linux-kernel - Re: [PATCH v4 7/8] mm: vmscan: remove shrinker_rwsem from synchronize

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Date:   Thu, 9 Mar 2023 15:06:25 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     Kirill Tkhai <tkhai@...ru>, akpm@...ux-foundation.org,
        hannes@...xchg.org, shakeelb@...gle.com, mhocko@...nel.org,
        roman.gushchin@...ux.dev, muchun.song@...ux.dev, david@...hat.com,
        shy828301@...il.com, rppt@...nel.org
Cc:     sultan@...neltoast.com, dave@...olabs.net,
        penguin-kernel@...ove.SAKURA.ne.jp, paulmck@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        christian.koenig@....com
Subject: Re: [PATCH v4 7/8] mm: vmscan: remove shrinker_rwsem from
 synchronize_shrinkers()

Hi Kirill,

On 2023/3/9 06:39, Kirill Tkhai wrote:
> On 07.03.2023 09:56, Qi Zheng wrote:
>> Now there are no readers of shrinker_rwsem, so
>> synchronize_shrinkers() does not need to hold the
>> writer of shrinker_rwsem to wait for all running
>> shinkers to complete, synchronize_srcu() is enough.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>> ---
>>   mm/vmscan.c | 8 ++------
>>   1 file changed, 2 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 7aaf6f94ac1b..ac7ab4aa344f 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -796,15 +796,11 @@ EXPORT_SYMBOL(unregister_shrinker);
>>   /**
>>    * synchronize_shrinkers - Wait for all running shrinkers to complete.
>>    *
>> - * This is equivalent to calling unregister_shrink() and register_shrinker(),
>> - * but atomically and with less overhead. This is useful to guarantee that all
>> - * shrinker invocations have seen an update, before freeing memory, similar to
>> - * rcu.
>> + * This is useful to guarantee that all shrinker invocations have seen an
>> + * update, before freeing memory.
>>    */
>>   void synchronize_shrinkers(void)
>>   {
>> -	down_write(&shrinker_rwsem);
>> -	up_write(&shrinker_rwsem);
>>   	atomic_inc(&shrinker_srcu_generation);
>>   	synchronize_srcu(&shrinker_srcu);
>>   }
> 
> Just curious, callers of synchronize_shrinkers() don't want to have parallel register_shrinker() and unregister_shrink() are completed?
> Here we only should wait for parallel shrink_slab(), correct?

I think yes.

The synchronize_shrinkers() is currently only used by TTM pool.

In TTM pool, a shrinker named "drm-ttm_pool" is registered, and
the scan_objects callback will pick an entry from its own shrinker_list:

ttm_pool_shrink
--> spin_lock(&shrinker_lock);
     pt = list_first_entry(&shrinker_list, typeof(*pt), shrinker_list);
     list_move_tail(&pt->shrinker_list, &shrinker_list);
     spin_unlock(&shrinker_lock);

These entries have been removed from shrinker_list before calling
synchronize_shrinkers():

ttm_pool_fini
--> ttm_pool_type_fini
     --> spin_lock(&shrinker_lock);
	list_del(&pt->shrinker_list);
	spin_unlock(&shrinker_lock);
     synchronize_shrinkers

So IIUC, we only need to wait for the parallel shrink_slab() here. Like
its comment says:

/* We removed the pool types from the LRU, but we need to also make sure
  * that no shrinker is concurrently freeing pages from the pool.
  */

+ CC: Christian König :)

Thanks,
Qi