linux-kernel - Re: [PATCH v2 1/3] slub: correct to calculate num of acquired objects in get_partial

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Date:	Tue, 19 Mar 2013 14:10:46 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	Christoph Lameter <cl@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] slub: correct to calculate num of acquired
 objects in get_partial_node()

Hello, Pekka.

Could you pick up 1/3, 3/3?
These are already acked by Christoph.
2/3 is same effect as Glauber's "slub: correctly bootstrap boot caches",
so should skip it.

Thanks.

On Mon, Jan 21, 2013 at 05:01:25PM +0900, Joonsoo Kim wrote:
> There is a subtle bug when calculating a number of acquired objects.
> 
> Currently, we calculate "available = page->objects - page->inuse",
> after acquire_slab() is called in get_partial_node().
> 
> In acquire_slab() with mode = 1, we always set new.inuse = page->objects.
> So,
> 
> 	acquire_slab(s, n, page, object == NULL);
> 
> 	if (!object) {
> 		c->page = page;
> 		stat(s, ALLOC_FROM_PARTIAL);
> 		object = t;
> 		available = page->objects - page->inuse;
> 
> 		!!! availabe is always 0 !!!
> 	...
> 
> Therfore, "available > s->cpu_partial / 2" is always false and
> we always go to second iteration.
> This patch correct this problem.
> 
> After that, we don't need return value of put_cpu_partial().
> So remove it.
> 
> v2: calculate nr of objects using new.objects and new.inuse.
> It is more accurate way than before.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index ba2ca53..7204c74 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1493,7 +1493,7 @@ static inline void remove_partial(struct kmem_cache_node *n,
>   */
>  static inline void *acquire_slab(struct kmem_cache *s,
>  		struct kmem_cache_node *n, struct page *page,
> -		int mode)
> +		int mode, int *objects)
>  {
>  	void *freelist;
>  	unsigned long counters;
> @@ -1507,6 +1507,7 @@ static inline void *acquire_slab(struct kmem_cache *s,
>  	freelist = page->freelist;
>  	counters = page->counters;
>  	new.counters = counters;
> +	*objects = new.objects - new.inuse;
>  	if (mode) {
>  		new.inuse = page->objects;
>  		new.freelist = NULL;
> @@ -1528,7 +1529,7 @@ static inline void *acquire_slab(struct kmem_cache *s,
>  	return freelist;
>  }
>  
> -static int put_cpu_partial(struct kmem_cache *s, struct page *page, int drain);
> +static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain);
>  static inline bool pfmemalloc_match(struct page *page, gfp_t gfpflags);
>  
>  /*
> @@ -1539,6 +1540,8 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
>  {
>  	struct page *page, *page2;
>  	void *object = NULL;
> +	int available = 0;
> +	int objects;
>  
>  	/*
>  	 * Racy check. If we mistakenly see no partial slabs then we
> @@ -1552,22 +1555,21 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
>  	spin_lock(&n->list_lock);
>  	list_for_each_entry_safe(page, page2, &n->partial, lru) {
>  		void *t;
> -		int available;
>  
>  		if (!pfmemalloc_match(page, flags))
>  			continue;
>  
> -		t = acquire_slab(s, n, page, object == NULL);
> +		t = acquire_slab(s, n, page, object == NULL, &objects);
>  		if (!t)
>  			break;
>  
> +		available += objects;
>  		if (!object) {
>  			c->page = page;
>  			stat(s, ALLOC_FROM_PARTIAL);
>  			object = t;
> -			available =  page->objects - page->inuse;
>  		} else {
> -			available = put_cpu_partial(s, page, 0);
> +			put_cpu_partial(s, page, 0);
>  			stat(s, CPU_PARTIAL_NODE);
>  		}
>  		if (kmem_cache_debug(s) || available > s->cpu_partial / 2)
> @@ -1946,7 +1948,7 @@ static void unfreeze_partials(struct kmem_cache *s,
>   * If we did not find a slot then simply move all the partials to the
>   * per node partial list.
>   */
> -static int put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
> +static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
>  {
>  	struct page *oldpage;
>  	int pages;
> @@ -1984,7 +1986,6 @@ static int put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
>  		page->next = oldpage;
>  
>  	} while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page) != oldpage);
> -	return pobjects;
>  }
>  
>  static inline void flush_slab(struct kmem_cache *s, struct kmem_cache_cpu *c)
> -- 
> 1.7.9.5
> 
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