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Message-ID: <af74599e-6384-4bcc-9773-d37b061eabdf@linux.dev>
Date: Sun, 3 Dec 2023 18:26:20 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc: vbabka@...e.cz, cl@...ux.com, penberg@...nel.org,
rientjes@...gle.com, iamjoonsoo.kim@....com,
akpm@...ux-foundation.org, roman.gushchin@...ux.dev,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [PATCH v5 7/9] slub: Optimize deactivate_slab()
On 2023/12/3 17:23, Hyeonggon Yoo wrote:
> On Thu, Nov 2, 2023 at 12:25 PM <chengming.zhou@...ux.dev> wrote:
>>
>> From: Chengming Zhou <zhouchengming@...edance.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@...edance.com>
>> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
>> Tested-by: Hyeonggon Yoo <42.hyeyoo@...il.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?
Thanks for review!
> 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.
>
> Otherwise it looks good to me!
>
>> if (!new.inuse && n->nr_partial >= s->min_partial) {
>> - mode = M_FREE;
>> + stat(s, DEACTIVATE_EMPTY);
>> + discard_slab(s, slab);
>> + stat(s, FREE_SLAB);
>> } else if (new.freelist) {
>> - mode = M_PARTIAL;
>> - /*
>> - * Taking the spinlock removes the possibility that
>> - * acquire_slab() will see a slab that is frozen
>> - */
>> spin_lock_irqsave(&n->list_lock, flags);
>> - } else {
>> - mode = M_FULL_NOLIST;
>> - }
>> -
>> -
>> - if (!slab_update_freelist(s, slab,
>> - old.freelist, old.counters,
>> - new.freelist, new.counters,
>> - "unfreezing slab")) {
>> - if (mode == M_PARTIAL)
>> - spin_unlock_irqrestore(&n->list_lock, flags);
>> - goto redo;
>> - }
>> -
>> -
>> - if (mode == M_PARTIAL) {
>> add_partial(n, slab, tail);
>> spin_unlock_irqrestore(&n->list_lock, flags);
>> stat(s, tail);
>> - } else if (mode == M_FREE) {
>> - stat(s, DEACTIVATE_EMPTY);
>> - discard_slab(s, slab);
>> - stat(s, FREE_SLAB);
>> - } else if (mode == M_FULL_NOLIST) {
>> + } else {
>> stat(s, DEACTIVATE_FULL);
>> }
>> }
>> --
>> 2.20.1
>>
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