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Date:   Mon, 4 Dec 2023 18:55:55 +0100
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Subject: Re: [PATCH v5 7/9] slub: Optimize deactivate_slab()
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To:     Hyeonggon Yoo <42.hyeyoo@gmail.com>, chengming.zhou@linux.dev
Cc:     cl@linux.com, penberg@kernel.org, rientjes@google.com,
        iamjoonsoo.kim@lge.com, akpm@linux-foundation.org,
        roman.gushchin@linux.dev, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Chengming Zhou <zhouchengming@bytedance.com>
References: <20231102032330.1036151-1-chengming.zhou@linux.dev>
 <20231102032330.1036151-8-chengming.zhou@linux.dev>
 <CAB=+i9RR-n4q5NU6LFqmhM8ys4kM0SPqwj0zYtr4twu=yYmPPA@mail.gmail.com>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <CAB=+i9RR-n4q5NU6LFqmhM8ys4kM0SPqwj0zYtr4twu=yYmPPA@mail.gmail.com>
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On 12/3/23 10:23, 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
>                                                spin_lock(&n->list_lock)
>                                                (slab's empty but not
> on partial list,
> 
> spin_unlock(&n->list_lock) and return)
> spin_lock(&n->list_lock)
> put slab into partial list
> spin_unlock(&n->list_lock)
> 
> IMHO it should be fine in the real world, but just wanted to
> mention as it doesn't seem to be intentional.

I've noticed it too during review, but then realized it's not a new
behavior, same thing could happen with deactivate_slab() already before the
series. Free slabs on partial list are supported, we even keep some
intentionally as long as "n->nr_partial < s->min_partial" (and that check is
racy too), so no need to try making this more strict.

> Otherwise it looks good to me!

Good enough for a reviewed-by? :)