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

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

On Wed, May 07, 2025 at 06:15:34PM +0900, Harry Yoo wrote:
> 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;

Uh, I shouldn't have deleted this return statement :)

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

-- 
Cheers,
Harry / Hyeonggon