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

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Date:   Thu, 9 Mar 2023 16:32:03 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     Christian König <christian.koenig@....com>,
        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
Subject: Re: [PATCH v4 7/8] mm: vmscan: remove shrinker_rwsem from
 synchronize_shrinkers()

Hi Christian,

On 2023/3/9 16:11, Christian König wrote:
> Am 09.03.23 um 08:06 schrieb Qi Zheng:
>> 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.
>>  */
> 
> Yes your analyses is completely correct.
> 
> I just didn't wanted to add another SRCU into the critical code paths of 
> the TTM pool just for device hot plug when I have that functionality 
> already.
> 
> We just make sure that no shrinker is running in parallel with 
> destruction of the pool. Registering and unregistering is harmless.

That's great, thanks for confirming.

Thanks,
Qi

> 
> Regards,
> Christian.
> 
>>
>> + CC: Christian König :)
>>
>> Thanks,
>> Qi
> 
>