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SubjectRe: [PATCH v2] mm/slab: Improve performance of gathering slabinfo stats
2016-08-05 4:01 GMT+09:00 Aruna Ramakrishna <aruna.ramakrishna@oracle.com>:
> On large systems, when some slab caches grow to millions of objects (and
> many gigabytes), running 'cat /proc/slabinfo' can take up to 1-2 seconds.
> During this time, interrupts are disabled while walking the slab lists
> (slabs_full, slabs_partial, and slabs_free) for each node, and this
> sometimes causes timeouts in other drivers (for instance, Infiniband).
>
> This patch optimizes 'cat /proc/slabinfo' by maintaining a counter for
> total number of allocated slabs per node, per cache. This counter is
> updated when a slab is created or destroyed. This enables us to skip
> traversing the slabs_full list while gathering slabinfo statistics, and
> since slabs_full tends to be the biggest list when the cache is large, it
> results in a dramatic performance improvement. Getting slabinfo statistics
> now only requires walking the slabs_free and slabs_partial lists, and
> those lists are usually much smaller than slabs_full. We tested this after
> growing the dentry cache to 70GB, and the performance improved from 2s to
> 5ms.
>
> Signed-off-by: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.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>
> ---
> Note: this has been tested only on x86_64.
>
> mm/slab.c | 25 ++++++++++++++++---------
> mm/slab.h | 15 ++++++++++++++-
> mm/slub.c | 19 +------------------
> 3 files changed, 31 insertions(+), 28 deletions(-)
>
> diff --git a/mm/slab.c b/mm/slab.c
> index 261147b..d683840 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -233,6 +233,7 @@ static void kmem_cache_node_init(struct kmem_cache_node *parent)
> spin_lock_init(&parent->list_lock);
> parent->free_objects = 0;
> parent->free_touched = 0;
> + atomic_long_set(&parent->nr_slabs, 0);
> }
>
> #define MAKE_LIST(cachep, listp, slab, nodeid) \
> @@ -2333,6 +2334,7 @@ static int drain_freelist(struct kmem_cache *cache,
> n->free_objects -= cache->num;
> spin_unlock_irq(&n->list_lock);
> slab_destroy(cache, page);
> + atomic_long_dec(&n->nr_slabs);
> nr_freed++;
> }

Please decrease counter when a slab is detached from the list.
Otherwise, there would be inconsistent between counter and
number of attached slab on the list.

> out:
> @@ -2736,6 +2738,8 @@ static struct page *cache_grow_begin(struct kmem_cache *cachep,
> if (gfpflags_allow_blocking(local_flags))
> local_irq_disable();
>
> + atomic_long_inc(&n->nr_slabs);
> +
> return page;

Please increase counter when a slab is attached to the list
in cache_grow_end().

> opps1:
> @@ -3455,6 +3459,7 @@ static void free_block(struct kmem_cache *cachep, void **objpp,
>
> page = list_last_entry(&n->slabs_free, struct page, lru);
> list_move(&page->lru, list);
> + atomic_long_dec(&n->nr_slabs);
> }
> }
>
> @@ -4111,6 +4116,8 @@ void get_slabinfo(struct kmem_cache *cachep, struct slabinfo *sinfo)
> unsigned long num_objs;
> unsigned long active_slabs = 0;
> unsigned long num_slabs, free_objects = 0, shared_avail = 0;
> + unsigned long num_slabs_partial = 0, num_slabs_free = 0;
> + unsigned long num_slabs_full = 0;
> const char *name;
> char *error = NULL;
> int node;
> @@ -4120,36 +4127,36 @@ void get_slabinfo(struct kmem_cache *cachep, struct slabinfo *sinfo)
> num_slabs = 0;
> for_each_kmem_cache_node(cachep, node, n) {
>
> + num_slabs += node_nr_slabs(n);
> check_irq_on();
> spin_lock_irq(&n->list_lock);
>
> - list_for_each_entry(page, &n->slabs_full, lru) {
> - if (page->active != cachep->num && !error)
> - error = "slabs_full accounting error";
> - active_objs += cachep->num;
> - active_slabs++;
> - }
> list_for_each_entry(page, &n->slabs_partial, lru) {
> if (page->active == cachep->num && !error)
> error = "slabs_partial accounting error";
> if (!page->active && !error)
> error = "slabs_partial accounting error";
> active_objs += page->active;
> - active_slabs++;
> + num_slabs_partial++;
> }
> +
> list_for_each_entry(page, &n->slabs_free, lru) {
> if (page->active && !error)
> error = "slabs_free accounting error";
> - num_slabs++;
> + num_slabs_free++;
> }
> +
> free_objects += n->free_objects;
> if (n->shared)
> shared_avail += n->shared->avail;
>
> spin_unlock_irq(&n->list_lock);
> }
> - num_slabs += active_slabs;
> num_objs = num_slabs * cachep->num;
> + active_slabs = num_slabs - num_slabs_free;
> + num_slabs_full = num_slabs - (num_slabs_partial + num_slabs_free);
> + active_objs += (num_slabs_full * cachep->num);
> +
> if (num_objs - active_objs != free_objects && !error)
> error = "free_objects accounting error";
>
> diff --git a/mm/slab.h b/mm/slab.h
> index 9653f2e..5740cec 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -427,6 +427,7 @@ static inline void slab_post_alloc_hook(struct kmem_cache *s, gfp_t flags,
> */
> struct kmem_cache_node {
> spinlock_t list_lock;
> + atomic_long_t nr_slabs;

If above my comments are fixed, all counting would be done with
holding a lock. So, atomic definition isn't needed for the SLAB.
I think that it's better not to commonize this counting.

Thanks.

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