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Date:   Tue, 27 Mar 2018 12:18:50 +0300
From:   Vladimir Davydov <vdavydov.dev@gmail.com>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     viro@zeniv.linux.org.uk, hannes@cmpxchg.org, mhocko@kernel.org,
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Subject: Re: [PATCH 02/10] mm: Maintain memcg-aware shrinkers in
 mcg_shrinkers array
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On Mon, Mar 26, 2018 at 06:20:55PM +0300, Kirill Tkhai wrote:
> On 24.03.2018 21:45, Vladimir Davydov wrote:
> > On Wed, Mar 21, 2018 at 04:21:29PM +0300, Kirill Tkhai wrote:
> >> The patch introduces mcg_shrinkers array to keep memcg-aware
> >> shrinkers in order of their shrinker::id.
> >>
> >> This allows to access the shrinkers dirrectly by the id,
> >> without iteration over shrinker_list list.
> > 
> > Why don't you simply use idr instead of ida? With idr you wouldn't need
> > the array mapping shrinker id to shrinker ptr. AFAIU you need this
> > mapping to look up the shrinker by id in shrink_slab. The latter doesn't
> > seem to be a hot path so using idr there should be acceptable. Since we
> > already have shrinker_rwsem, which is taken for reading by shrink_slab,
> > we wouldn't even need any additional locking for it.
> 
> The reason is ida may allocate memory, and since list_lru_add() can't fail,
> we can't do that there. If we allocate all the ida memory at the time of
> memcg creation (i.e., preallocate it), this is not different to the way
> the bitmap makes.
> 
> While bitmap has the agvantage, since it's simplest data structure (while
> ida has some radix tree overhead).
> 
> Also, bitmap does not require a lock, there is single atomic operation
> to set or clear a bit, and it scales better, when anything.

I didn't mean the per-memcg bitmaps - I think it's OK to use plain
arrays for them and reallocate them with the aid of RCU.

What I actually mean is the mapping shrink_id => shrinker. AFAIU it
isn't accessed from list_lru, it is only needed to look up a shrinker
by id from shrink_slab(). The latter is rather a slow path so I think
we can use an IDR for this mapping instead of IDA + plain array.