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Subject: Re: [PATCH RFC v2 02/10] mm: Make shrink_slab() lockless
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
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From:   Kirill Tkhai <ktkhai@virtuozzo.com>
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Date:   Thu, 9 Aug 2018 13:58:16 +0300
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On 09.08.2018 13:37, Tetsuo Handa wrote:
> On 2018/08/09 18:21, Kirill Tkhai wrote:
>> 2)SRCU. Pros are there are no the above problems; we will have completely unlocked and
>>   scalable shrink_slab(). We will also have a possibility to avoid unregistering delays,
>>   like I did for superblock shrinker. There will be full scalability.
>>   Cons is enabling SRCU.
>>
> 
> How unregistering delays can be avoided? Since you traverse all shrinkers
> using one shrinker_srcu, synchronize_srcu(&shrinker_srcu) will block
> unregistering threads until longest inflight srcu_read_lock() user calls
> srcu_read_unlock().

Yes, but we can do synchronize_srcu() from async work like I did for the further patches.
The only thing we need is to teach shrinker::count_objects() and shrinker::scan_objects()
be safe to be called on unregistering shrinker user. The next patches do this for superblock
shrinker.

> Unless you use per shrinker counter like below, I wonder whether
> unregistering delays can be avoided...
> 
>   https://marc.info/?l=linux-mm&m=151060909613004
>   https://marc.info/?l=linux-mm&m=151060909713005

I'm afraid these atomic_{inc,dec}(&shrinker->nr_active) may regulary drop CPU caches
on another CPUs on some workloads. Also, synchronize_rcu() is also a heavy delay.