Re: [PATCH 06/30] mm: kmem_alloc_estimate()

[Pekka Enberg <penberg@cs.helsinki.fi>]

Hi Peter,

On Thu, 2008-07-24 at 16:00 +0200, Peter Zijlstra wrote:
> plain text document attachment (mm-kmem_estimate_pages.patch)
> Provide a method to get the upper bound on the pages needed to allocate
> a given number of objects from a given kmem_cache.
> 
> This lays the foundation for a generic reserve framework as presented in
> a later patch in this series. This framework needs to convert object demand
> (kmalloc() bytes, kmem_cache_alloc() objects) to pages.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
>  include/linux/slab.h |    4 ++
>  mm/slab.c            |   75 +++++++++++++++++++++++++++++++++++++++++++
>  mm/slub.c            |   87 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 166 insertions(+)
> 
> Index: linux-2.6/include/linux/slab.h
> ===================================================================
> --- linux-2.6.orig/include/linux/slab.h
> +++ linux-2.6/include/linux/slab.h
> @@ -65,6 +65,8 @@ void kmem_cache_free(struct kmem_cache *
>  unsigned int kmem_cache_size(struct kmem_cache *);
>  const char *kmem_cache_name(struct kmem_cache *);
>  int kmem_ptr_validate(struct kmem_cache *cachep, const void *ptr);
> +unsigned kmem_alloc_estimate(struct kmem_cache *cachep,
> +			gfp_t flags, int objects);
>  
>  /*
>   * Please use this macro to create slab caches. Simply specify the
> @@ -99,6 +101,8 @@ int kmem_ptr_validate(struct kmem_cache 
>  void * __must_check krealloc(const void *, size_t, gfp_t);
>  void kfree(const void *);
>  size_t ksize(const void *);

Just a nitpick, but:

> +unsigned kmalloc_estimate_fixed(size_t, gfp_t, int);

kmalloc_estimate_objs()?

> +unsigned kmalloc_estimate_variable(gfp_t, size_t);

kmalloc_estimate_bytes()?

>  
>  /*
>   * Allocator specific definitions. These are mainly used to establish optimized
> Index: linux-2.6/mm/slub.c
> ===================================================================
> --- linux-2.6.orig/mm/slub.c
> +++ linux-2.6/mm/slub.c
> @@ -2412,6 +2412,42 @@ const char *kmem_cache_name(struct kmem_
>  }
>  EXPORT_SYMBOL(kmem_cache_name);
>  
> +/*
> + * Calculate the upper bound of pages required to sequentially allocate
> + * @objects objects from @cachep.
> + *
> + * We should use s->min_objects because those are the least efficient.
> + */
> +unsigned kmem_alloc_estimate(struct kmem_cache *s, gfp_t flags, int objects)
> +{
> +	unsigned long pages;
> +	struct kmem_cache_order_objects x;
> +
> +	if (WARN_ON(!s) || WARN_ON(!oo_objects(s->min)))
> +		return 0;
> +
> +	x = s->min;
> +	pages = DIV_ROUND_UP(objects, oo_objects(x)) << oo_order(x);
> +
> +	/*
> +	 * Account the possible additional overhead if the slab holds more that
> +	 * one object. Use s->max_objects because that's the worst case.
> +	 */
> +	x = s->oo;
> +	if (oo_objects(x) > 1) {

Hmm, I'm not sure why slab with just one object is treated separately
here. Surely you have per-CPU slabs then as well?

> +		/*
> +		 * Account the possible additional overhead if per cpu slabs
> +		 * are currently empty and have to be allocated. This is very
> +		 * unlikely but a possible scenario immediately after
> +		 * kmem_cache_shrink.
> +		 */
> +		pages += num_online_cpus() << oo_order(x);

Isn't this problematic with CPU hotplug? Shouldn't we use
num_possible_cpus() here?

> +	}
> +
> +	return pages;
> +}
> +EXPORT_SYMBOL_GPL(kmem_alloc_estimate);
> +
>  static void list_slab_objects(struct kmem_cache *s, struct page *page,
>  							const char *text)
>  {
> @@ -2789,6 +2825,57 @@ void kfree(const void *x)
>  EXPORT_SYMBOL(kfree);
>  
>  /*
> + * Calculate the upper bound of pages required to sequentially allocate
> + * @count objects of @size bytes from kmalloc given @flags.
> + */
> +unsigned kmalloc_estimate_fixed(size_t size, gfp_t flags, int count)
> +{
> +	struct kmem_cache *s = get_slab(size, flags);
> +	if (!s)
> +		return 0;
> +
> +	return kmem_alloc_estimate(s, flags, count);
> +
> +}
> +EXPORT_SYMBOL_GPL(kmalloc_estimate_fixed);
> +
> +/*
> + * Calculate the upper bound of pages requires to sequentially allocate @bytes
> + * from kmalloc in an unspecified number of allocations of nonuniform size.
> + */
> +unsigned kmalloc_estimate_variable(gfp_t flags, size_t bytes)
> +{
> +	int i;
> +	unsigned long pages;
> +
> +	/*
> +	 * multiply by two, in order to account the worst case slack space
> +	 * due to the power-of-two allocation sizes.
> +	 */
> +	pages = DIV_ROUND_UP(2 * bytes, PAGE_SIZE);

For bytes > PAGE_SIZE this doesn't look right (for SLUB). We do page
allocator pass-through which means that we'll be grabbing high order
pages which can be bigger than what 'pages' is here.

> +
> +	/*
> +	 * add the kmem_cache overhead of each possible kmalloc cache
> +	 */
> +	for (i = 1; i < PAGE_SHIFT; i++) {
> +		struct kmem_cache *s;
> +
> +#ifdef CONFIG_ZONE_DMA
> +		if (unlikely(flags & SLUB_DMA))
> +			s = dma_kmalloc_cache(i, flags);
> +		else
> +#endif
> +			s = &kmalloc_caches[i];
> +
> +		if (s)
> +			pages += kmem_alloc_estimate(s, flags, 0);
> +	}
> +
> +	return pages;
> +}
> +EXPORT_SYMBOL_GPL(kmalloc_estimate_variable);
> +
> +/*
>   * kmem_cache_shrink removes empty slabs from the partial lists and sorts
>   * the remaining slabs by the number of items in use. The slabs with the
>   * most items in use come first. New allocations will then fill those up
> Index: linux-2.6/mm/slab.c
> ===================================================================
> --- linux-2.6.orig/mm/slab.c
> +++ linux-2.6/mm/slab.c
> @@ -3854,6 +3854,81 @@ const char *kmem_cache_name(struct kmem_
>  EXPORT_SYMBOL_GPL(kmem_cache_name);
>  
>  /*
> + * Calculate the upper bound of pages required to sequentially allocate
> + * @objects objects from @cachep.
> + */
> +unsigned kmem_alloc_estimate(struct kmem_cache *cachep,
> +		gfp_t flags, int objects)
> +{
> +	/*
> +	 * (1) memory for objects,
> +	 */
> +	unsigned nr_slabs = DIV_ROUND_UP(objects, cachep->num);
> +	unsigned nr_pages = nr_slabs << cachep->gfporder;
> +
> +	/*
> +	 * (2) memory for each per-cpu queue (nr_cpu_ids),
> +	 * (3) memory for each per-node alien queues (nr_cpu_ids), and
> +	 * (4) some amount of memory for the slab management structures
> +	 *
> +	 * XXX: truely account these

Heh, yes please. Or add a comment why it doesn't matter.

> +	 */
> +	nr_pages += 1 + ilog2(nr_pages);
> +
> +	return nr_pages;
> +}
> +
> +/*
> + * Calculate the upper bound of pages required to sequentially allocate
> + * @count objects of @size bytes from kmalloc given @flags.
> + */
> +unsigned kmalloc_estimate_fixed(size_t size, gfp_t flags, int count)
> +{
> +	struct kmem_cache *s = kmem_find_general_cachep(size, flags);
> +	if (!s)
> +		return 0;
> +
> +	return kmem_alloc_estimate(s, flags, count);
> +}
> +EXPORT_SYMBOL_GPL(kmalloc_estimate_fixed);
> +
> +/*
> + * Calculate the upper bound of pages requires to sequentially allocate @bytes
> + * from kmalloc in an unspecified number of allocations of nonuniform size.
> + */
> +unsigned kmalloc_estimate_variable(gfp_t flags, size_t bytes)
> +{
> +	unsigned long pages;
> +	struct cache_sizes *csizep = malloc_sizes;
> +
> +	/*
> +	 * multiply by two, in order to account the worst case slack space
> +	 * due to the power-of-two allocation sizes.
> +	 */
> +	pages = DIV_ROUND_UP(2 * bytes, PAGE_SIZE);
> +
> +	/*
> +	 * add the kmem_cache overhead of each possible kmalloc cache
> +	 */
> +	for (csizep = malloc_sizes; csizep->cs_cachep; csizep++) {
> +		struct kmem_cache *s;
> +
> +#ifdef CONFIG_ZONE_DMA
> +		if (unlikely(flags & __GFP_DMA))
> +			s = csizep->cs_dmacachep;
> +		else
> +#endif
> +			s = csizep->cs_cachep;
> +
> +		if (s)
> +			pages += kmem_alloc_estimate(s, flags, 0);
> +	}
> +
> +	return pages;
> +}
> +EXPORT_SYMBOL_GPL(kmalloc_estimate_variable);
> +
> +/*
>   * This initializes kmem_list3 or resizes various caches for all nodes.
>   */
>  static int alloc_kmemlist(struct kmem_cache *cachep)
> 

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