Re: [PATCH v6 42/45] mm: shrinker: make global slab shrink lockless

[Qi Zheng <zhengqi.arch@bytedance.com>]

Hi,

On 2023/12/6 15:47, Lai Jiangshan wrote:
> On Tue, Sep 12, 2023 at 9:57 PM Qi Zheng <zhengqi.arch@bytedance.com> wrote:
> 
>> -       if (!down_read_trylock(&shrinker_rwsem))
>> -               goto out;
>> -
>> -       list_for_each_entry(shrinker, &shrinker_list, list) {
>> +       /*
>> +        * lockless algorithm of global shrink.
>> +        *
>> +        * In the unregistration setp, the shrinker will be freed asynchronously
>> +        * via RCU after its refcount reaches 0. So both rcu_read_lock() and
>> +        * shrinker_try_get() can be used to ensure the existence of the shrinker.
>> +        *
>> +        * So in the global shrink:
>> +        *  step 1: use rcu_read_lock() to guarantee existence of the shrinker
>> +        *          and the validity of the shrinker_list walk.
>> +        *  step 2: use shrinker_try_get() to try get the refcount, if successful,
>> +        *          then the existence of the shrinker can also be guaranteed,
>> +        *          so we can release the RCU lock to do do_shrink_slab() that
>> +        *          may sleep.
>> +        *  step 3: *MUST* to reacquire the RCU lock before calling shrinker_put(),
>> +        *          which ensures that neither this shrinker nor the next shrinker
>> +        *          will be freed in the next traversal operation.
> 
> Hello, Qi, Andrew, Paul,
> 
> I wonder know how RCU can ensure the lifespan of the next shrinker.
> it seems it is diverged from the common pattern usage of RCU+reference.
> 
> cpu1:
> rcu_read_lock();
> shrinker_try_get(this_shrinker);
> rcu_read_unlock();
>      cpu2: shrinker_free(this_shrinker);
>      cpu2: shrinker_free(next_shrinker); and free the memory of next_shrinker
>      cpu2: when shrinker_free(next_shrinker), no one updates this_shrinker's next
>      cpu2: since this_shrinker has been removed first.

No, this_shrinker will not be removed from the shrinker_list until the
last refcount is released. See below:

> rcu_read_lock();
> shrinker_put(this_shrinker);

	CPU 1                                      CPU 2

   --> if (refcount_dec_and_test(&shrinker->refcount))
		complete(&shrinker->done);

				wait_for_completion(&shrinker->done);
                                 list_del_rcu(&shrinker->list);

> travel to the freed next_shrinker.
> 
> a quick simple fix:
> 
> // called with other references other than RCU (i.e. refcount)
> static inline rcu_list_deleted(struct list_head *entry)
> {
>     // something like this:
>     return entry->prev == LIST_POISON2;
> }
> 
> // in the loop
> if (rcu_list_deleted(&shrinker->list)) {
>     shrinker_put(shrinker);
>     goto restart;
> }
> rcu_read_lock();
> shrinker_put(shrinker);
> 
> Thanks
> Lai
> 
>> +        *  step 4: do shrinker_put() paired with step 2 to put the refcount,
>> +        *          if the refcount reaches 0, then wake up the waiter in
>> +        *          shrinker_free() by calling complete().
>> +        */
>> +       rcu_read_lock();
>> +       list_for_each_entry_rcu(shrinker, &shrinker_list, list) {
>>                  struct shrink_control sc = {
>>                          .gfp_mask = gfp_mask,
>>                          .nid = nid,
>>                          .memcg = memcg,
>>                  };
>>
>> +               if (!shrinker_try_get(shrinker))
>> +                       continue;
>> +
>> +               rcu_read_unlock();
>> +
>>                  ret = do_shrink_slab(&sc, shrinker, priority);
>>                  if (ret == SHRINK_EMPTY)
>>                          ret = 0;
>>                  freed += ret;
>> -               /*
>> -                * Bail out if someone want to register a new shrinker to
>> -                * prevent the registration from being stalled for long periods
>> -                * by parallel ongoing shrinking.
>> -                */
>> -               if (rwsem_is_contended(&shrinker_rwsem)) {
>> -                       freed = freed ? : 1;
>> -                       break;
>> -               }
>> +
>> +               rcu_read_lock();
>> +               shrinker_put(shrinker);
>>          }
>>