Re: [PATCH v5 6/9] slub: Delay freezing of partial slabs

[Chengming Zhou]

On 2023/12/3 14:53, Hyeonggon Yoo wrote:
> On Thu, Nov 2, 2023 at 12:25 PM <chengming.zhou@xxxxxxxxx> wrote:
>>
>> From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
>>
>> Now we will freeze slabs when moving them out of node partial list to
>> cpu partial list, this method needs two cmpxchg_double operations:
>>
>> 1. freeze slab (acquire_slab()) under the node list_lock
>> 2. get_freelist() when pick used in ___slab_alloc()
>>
>> Actually we don't need to freeze when moving slabs out of node partial
>> list, we can delay freezing to when use slab freelist in ___slab_alloc(),
>> so we can save one cmpxchg_double().
>>
>> And there are other good points:
>>  - The moving of slabs between node partial list and cpu partial list
>>    becomes simpler, since we don't need to freeze or unfreeze at all.
>>
>>  - The node list_lock contention would be less, since we don't need to
>>    freeze any slab under the node list_lock.
>>
>> We can achieve this because there is no concurrent path would manipulate
>> the partial slab list except the __slab_free() path, which is now
>> serialized by slab_test_node_partial() under the list_lock.
>>
>> Since the slab returned by get_partial() interfaces is not frozen anymore
>> and no freelist is returned in the partial_context, so we need to use the
>> introduced freeze_slab() to freeze it and get its freelist.
>>
>> Similarly, the slabs on the CPU partial list are not frozen anymore,
>> we need to freeze_slab() on it before use.
>>
>> We can now delete acquire_slab() as it became unused.
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
>> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
>> Tested-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
>> ---
>>  mm/slub.c | 113 +++++++++++-------------------------------------------
>>  1 file changed, 23 insertions(+), 90 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index edf567971679..bcb5b2c4e213 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2234,51 +2234,6 @@ static void *alloc_single_from_new_slab(struct kmem_cache *s,
>>         return object;
>>  }
>>
>> -/*
>> - * Remove slab from the partial list, freeze it and
>> - * return the pointer to the freelist.
>> - *
>> - * Returns a list of objects or NULL if it fails.
>> - */
>> -static inline void *acquire_slab(struct kmem_cache *s,
>> -               struct kmem_cache_node *n, struct slab *slab,
>> -               int mode)
> 
> Nit: alloc_single_from_partial()'s comment still refers to acquire_slab().
> 

Ah, right! It should be changed to remove_partial().

diff --git a/mm/slub.c b/mm/slub.c
index 437485a2408d..623c17a4cdd6 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2463,7 +2463,7 @@ static inline void remove_partial(struct kmem_cache_node *n,
 }

 /*
- * Called only for kmem_cache_debug() caches instead of acquire_slab(), with a
+ * Called only for kmem_cache_debug() caches instead of remove_partial(), with a
  * slab from the n->partial list. Remove only a single object from the slab, do
  * the alloc_debug_processing() checks and leave the slab on the list, or move
  * it to full list if it was the last free object.

Hi Vlastimil, could you please help to fold it?

Thanks!

>> -{
>> -       void *freelist;
>> -       unsigned long counters;
>> -       struct slab new;
>> -
>> -       lockdep_assert_held(&n->list_lock);
>> -
>> -       /*
>> -        * Zap the freelist and set the frozen bit.
>> -        * The old freelist is the list of objects for the
>> -        * per cpu allocation list.
>> -        */
>> -       freelist = slab->freelist;
>> -       counters = slab->counters;
>> -       new.counters = counters;
>> -       if (mode) {
>> -               new.inuse = slab->objects;
>> -               new.freelist = NULL;
>> -       } else {
>> -               new.freelist = freelist;
>> -       }
>> -
>> -       VM_BUG_ON(new.frozen);
>> -       new.frozen = 1;
>> -
>> -       if (!__slab_update_freelist(s, slab,
>> -                       freelist, counters,
>> -                       new.freelist, new.counters,
>> -                       "acquire_slab"))
>> -               return NULL;
>> -
>> -       remove_partial(n, slab);
>> -       WARN_ON(!freelist);
>> -       return freelist;
>> -}
>> -
>>  #ifdef CONFIG_SLUB_CPU_PARTIAL
>>  static void put_cpu_partial(struct kmem_cache *s, struct slab *slab, int drain);
>>  #else
>> @@ -2295,7 +2250,6 @@ static struct slab *get_partial_node(struct kmem_cache *s,
>>                                      struct partial_context *pc)
>>  {
>>         struct slab *slab, *slab2, *partial = NULL;
>> -       void *object = NULL;
>>         unsigned long flags;
>>         unsigned int partial_slabs = 0;
>>
>> @@ -2314,7 +2268,7 @@ static struct slab *get_partial_node(struct kmem_cache *s,
>>                         continue;
>>
>>                 if (IS_ENABLED(CONFIG_SLUB_TINY) || kmem_cache_debug(s)) {
>> -                       object = alloc_single_from_partial(s, n, slab,
>> +                       void *object = alloc_single_from_partial(s, n, slab,
>>                                                         pc->orig_size);
>>                         if (object) {
>>                                 partial = slab;
>> @@ -2324,13 +2278,10 @@ static struct slab *get_partial_node(struct kmem_cache *s,
>>                         continue;
>>                 }
>>
>> -               object = acquire_slab(s, n, slab, object == NULL);
>> -               if (!object)
>> -                       break;
>> +               remove_partial(n, slab);
>>
>>                 if (!partial) {
>>                         partial = slab;
>> -                       pc->object = object;
>>                         stat(s, ALLOC_FROM_PARTIAL);
>>                 } else {
>>                         put_cpu_partial(s, slab, 0);
>> @@ -2629,9 +2580,6 @@ static void __unfreeze_partials(struct kmem_cache *s, struct slab *partial_slab)
>>         unsigned long flags = 0;
>>
>>         while (partial_slab) {
>> -               struct slab new;
>> -               struct slab old;
>> -
>>                 slab = partial_slab;
>>                 partial_slab = slab->next;
>>
>> @@ -2644,23 +2592,7 @@ static void __unfreeze_partials(struct kmem_cache *s, struct slab *partial_slab)
>>                         spin_lock_irqsave(&n->list_lock, flags);
>>                 }
>>
>> -               do {
>> -
>> -                       old.freelist = slab->freelist;
>> -                       old.counters = slab->counters;
>> -                       VM_BUG_ON(!old.frozen);
>> -
>> -                       new.counters = old.counters;
>> -                       new.freelist = old.freelist;
>> -
>> -                       new.frozen = 0;
>> -
>> -               } while (!__slab_update_freelist(s, slab,
>> -                               old.freelist, old.counters,
>> -                               new.freelist, new.counters,
>> -                               "unfreezing slab"));
>> -
>> -               if (unlikely(!new.inuse && n->nr_partial >= s->min_partial)) {
>> +               if (unlikely(!slab->inuse && n->nr_partial >= s->min_partial)) {
>>                         slab->next = slab_to_discard;
>>                         slab_to_discard = slab;
>>                 } else {
>> @@ -3167,7 +3099,6 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>>                         node = NUMA_NO_NODE;
>>                 goto new_slab;
>>         }
>> -redo:
>>
>>         if (unlikely(!node_match(slab, node))) {
>>                 /*
>> @@ -3243,7 +3174,8 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>>
>>  new_slab:
>>
>> -       if (slub_percpu_partial(c)) {
>> +#ifdef CONFIG_SLUB_CPU_PARTIAL
>> +       while (slub_percpu_partial(c)) {
>>                 local_lock_irqsave(&s->cpu_slab->lock, flags);
>>                 if (unlikely(c->slab)) {
>>                         local_unlock_irqrestore(&s->cpu_slab->lock, flags);
>> @@ -3255,12 +3187,22 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>>                         goto new_objects;
>>                 }
>>
>> -               slab = c->slab = slub_percpu_partial(c);
>> +               slab = slub_percpu_partial(c);
>>                 slub_set_percpu_partial(c, slab);
>>                 local_unlock_irqrestore(&s->cpu_slab->lock, flags);
>>                 stat(s, CPU_PARTIAL_ALLOC);
>> -               goto redo;
>> +
>> +               if (unlikely(!node_match(slab, node) ||
>> +                            !pfmemalloc_match(slab, gfpflags))) {
>> +                       slab->next = NULL;
>> +                       __unfreeze_partials(s, slab);
>> +                       continue;
>> +               }
>> +
>> +               freelist = freeze_slab(s, slab);
>> +               goto retry_load_slab;
>>         }
>> +#endif
>>
>>  new_objects:
>>
>> @@ -3268,8 +3210,8 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>>         pc.orig_size = orig_size;
>>         slab = get_partial(s, node, &pc);
>>         if (slab) {
>> -               freelist = pc.object;
>>                 if (kmem_cache_debug(s)) {
>> +                       freelist = pc.object;
>>                         /*
>>                          * For debug caches here we had to go through
>>                          * alloc_single_from_partial() so just store the
>> @@ -3281,6 +3223,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>>                         return freelist;
>>                 }
>>
>> +               freelist = freeze_slab(s, slab);
>>                 goto retry_load_slab;
>>         }
>>
>> @@ -3682,18 +3625,8 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>>                 was_frozen = new.frozen;
>>                 new.inuse -= cnt;
>>                 if ((!new.inuse || !prior) && !was_frozen) {
>> -
>> -                       if (kmem_cache_has_cpu_partial(s) && !prior) {
>> -
>> -                               /*
>> -                                * Slab was on no list before and will be
>> -                                * partially empty
>> -                                * We can defer the list move and instead
>> -                                * freeze it.
>> -                                */
>> -                               new.frozen = 1;
>> -
>> -                       } else { /* Needs to be taken off a list */
>> +                       /* Needs to be taken off a list */
>> +                       if (!kmem_cache_has_cpu_partial(s) || prior) {
>>
>>                                 n = get_node(s, slab_nid(slab));
>>                                 /*
>> @@ -3723,9 +3656,9 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>>                          * activity can be necessary.
>>                          */
>>                         stat(s, FREE_FROZEN);
>> -               } else if (new.frozen) {
>> +               } else if (kmem_cache_has_cpu_partial(s) && !prior) {
>>                         /*
>> -                        * If we just froze the slab then put it onto the
>> +                        * If we started with a full slab then put it onto the
>>                          * per cpu partial list.
>>                          */
>>                         put_cpu_partial(s, slab, 1);
>> --
> 
> Looks good to me,
> Reviewed-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> 
> Thanks!
> 
>> 2.20.1
>>