On Fri, Jun 13, 2014 at 12:38:22AM +0400, Vladimir Davydov wrote:
> Since a dead memcg cache is destroyed only after the last slab allocated
> to it is freed, we must disable caching of free objects/slabs for such
> caches, otherwise they will be hanging around forever.
>
> For SLAB that means we must disable per cpu free object arrays and make
> free_block always discard empty slabs irrespective of node's free_limit.
>
> To disable per cpu arrays, we free them on kmem_cache_shrink (see
> drain_cpu_caches -> do_drain) and make __cache_free fall back to
> free_block if there is no per cpu array. Also, we have to disable
> allocation of per cpu arrays on cpu hotplug for dead caches (see
> cpuup_prepare, __do_tune_cpucache).
>
> After we disabled free objects/slabs caching, there is no need to reap
> those caches periodically. Moreover, it will only result in slowdown. So
> we also make cache_reap skip then.
>
> Signed-off-by: Vladimir Davydov <vdavy...@parallels.com>
> ---
> mm/slab.c | 31 ++++++++++++++++++++++++++++++-
> 1 file changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/mm/slab.c b/mm/slab.c
> index b3af82419251..7e91f5f1341d 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -1210,6 +1210,9 @@ static int cpuup_prepare(long cpu)
> struct array_cache *shared = NULL;
> struct array_cache **alien = NULL;
>
> + if (memcg_cache_dead(cachep))
> + continue;
> +
> nc = alloc_arraycache(node, cachep->limit,
> cachep->batchcount, GFP_KERNEL);
> if (!nc)
> @@ -2411,10 +2414,18 @@ static void do_drain(void *arg)
>
> check_irq_off();
> ac = cpu_cache_get(cachep);
> + if (!ac)
> + return;
> +
> spin_lock(&cachep->node[node]->list_lock);
> free_block(cachep, ac->entry, ac->avail, node);
> spin_unlock(&cachep->node[node]->list_lock);
> ac->avail = 0;
> +
> + if (memcg_cache_dead(cachep)) {
> + cachep->array[smp_processor_id()] = NULL;
> + kfree(ac);
> + }
> }
>
> static void drain_cpu_caches(struct kmem_cache *cachep)
> @@ -3368,7 +3379,8 @@ static void free_block(struct kmem_cache *cachep, void
> **objpp, int nr_objects,
>
> /* fixup slab chains */
> if (page->active == 0) {
> - if (n->free_objects > n->free_limit) {
> + if (n->free_objects > n->free_limit ||
> + memcg_cache_dead(cachep)) {
> n->free_objects -= cachep->num;
> /* No need to drop any previously held
> * lock here, even if we have a off-slab slab
> @@ -3462,6 +3474,17 @@ static inline void __cache_free(struct kmem_cache
> *cachep, void *objp,
>
> kmemcheck_slab_free(cachep, objp, cachep->object_size);
>
> +#ifdef CONFIG_MEMCG_KMEM
> + if (unlikely(!ac)) {
> + int nodeid = page_to_nid(virt_to_page(objp));
> +
> + spin_lock(&cachep->node[nodeid]->list_lock);
> + free_block(cachep, &objp, 1, nodeid);
> + spin_unlock(&cachep->node[nodeid]->list_lock);
> + return;
> + }
> +#endif
> +
And, please document intention of this code. :)
And, you said that this way of implementation would be slow because
there could be many object in dead caches and this implementation
needs node spin_lock on each object freeing. Is it no problem now?
If you have any performance data about this implementation and
alternative one, could you share it?
Thanks.
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