Commit 80cbd911 authored Nov 29, 2007 by Matthew Wilcox Committed by Linus Torvalds Nov 30, 2007

Fix kmem_cache_free performance regression in slab



The database performance group have found that half the cycles spent
in kmem_cache_free are spent in this one call to BUG_ON.  Moving it
into the CONFIG_SLAB_DEBUG-only function cache_free_debugcheck() is a
performance win of almost 0.5% on their particular benchmark.

The call was added as part of commit ddc2e812
with the comment that "overhead should be minimal".  It may have been
minimal at the time, but it isn't now.

[ Quoth Pekka Enberg: "I don't think the BUG_ON per se caused the
  performance regression but rather the virt_to_head_page() changes to
  virt_to_cache() that were added later." ]

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Acked-by: Pekka J Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent e1cca7e8

mm/slab.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -2881,6 +2881,8 @@ static void cache_free_debugcheck(struct kmem_cache cachep, void *objp,
		unsigned int objnr;
		struct slab *slabp;

		BUG_ON(virt_to_cache(objp) != cachep);

		objp -= obj_offset(cachep);
		kfree_debugcheck(objp);
		page = virt_to_head_page(objp);
		@@ -3759,8 +3761,6 @@ void kmem_cache_free(struct kmem_cache cachep, void objp)
		{
		unsigned long flags;

		BUG_ON(virt_to_cache(objp) != cachep);

		local_irq_save(flags);
		debug_check_no_locks_freed(objp, obj_size(cachep));
		__cache_free(cachep, objp);

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