Re: [PATCH RFC v2 05/10] slab: switch percpu sheaves locking to localtry_lock

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Feb 14, 2025 at 8:27 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> Instead of local_lock_irqsave(), use localtry_trylock() when potential
> callers include irq context, and localtry_lock() otherwise (such as when
> we already know the gfp flags allow blocking).
>
> This should reduce the locking (due to irq disabling/enabling) overhead.
> Failing to use percpu sheaves in an irq due to preempting an already
> locked user of sheaves should be rare so it's a favorable tradeoff.
>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  mm/slub.c | 122 ++++++++++++++++++++++++++++++++++++++------------------------
>  1 file changed, 76 insertions(+), 46 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 40175747212fefb27137309b27571abe8d0966e2..3d7345e7e938d53950ed0d6abe8eb0e93cf8f5b1 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -450,7 +450,7 @@ struct slab_sheaf {
>  };
>
>  struct slub_percpu_sheaves {
> -       local_lock_t lock;
> +       localtry_lock_t lock;
>         struct slab_sheaf *main; /* never NULL when unlocked */
>         struct slab_sheaf *spare; /* empty or full, may be NULL */
>         struct slab_sheaf *rcu_free;
> @@ -2529,16 +2529,19 @@ static struct slab_sheaf *alloc_full_sheaf(struct kmem_cache *s, gfp_t gfp)
>
>  static void __kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p);
>
> -static void sheaf_flush_main(struct kmem_cache *s)
> +/* returns true if at least partially flushed */
> +static bool sheaf_flush_main(struct kmem_cache *s)
>  {
>         struct slub_percpu_sheaves *pcs;
>         unsigned int batch, remaining;
>         void *objects[PCS_BATCH_MAX];
>         struct slab_sheaf *sheaf;
> -       unsigned long flags;
> +       bool ret = false;
>
>  next_batch:
> -       local_lock_irqsave(&s->cpu_sheaves->lock, flags);
> +       if (!localtry_trylock(&s->cpu_sheaves->lock))
> +               return ret;
> +
>         pcs = this_cpu_ptr(s->cpu_sheaves);
>         sheaf = pcs->main;
>
> @@ -2549,14 +2552,18 @@ static void sheaf_flush_main(struct kmem_cache *s)
>
>         remaining = sheaf->size;
>
> -       local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
> +       localtry_unlock(&s->cpu_sheaves->lock);
>
>         __kmem_cache_free_bulk(s, batch, &objects[0]);
>
>         stat_add(s, SHEAF_FLUSH_MAIN, batch);
>
> +       ret = true;
> +
>         if (remaining)
>                 goto next_batch;
> +
> +       return ret;
>  }
>
>  static void sheaf_flush(struct kmem_cache *s, struct slab_sheaf *sheaf)
> @@ -2593,6 +2600,8 @@ static void rcu_free_sheaf_nobarn(struct rcu_head *head)
>   * Caller needs to make sure migration is disabled in order to fully flush
>   * single cpu's sheaves
>   *
> + * must not be called from an irq
> + *
>   * flushing operations are rare so let's keep it simple and flush to slabs
>   * directly, skipping the barn
>   */
> @@ -2600,9 +2609,8 @@ static void pcs_flush_all(struct kmem_cache *s)
>  {
>         struct slub_percpu_sheaves *pcs;
>         struct slab_sheaf *spare, *rcu_free;
> -       unsigned long flags;
>
> -       local_lock_irqsave(&s->cpu_sheaves->lock, flags);
> +       localtry_lock(&s->cpu_sheaves->lock);
>         pcs = this_cpu_ptr(s->cpu_sheaves);
>
>         spare = pcs->spare;
> @@ -2611,7 +2619,7 @@ static void pcs_flush_all(struct kmem_cache *s)
>         rcu_free = pcs->rcu_free;
>         pcs->rcu_free = NULL;
>
> -       local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
> +       localtry_unlock(&s->cpu_sheaves->lock);
>
>         if (spare) {
>                 sheaf_flush(s, spare);
> @@ -4554,10 +4562,11 @@ static __fastpath_inline
>  void *alloc_from_pcs(struct kmem_cache *s, gfp_t gfp)
>  {
>         struct slub_percpu_sheaves *pcs;
> -       unsigned long flags;
>         void *object;
>
> -       local_lock_irqsave(&s->cpu_sheaves->lock, flags);
> +       if (!localtry_trylock(&s->cpu_sheaves->lock))
> +               return NULL;
> +
>         pcs = this_cpu_ptr(s->cpu_sheaves);
>
>         if (unlikely(pcs->main->size == 0)) {
> @@ -4590,7 +4599,7 @@ void *alloc_from_pcs(struct kmem_cache *s, gfp_t gfp)
>                         }
>                 }
>
> -               local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
> +               localtry_unlock(&s->cpu_sheaves->lock);
>
>                 if (!can_alloc)
>                         return NULL;
> @@ -4612,7 +4621,11 @@ void *alloc_from_pcs(struct kmem_cache *s, gfp_t gfp)
>                 if (!full)
>                         return NULL;
>
> -               local_lock_irqsave(&s->cpu_sheaves->lock, flags);
> +               /*
> +                * we can reach here only when gfpflags_allow_blocking
> +                * so this must not be an irq
> +                */
> +               localtry_lock(&s->cpu_sheaves->lock);
>                 pcs = this_cpu_ptr(s->cpu_sheaves);
>
>                 /*
> @@ -4646,7 +4659,7 @@ void *alloc_from_pcs(struct kmem_cache *s, gfp_t gfp)
>  do_alloc:
>         object = pcs->main->objects[--pcs->main->size];
>
> -       local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
> +       localtry_unlock(&s->cpu_sheaves->lock);
>
>         stat(s, ALLOC_PCS);
>
> @@ -4658,12 +4671,13 @@ unsigned int alloc_from_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
>  {
>         struct slub_percpu_sheaves *pcs;
>         struct slab_sheaf *main;
> -       unsigned long flags;
>         unsigned int allocated = 0;
>         unsigned int batch;
>
>  next_batch:
> -       local_lock_irqsave(&s->cpu_sheaves->lock, flags);
> +       if (!localtry_trylock(&s->cpu_sheaves->lock))
> +               return allocated;
> +
>         pcs = this_cpu_ptr(s->cpu_sheaves);
>
>         if (unlikely(pcs->main->size == 0)) {
> @@ -4683,7 +4697,7 @@ unsigned int alloc_from_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
>                         goto do_alloc;
>                 }
>
> -               local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
> +               localtry_unlock(&s->cpu_sheaves->lock);
>
>                 /*
>                  * Once full sheaves in barn are depleted, let the bulk
> @@ -4701,7 +4715,7 @@ unsigned int alloc_from_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
>         main->size -= batch;
>         memcpy(p, main->objects + main->size, batch * sizeof(void *));
>
> -       local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
> +       localtry_unlock(&s->cpu_sheaves->lock);
>
>         stat_add(s, ALLOC_PCS, batch);
>
> @@ -5121,13 +5135,14 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>   * The object is expected to have passed slab_free_hook() already.
>   */
>  static __fastpath_inline
> -void free_to_pcs(struct kmem_cache *s, void *object)
> +bool free_to_pcs(struct kmem_cache *s, void *object)
>  {
>         struct slub_percpu_sheaves *pcs;
> -       unsigned long flags;
>
>  restart:
> -       local_lock_irqsave(&s->cpu_sheaves->lock, flags);
> +       if (!localtry_trylock(&s->cpu_sheaves->lock))
> +               return false;
> +
>         pcs = this_cpu_ptr(s->cpu_sheaves);
>
>         if (unlikely(pcs->main->size == s->sheaf_capacity)) {
> @@ -5162,7 +5177,7 @@ void free_to_pcs(struct kmem_cache *s, void *object)
>                         struct slab_sheaf *to_flush = pcs->spare;
>
>                         pcs->spare = NULL;
> -                       local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
> +                       localtry_unlock(&s->cpu_sheaves->lock);
>
>                         sheaf_flush(s, to_flush);
>                         empty = to_flush;
> @@ -5170,17 +5185,27 @@ void free_to_pcs(struct kmem_cache *s, void *object)
>                 }
>
>  alloc_empty:
> -               local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
> +               localtry_unlock(&s->cpu_sheaves->lock);
>
>                 empty = alloc_empty_sheaf(s, GFP_NOWAIT);
>
>                 if (!empty) {
> -                       sheaf_flush_main(s);
> -                       goto restart;
> +                       if (sheaf_flush_main(s))
> +                               goto restart;
> +                       else
> +                               return false;
>                 }
>
>  got_empty:
> -               local_lock_irqsave(&s->cpu_sheaves->lock, flags);
> +               if (!localtry_trylock(&s->cpu_sheaves->lock)) {
> +                       struct node_barn *barn;
> +
> +                       barn = get_node(s, numa_mem_id())->barn;
> +
> +                       barn_put_empty_sheaf(barn, empty, true);
> +                       return false;
> +               }
> +
>                 pcs = this_cpu_ptr(s->cpu_sheaves);
>
>                 /*
> @@ -5209,9 +5234,11 @@ void free_to_pcs(struct kmem_cache *s, void *object)
>  do_free:
>         pcs->main->objects[pcs->main->size++] = object;
>
> -       local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
> +       localtry_unlock(&s->cpu_sheaves->lock);
>
>         stat(s, FREE_PCS);
> +
> +       return true;
>  }
>
>  static void __rcu_free_sheaf_prepare(struct kmem_cache *s,
> @@ -5270,9 +5297,10 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
>  {
>         struct slub_percpu_sheaves *pcs;
>         struct slab_sheaf *rcu_sheaf;
> -       unsigned long flags;
>
> -       local_lock_irqsave(&s->cpu_sheaves->lock, flags);
> +       if (!localtry_trylock(&s->cpu_sheaves->lock))
> +               goto fail;
> +
>         pcs = this_cpu_ptr(s->cpu_sheaves);
>
>         if (unlikely(!pcs->rcu_free)) {
> @@ -5286,16 +5314,16 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
>                         goto do_free;
>                 }
>
> -               local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
> +               localtry_unlock(&s->cpu_sheaves->lock);
>
>                 empty = alloc_empty_sheaf(s, GFP_NOWAIT);
>
> -               if (!empty) {
> -                       stat(s, FREE_RCU_SHEAF_FAIL);
> -                       return false;
> -               }
> +               if (!empty)
> +                       goto fail;
> +
> +               if (!localtry_trylock(&s->cpu_sheaves->lock))
> +                       goto fail;
>
> -               local_lock_irqsave(&s->cpu_sheaves->lock, flags);
>                 pcs = this_cpu_ptr(s->cpu_sheaves);
>
>                 if (unlikely(pcs->rcu_free))
> @@ -5311,19 +5339,22 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
>         rcu_sheaf->objects[rcu_sheaf->size++] = obj;
>
>         if (likely(rcu_sheaf->size < s->sheaf_capacity)) {
> -               local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
> +               localtry_unlock(&s->cpu_sheaves->lock);
>                 stat(s, FREE_RCU_SHEAF);
>                 return true;
>         }
>
>         pcs->rcu_free = NULL;
> -       local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
> +       localtry_unlock(&s->cpu_sheaves->lock);
>
>         call_rcu(&rcu_sheaf->rcu_head, rcu_free_sheaf);
>
>         stat(s, FREE_RCU_SHEAF);
> -
>         return true;
> +
> +fail:
> +       stat(s, FREE_RCU_SHEAF_FAIL);
> +       return false;
>  }
>
>  /*
> @@ -5335,7 +5366,6 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
>  {
>         struct slub_percpu_sheaves *pcs;
>         struct slab_sheaf *main;
> -       unsigned long flags;
>         unsigned int batch, i = 0;
>         bool init;
>
> @@ -5358,7 +5388,9 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
>         }
>
>  next_batch:
> -       local_lock_irqsave(&s->cpu_sheaves->lock, flags);
> +       if (!localtry_trylock(&s->cpu_sheaves->lock))
> +               goto fallback;
> +
>         pcs = this_cpu_ptr(s->cpu_sheaves);
>
>         if (unlikely(pcs->main->size == s->sheaf_capacity)) {
> @@ -5389,13 +5421,13 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
>                 }
>
>  no_empty:
> -               local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
> +               localtry_unlock(&s->cpu_sheaves->lock);
>
>                 /*
>                  * if we depleted all empty sheaves in the barn or there are too
>                  * many full sheaves, free the rest to slab pages
>                  */
> -
> +fallback:
>                 __kmem_cache_free_bulk(s, size, p);
>                 return;
>         }
> @@ -5407,7 +5439,7 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
>         memcpy(main->objects + main->size, p, batch * sizeof(void *));
>         main->size += batch;
>
> -       local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
> +       localtry_unlock(&s->cpu_sheaves->lock);
>
>         stat_add(s, FREE_PCS, batch);
>
> @@ -5507,9 +5539,7 @@ void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
>         if (unlikely(!slab_free_hook(s, object, slab_want_init_on_free(s), false)))
>                 return;
>
> -       if (s->cpu_sheaves)
> -               free_to_pcs(s, object);
> -       else
> +       if (!s->cpu_sheaves || !free_to_pcs(s, object))
>                 do_slab_free(s, slab, object, object, 1, addr);
>  }
>
> @@ -6288,7 +6318,7 @@ static int init_percpu_sheaves(struct kmem_cache *s)
>
>                 pcs = per_cpu_ptr(s->cpu_sheaves, cpu);
>
> -               local_lock_init(&pcs->lock);
> +               localtry_lock_init(&pcs->lock);
>
>                 nid = cpu_to_mem(cpu);
>
>
> --
> 2.48.1
>