Re: [PATCH v4 06/22] slab: add sheaves to most caches

[Liam R. Howlett]

* Vlastimil Babka <vbabka@xxxxxxx> [260123 01:53]:
> In the first step to replace cpu (partial) slabs with sheaves, enable
> sheaves for almost all caches. Treat args->sheaf_capacity as a minimum,
> and calculate sheaf capacity with a formula that roughly follows the
> formula for number of objects in cpu partial slabs in set_cpu_partial().
> 
> This should achieve roughly similar contention on the barn spin lock as
> there's currently for node list_lock without sheaves, to make
> benchmarking results comparable. It can be further tuned later.
> 
> Don't enable sheaves for bootstrap caches as that wouldn't work. In
> order to recognize them by SLAB_NO_OBJ_EXT, make sure the flag exists
> even for !CONFIG_SLAB_OBJ_EXT.
> 
> This limitation will be lifted for kmalloc caches after the necessary
> bootstrapping changes.
> 
> Also do not enable sheaves for SLAB_NOLEAKTRACE caches to avoid
> recursion with kmemleak tracking (thanks to Breno Leitao).
> 
> Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Is there a way to force a specific limit to the sheaf capacity if you
want a lower number than what is calculated in
calculate_sheaf_capacity()?  That is, it seems your code always decides
if the specified sheaf number is smaller right now.  I'm not sure it's
practical to want a smaller number though.

Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>

> ---
>  include/linux/slab.h |  6 ------
>  mm/slub.c            | 56 ++++++++++++++++++++++++++++++++++++++++++++++++----
>  2 files changed, 52 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 2482992248dc..2682ee57ec90 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -57,9 +57,7 @@ enum _slab_flag_bits {
>  #endif
>  	_SLAB_OBJECT_POISON,
>  	_SLAB_CMPXCHG_DOUBLE,
> -#ifdef CONFIG_SLAB_OBJ_EXT
>  	_SLAB_NO_OBJ_EXT,
> -#endif
>  	_SLAB_FLAGS_LAST_BIT
>  };
>  
> @@ -238,11 +236,7 @@ enum _slab_flag_bits {
>  #define SLAB_TEMPORARY		SLAB_RECLAIM_ACCOUNT	/* Objects are short-lived */
>  
>  /* Slab created using create_boot_cache */
> -#ifdef CONFIG_SLAB_OBJ_EXT
>  #define SLAB_NO_OBJ_EXT		__SLAB_FLAG_BIT(_SLAB_NO_OBJ_EXT)
> -#else
> -#define SLAB_NO_OBJ_EXT		__SLAB_FLAG_UNUSED
> -#endif
>  
>  /*
>   * ZERO_SIZE_PTR will be returned for zero sized kmalloc requests.
> diff --git a/mm/slub.c b/mm/slub.c
> index 9d86c0505dcd..594f5fac39b3 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -7880,6 +7880,53 @@ static void set_cpu_partial(struct kmem_cache *s)
>  #endif
>  }
>  
> +static unsigned int calculate_sheaf_capacity(struct kmem_cache *s,
> +					     struct kmem_cache_args *args)
> +
> +{
> +	unsigned int capacity;
> +	size_t size;
> +
> +
> +	if (IS_ENABLED(CONFIG_SLUB_TINY) || s->flags & SLAB_DEBUG_FLAGS)
> +		return 0;
> +
> +	/*
> +	 * Bootstrap caches can't have sheaves for now (SLAB_NO_OBJ_EXT).
> +	 * SLAB_NOLEAKTRACE caches (e.g., kmemleak's object_cache) must not
> +	 * have sheaves to avoid recursion when sheaf allocation triggers
> +	 * kmemleak tracking.
> +	 */
> +	if (s->flags & (SLAB_NO_OBJ_EXT | SLAB_NOLEAKTRACE))
> +		return 0;
> +
> +	/*
> +	 * For now we use roughly similar formula (divided by two as there are
> +	 * two percpu sheaves) as what was used for percpu partial slabs, which
> +	 * should result in similar lock contention (barn or list_lock)
> +	 */
> +	if (s->size >= PAGE_SIZE)
> +		capacity = 4;
> +	else if (s->size >= 1024)
> +		capacity = 12;
> +	else if (s->size >= 256)
> +		capacity = 26;
> +	else
> +		capacity = 60;
> +
> +	/* Increment capacity to make sheaf exactly a kmalloc size bucket */
> +	size = struct_size_t(struct slab_sheaf, objects, capacity);
> +	size = kmalloc_size_roundup(size);
> +	capacity = (size - struct_size_t(struct slab_sheaf, objects, 0)) / sizeof(void *);
> +
> +	/*
> +	 * Respect an explicit request for capacity that's typically motivated by
> +	 * expected maximum size of kmem_cache_prefill_sheaf() to not end up
> +	 * using low-performance oversize sheaves
> +	 */
> +	return max(capacity, args->sheaf_capacity);
> +}
> +
>  /*
>   * calculate_sizes() determines the order and the distribution of data within
>   * a slab object.
> @@ -8014,6 +8061,10 @@ static int calculate_sizes(struct kmem_cache_args *args, struct kmem_cache *s)
>  	if (s->flags & SLAB_RECLAIM_ACCOUNT)
>  		s->allocflags |= __GFP_RECLAIMABLE;
>  
> +	/* kmalloc caches need extra care to support sheaves */
> +	if (!is_kmalloc_cache(s))
> +		s->sheaf_capacity = calculate_sheaf_capacity(s, args);
> +
>  	/*
>  	 * Determine the number of objects per slab
>  	 */
> @@ -8618,15 +8669,12 @@ int do_kmem_cache_create(struct kmem_cache *s, const char *name,
>  
>  	set_cpu_partial(s);
>  
> -	if (args->sheaf_capacity && !IS_ENABLED(CONFIG_SLUB_TINY)
> -					&& !(s->flags & SLAB_DEBUG_FLAGS)) {
> +	if (s->sheaf_capacity) {
>  		s->cpu_sheaves = alloc_percpu(struct slub_percpu_sheaves);
>  		if (!s->cpu_sheaves) {
>  			err = -ENOMEM;
>  			goto out;
>  		}
> -		// TODO: increase capacity to grow slab_sheaf up to next kmalloc size?
> -		s->sheaf_capacity = args->sheaf_capacity;
>  	}
>  
>  #ifdef CONFIG_NUMA
> 
> -- 
> 2.52.0
> 
>