only memcg-aware shrinkers, so all bits from memcg map
> may be potentially populated. In case of memcg-aware shrinkers and !memcg-aware
> shrinkers share the same numbers like you suggest, this will lead to increasing
> size of memcg maps, which is bad for memory consumption. So, memcg-aware shrinkers
> should to have its own IDR and its own numbers. The tricks like allocation big
> IDs for !memcg-aware shrinkers seem bad for me, since they make the code more
> complicated.

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.

> > This will actually reduce the size of each shrinker and be more
> > cache-efficient when calling the shrinkers.  I think we can also get
> > rid of the shrinker_rwsem eventually, but let's leave it for now.
> 
> This patchset does not make the cache-efficient bad, since without the patchset the situation
> is so bad, that it's just impossible to talk about the cache efficiently,
> so let's leave lockless iteration/etc for the future works.

The situation is that bad /for your use case/.  Not so much for others.
You're introducing additional complexity here, and it'd be nice if we
can remove some of the complexity that's already there.