Re: [PATCH v4 3/9] slab: sheaf prefilling for guaranteed allocations

[Harry Yoo <harry.yoo@oracle.com>]

On Fri, Apr 25, 2025 at 10:27:23AM +0200, Vlastimil Babka wrote:
> Add functions for efficient guaranteed allocations e.g. in a critical
> section that cannot sleep, when the exact number of allocations is not
> known beforehand, but an upper limit can be calculated.
> 
> kmem_cache_prefill_sheaf() returns a sheaf containing at least given
> number of objects.
> 
> kmem_cache_alloc_from_sheaf() will allocate an object from the sheaf
> and is guaranteed not to fail until depleted.
> 
> kmem_cache_return_sheaf() is for giving the sheaf back to the slab
> allocator after the critical section. This will also attempt to refill
> it to cache's sheaf capacity for better efficiency of sheaves handling,
> but it's not stricly necessary to succeed.
> 
> kmem_cache_refill_sheaf() can be used to refill a previously obtained
> sheaf to requested size. If the current size is sufficient, it does
> nothing. If the requested size exceeds cache's sheaf_capacity and the
> sheaf's current capacity, the sheaf will be replaced with a new one,
> hence the indirect pointer parameter.
> 
> kmem_cache_sheaf_size() can be used to query the current size.
> 
> The implementation supports requesting sizes that exceed cache's
> sheaf_capacity, but it is not efficient - such "oversize" sheaves are
> allocated fresh in kmem_cache_prefill_sheaf() and flushed and freed
> immediately by kmem_cache_return_sheaf(). kmem_cache_refill_sheaf()
> might be especially ineffective when replacing a sheaf with a new one of
> a larger capacity. It is therefore better to size cache's
> sheaf_capacity accordingly to make oversize sheaves exceptional.
> 
> CONFIG_SLUB_STATS counters are added for sheaf prefill and return
> operations. A prefill or return is considered _fast when it is able to
> grab or return a percpu spare sheaf (even if the sheaf needs a refill to
> satisfy the request, as those should amortize over time), and _slow
> otherwise (when the barn or even sheaf allocation/freeing has to be
> involved). sheaf_prefill_oversize is provided to determine how many
> prefills were oversize (counter for oversize returns is not necessary as
> all oversize refills result in oversize returns).
> 
> When slub_debug is enabled for a cache with sheaves, no percpu sheaves
> exist for it, but the prefill functionality is still provided simply by
> all prefilled sheaves becoming oversize. If percpu sheaves are not
> created for a cache due to not passing the sheaf_capacity argument on
> cache creation, the prefills also work through oversize sheaves, but
> there's a WARN_ON_ONCE() to indicate the omission.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> ---

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

with a nit below.

> +/*
> + * Use this to return a sheaf obtained by kmem_cache_prefill_sheaf()
> + *
> + * If the sheaf cannot simply become the percpu spare sheaf, but there's space
> + * for a full sheaf in the barn, we try to refill the sheaf back to the cache's
> + * sheaf_capacity to avoid handling partially full sheaves.
> + *
> + * If the refill fails because gfp is e.g. GFP_NOWAIT, or the barn is full, the
> + * sheaf is instead flushed and freed.
> + */
> +void kmem_cache_return_sheaf(struct kmem_cache *s, gfp_t gfp,
> +			     struct slab_sheaf *sheaf)
> +{
> +	struct slub_percpu_sheaves *pcs;
> +	bool refill = false;
> +	struct node_barn *barn;
> +
> +	if (unlikely(sheaf->capacity != s->sheaf_capacity)) {
> +		sheaf_flush_unused(s, sheaf);
> +		kfree(sheaf);
> +		return;
> +	}
> +
> +	local_lock(&s->cpu_sheaves->lock);
> +	pcs = this_cpu_ptr(s->cpu_sheaves);
> +
> +	if (!pcs->spare) {
> +		pcs->spare = sheaf;
> +		sheaf = NULL;
> +		stat(s, SHEAF_RETURN_FAST);
> +	} else if (data_race(pcs->barn->nr_full) < MAX_FULL_SHEAVES) {
> +		barn = pcs->barn;
> +		refill = true;
> +	}
> +
> +	local_unlock(&s->cpu_sheaves->lock);
> +
> +	if (!sheaf)
> +		return;
> +
> +	stat(s, SHEAF_RETURN_SLOW);
> +
> +	/*
> +	 * if the barn is full of full sheaves or we fail to refill the sheaf,
> +	 * simply flush and free it
> +	 */
> +	if (!refill || refill_sheaf(s, sheaf, gfp)) {
> +		sheaf_flush_unused(s, sheaf);
> +		free_empty_sheaf(s, sheaf);
> +		return;
> +	}
> +
> +	/* we racily determined the sheaf would fit, so now force it */
> +	barn_put_full_sheaf(barn, sheaf);
> +	stat(s, BARN_PUT);
> +}

nit: as accessing pcs->barn outside local_lock is safe (it does not go
away until the cache is destroyed...), this could be simplified a little
bit:

diff --git a/mm/slub.c b/mm/slub.c
index 2bf83e2b85b2..4e1daba4d13e 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5043,7 +5043,6 @@ void kmem_cache_return_sheaf(struct kmem_cache *s, gfp_t gfp,
 			     struct slab_sheaf *sheaf)
 {
 	struct slub_percpu_sheaves *pcs;
-	bool refill = false;
 	struct node_barn *barn;

 	if (unlikely(sheaf->capacity != s->sheaf_capacity)) {
@@ -5059,9 +5058,6 @@ void kmem_cache_return_sheaf(struct kmem_cache *s, gfp_t gfp,
 		pcs->spare = sheaf;
 		sheaf = NULL;
 		stat(s, SHEAF_RETURN_FAST);
-	} else if (data_race(pcs->barn->nr_full) < MAX_FULL_SHEAVES) {
-		barn = pcs->barn;
-		refill = true;
 	}

 	local_unlock(&s->cpu_sheaves->lock);
@@ -5071,17 +5067,19 @@ void kmem_cache_return_sheaf(struct kmem_cache *s, gfp_t gfp,

 	stat(s, SHEAF_RETURN_SLOW);

+	/* Accessing pcs->barn outside local_lock is safe */
+	barn = pcs->barn;
+
 	/*
 	 * if the barn is full of full sheaves or we fail to refill the sheaf,
 	 * simply flush and free it
 	 */
-	if (!refill || refill_sheaf(s, sheaf, gfp)) {
+	if (data_race(barn->nr_full) >= MAX_FULL_SHEAVES ||
+			refill_sheaf(s, sheaf, gfp)) {
 		sheaf_flush_unused(s, sheaf);
 		free_empty_sheaf(s, sheaf);
-		return;
 	}

-	/* we racily determined the sheaf would fit, so now force it */
 	barn_put_full_sheaf(barn, sheaf);
 	stat(s, BARN_PUT);
 }

-- 
Cheers,
Harry / Hyeonggon