On 09/21/2012 01:33 PM, Pekka Enberg wrote:

On Wed, Sep 19, 2012 at 10:42 AM, Glauber Costa [off-list ref] wrote:

quoted

quoted

quoted

index f2d760c..18de3f6 100644

--- a/mm/slab.c
+++ b/mm/slab.c

@@ -3938,9 +3938,12 @@ EXPORT_SYMBOL(__kmalloc);
  * Free an object which was previously allocated from this
  * cache.
  */
-void kmem_cache_free(struct kmem_cache *cachep, void *objp)
+void kmem_cache_free(struct kmem_cache *s, void *objp)
 {
     unsigned long flags;
+    struct kmem_cache *cachep = virt_to_cache(objp);
+
+    VM_BUG_ON(!slab_equal_or_parent(cachep, s));

This is an extremely hot path of the kernel and you are adding significant
processing. Check how the benchmarks are influenced by this change.
virt_to_cache can be a bit expensive.

Would it be enough for you to have a separate code path for
!CONFIG_MEMCG_KMEM?

I don't really see another way to do it, aside from deriving the cache
from the object in our case. I am open to suggestions if you do.

We should assume that most distributions enable CONFIG_MEMCG_KMEM,
right? Therfore, any performance impact should be dependent on whether
or not kmem memcg is *enabled* at runtime or not.

Can we use the "static key" thingy introduced by tracing folks for this?

Yes.

I am already using static keys extensively in this patchset, and that is
how I intend to handle this particular case.


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