linux-kernel - Re: [PATCH RFC v2 02/10] mm: Make shrink

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Date:   Thu, 9 Aug 2018 13:58:16 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Michal Hocko <mhocko@...nel.org>, akpm@...ux-foundation.org,
        gregkh@...uxfoundation.org, rafael@...nel.org,
        viro@...iv.linux.org.uk, darrick.wong@...cle.com,
        paulmck@...ux.vnet.ibm.com, josh@...htriplett.org,
        rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, hughd@...gle.com, shuah@...nel.org,
        robh@...nel.org, ulf.hansson@...aro.org, aspriel@...il.com,
        vivek.gautam@...eaurora.org, robin.murphy@....com, joe@...ches.com,
        heikki.krogerus@...ux.intel.com, sfr@...b.auug.org.au,
        vdavydov.dev@...il.com, chris@...is-wilson.co.uk,
        aryabinin@...tuozzo.com, willy@...radead.org, ying.huang@...el.com,
        shakeelb@...gle.com, jbacik@...com, mingo@...nel.org,
        mhiramat@...nel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH RFC v2 02/10] mm: Make shrink_slab() lockless

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.