Re: [PATCH 3/9] slab: simplify shutdown_memcg_caches()

Vladimir Davydov
2017-01-14T13:27:22Z

On Sat, Jan 14, 2017 at 12:54:43AM -0500, Tejun Heo wrote:
> shutdown_memcg_caches() shuts down all memcg caches associated with a
> root cache. It first walks the index table clearing and shutting down
> each entry and then shuts down the ones on
> root_cache->memcg_params.list. As active caches are on both the table
> and the list, they're stashed away from the list to avoid shutting
> down twice and then get spliced back later.
>
> This is unnecessarily complication. All memcg caches are on
> root_cache->memcg_params.list. The function can simply clear the
> index table and shut down all caches on the list. There's no need to
> muck with temporary stashing.
>
> Simplify the code.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> ---
> mm/slab_common.c | 32 +++++---------------------------
> 1 file changed, 5 insertions(+), 27 deletions(-)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 851c75e..45aa67c 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -634,48 +634,26 @@ static int shutdown_memcg_caches(struct kmem_cache *s)
> {
> struct memcg_cache_array *arr;
> struct kmem_cache *c, *c2;
> - LIST_HEAD(busy);
> int i;
>
> BUG_ON(!is_root_cache(s));
>
> /*
> - * First, shutdown active caches, i.e. caches that belong to online
> - * memory cgroups.
> + * First, clear the pointers to all memcg caches so that they will
> + * never be accessed even if the root cache stays alive.
> */
> arr = rcu_dereference_protected(s->memcg_params.memcg_caches,
> lockdep_is_held(&slab_mutex));
> - for_each_memcg_cache_index(i) {
> - c = arr->entries[i];
> - if (!c)
> - continue;
> - if (shutdown_cache(c))
> - /*
> - * The cache still has objects. Move it to a temporary
> - * list so as not to try to destroy it for a second
> - * time while iterating over inactive caches below.
> - */
> - list_move(&c->memcg_params.list, &busy);
> - else
> - /*
> - * The cache is empty and will be destroyed soon. Clear
> - * the pointer to it in the memcg_caches array so that
> - * it will never be accessed even if the root cache
> - * stays alive.
> - */
> - arr->entries[i] = NULL;
> - }
> + for_each_memcg_cache_index(i)
> + arr->entries[i] = NULL;
>
> /*
> - * Second, shutdown all caches left from memory cgroups that are now
> - * offline.
> + * Shutdown all caches.
> */
> list_for_each_entry_safe(c, c2, &s->memcg_params.list,
> memcg_params.list)
> shutdown_cache(c);

The point of this complexity was to leave caches that happen to have
objects when kmem_cache_destroy() is called on the list, so that they
could be reused later. This behavior was inherited from the global
caches - if kmem_cache_destroy() is called on a cache that still has
object, we print a warning message and don't destroy the cache. This
patch changes this behavior.

>
> - list_splice(&busy, &s->memcg_params.list);
> -
> /*
> * A cache being destroyed must be empty. In particular, this means
> * that all per memcg caches attached to it must be empty too.