Re: [PATCH v5 7/9] slub: Optimize deactivate_slab()

[Chengming Zhou <chengming.zhou@linux.dev>]

On 2023/12/3 19:19, Hyeonggon Yoo wrote:
> On Sun, Dec 3, 2023 at 7:26 PM Chengming Zhou <chengming.zhou@linux.dev> wrote:
>>
>> On 2023/12/3 17:23, Hyeonggon Yoo wrote:
>>> On Thu, Nov 2, 2023 at 12:25 PM <chengming.zhou@linux.dev> wrote:
>>>>
>>>> From: Chengming Zhou <zhouchengming@bytedance.com>
>>>>
>>>> Since the introduce of unfrozen slabs on cpu partial list, we don't
>>>> need to synchronize the slab frozen state under the node list_lock.
>>>>
>>>> The caller of deactivate_slab() and the caller of __slab_free() won't
>>>> manipulate the slab list concurrently.
>>>>
>>>> So we can get node list_lock in the last stage if we really need to
>>>> manipulate the slab list in this path.
>>>>
>>>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
>>>> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
>>>> Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
>>>> ---
>>>>  mm/slub.c | 79 ++++++++++++++++++-------------------------------------
>>>>  1 file changed, 26 insertions(+), 53 deletions(-)
>>>>
>>>> diff --git a/mm/slub.c b/mm/slub.c
>>>> index bcb5b2c4e213..d137468fe4b9 100644
>>>> --- a/mm/slub.c
>>>> +++ b/mm/slub.c
>>>> @@ -2468,10 +2468,8 @@ static void init_kmem_cache_cpus(struct kmem_cache *s)
>>>>  static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
>>>>                             void *freelist)
>>>>  {
>>>> -       enum slab_modes { M_NONE, M_PARTIAL, M_FREE, M_FULL_NOLIST };
>>>>         struct kmem_cache_node *n = get_node(s, slab_nid(slab));
>>>>         int free_delta = 0;
>>>> -       enum slab_modes mode = M_NONE;
>>>>         void *nextfree, *freelist_iter, *freelist_tail;
>>>>         int tail = DEACTIVATE_TO_HEAD;
>>>>         unsigned long flags = 0;
>>>> @@ -2509,65 +2507,40 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
>>>>         /*
>>>>          * Stage two: Unfreeze the slab while splicing the per-cpu
>>>>          * freelist to the head of slab's freelist.
>>>> -        *
>>>> -        * Ensure that the slab is unfrozen while the list presence
>>>> -        * reflects the actual number of objects during unfreeze.
>>>> -        *
>>>> -        * We first perform cmpxchg holding lock and insert to list
>>>> -        * when it succeed. If there is mismatch then the slab is not
>>>> -        * unfrozen and number of objects in the slab may have changed.
>>>> -        * Then release lock and retry cmpxchg again.
>>>>          */
>>>> -redo:
>>>> -
>>>> -       old.freelist = READ_ONCE(slab->freelist);
>>>> -       old.counters = READ_ONCE(slab->counters);
>>>> -       VM_BUG_ON(!old.frozen);
>>>> -
>>>> -       /* Determine target state of the slab */
>>>> -       new.counters = old.counters;
>>>> -       if (freelist_tail) {
>>>> -               new.inuse -= free_delta;
>>>> -               set_freepointer(s, freelist_tail, old.freelist);
>>>> -               new.freelist = freelist;
>>>> -       } else
>>>> -               new.freelist = old.freelist;
>>>> -
>>>> -       new.frozen = 0;
>>>> +       do {
>>>> +               old.freelist = READ_ONCE(slab->freelist);
>>>> +               old.counters = READ_ONCE(slab->counters);
>>>> +               VM_BUG_ON(!old.frozen);
>>>> +
>>>> +               /* Determine target state of the slab */
>>>> +               new.counters = old.counters;
>>>> +               new.frozen = 0;
>>>> +               if (freelist_tail) {
>>>> +                       new.inuse -= free_delta;
>>>> +                       set_freepointer(s, freelist_tail, old.freelist);
>>>> +                       new.freelist = freelist;
>>>> +               } else {
>>>> +                       new.freelist = old.freelist;
>>>> +               }
>>>> +       } while (!slab_update_freelist(s, slab,
>>>> +               old.freelist, old.counters,
>>>> +               new.freelist, new.counters,
>>>> +               "unfreezing slab"));
>>>>
>>>> +       /*
>>>> +        * Stage three: Manipulate the slab list based on the updated state.
>>>> +        */
>>>
>>> deactivate_slab() might unconsciously put empty slabs into partial list, like:
>>>
>>> deactivate_slab()                    __slab_free()
>>> cmpxchg(), slab's not empty
>>>                                                cmpxchg(), slab's empty
>>> and unfrozen
>>
>> Hi,
>>
>> Sorry, but I don't get it here how __slab_free() can see the slab empty,
>> since the slab is not empty from deactivate_slab() path, and it can't be
>> used by any CPU at that time?
> 
> The scenario is CPU B previously allocated an object from slab X, but
> put it into node partial list and then CPU A have taken slab X into cpu slab.
> 
> While slab X is CPU A's cpu slab, when CPU B frees an object from slab X,
> it puts the object into slab X's freelist using cmpxchg.
> 
> Let's say in CPU A the deactivation path performs cmpxchg and X.inuse was 1,
> and then CPU B frees (__slab_free()) to slab X's freelist using cmpxchg,
> _before_ slab X's put into partial list by CPU A.
> 
> Then CPU A thinks it's not empty so put it into partial list, but by CPU B
> the slab has become empty.
> 
> Maybe I am confused, in that case please tell me I'm wrong :)
> 

Ah, you're right! I misunderstood the slab "empty" with "full". :)

Yes, in this case the "empty" slab would be put into the node partial list,
and it should be fine in the real world as you noted earlier.

Thanks!