On 09/21/2012 08:48 AM, JoonSoo Kim wrote:

Hi Glauber.

Hi

2012/9/18 Glauber Costa [off-list ref]:

quoted

diff --git a/mm/slub.c b/mm/slub.c
index 0b68d15..9d79216 100644
--- a/mm/slub.c
+++ b/mm/slub.c

@@ -2602,6 +2602,7 @@ redo:
        } else
                __slab_free(s, page, x, addr);

+       kmem_cache_verify_dead(s);
 }

As far as u know, I am not a expert and don't know anything about memcg.
IMHO, this implementation may hurt system performance in some case.

In case of memcg is destoried, remained kmem_cache is marked "dead".
After it is marked,
every free operation to this "dead" kmem_cache call
kmem_cache_verify_dead() and finally call kmem_cache_shrink().

As long as it is restricted to that cache, this is a non issue.
dead caches are exactly what they name imply: dead.

Means that we actively want them to go away, and just don't kill them
right away because they have some inflight objects - which we expect not
to be too much.

kmem_cache_shrink() do invoking kmalloc and flush_all() and taking a
lock for online node and invoking kfree.
Especially, flush_all() may hurt performance largely, because it call
has_cpu_slab() against all the cpus.

Again, this is all right, but being a dead cache, it shouldn't be on any
hot path.

And, I found one case that destroying memcg's kmem_cache don't works properly.
If we destroy memcg after all object is freed, current implementation
doesn't destroy kmem_cache.
kmem_cache_destroy_work_func() check "cachep->memcg_params.nr_pages == 0",
but in this case, it return false, because kmem_cache may have
cpu_slab, and cpu_partials_slabs.
As we already free all objects, kmem_cache_verify_dead() is not invoked forever.
I think that we need another kmem_cache_shrink() in
kmem_cache_destroy_work_func().

I'll take a look here. What you describe makes sense, and can
potentially happen. I tried to handle this case with care in
destroy_all_caches, but I may have always made a mistake...

Did you see this actively happening, or are you just assuming this can
happen from your read of the code?

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