[PATCH -mm 3/3] slub: make dead caches discard free slabs immediately

From: Vladimir Davydov
Date: Mon Jan 26 2015 - 07:56:36 EST

To speed up further allocations SLUB may store empty slabs in per
cpu/node partial lists instead of freeing them immediately. This
prevents per memcg caches destruction, because kmem caches created for a
memory cgroup are only destroyed after the last page charged to the
cgroup is freed.

To fix this issue, this patch resurrects approach first proposed in [1].
It forbids SLUB to cache empty slabs after the memory cgroup that the
cache belongs to was destroyed. It is achieved by setting kmem_cache's
cpu_partial and min_partial constants to 0 and tuning put_cpu_partial()
so that it would drop frozen empty slabs immediately if cpu_partial = 0.

The runtime overhead is minimal. From all the hot functions, we only
touch relatively cold put_cpu_partial(): we make it call
unfreeze_partials() after freezing a slab that belongs to an offline
memory cgroup. Since slab freezing exists to avoid moving slabs from/to
a partial list on free/alloc, and there can't be allocations from dead
caches, it shouldn't cause any overhead. We do have to disable
preemption for put_cpu_partial() to achieve that though.

The original patch was accepted well and even merged to the mm tree.
However, I decided to withdraw it due to changes happening to the memcg
core at that time. I had an idea of introducing per-memcg shrinkers for
kmem caches, but now, as memcg has finally settled down, I do not see it
as an option, because SLUB shrinker would be too costly to call since
SLUB does not keep free slabs on a separate list. Besides, we currently
do not even call per-memcg shrinkers for offline memcgs. Overall, it
would introduce much more complexity to both SLUB and memcg than this
small patch.

Regarding to SLAB, there's no problem with it, because it shrinks
per-cpu/node caches periodically. Thanks to list_lru reparenting, we no
longer keep entries for offline cgroups in per-memcg arrays (such as
memcg_cache_params->memcg_caches), so we do not have to bother if a
per-memcg cache will be shrunk a bit later than it could be.

[1] http://thread.gmane.org/gmane.linux.kernel.mm/118649/focus=118650

Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
---
mm/slab.c | 2 +-
mm/slab.h | 2 +-
mm/slab_common.c | 15 +++++++++++++--
mm/slob.c | 2 +-
mm/slub.c | 25 ++++++++++++++++++++-----
5 files changed, 36 insertions(+), 10 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index 279c44d6d8e1..f0514df07b85 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2400,7 +2400,7 @@ static int __cache_shrink(struct kmem_cache *cachep)
return (ret ? 1 : 0);
}

-void __kmem_cache_shrink(struct kmem_cache *cachep)
+void __kmem_cache_shrink(struct kmem_cache *cachep, bool deactivate)
{
__cache_shrink(cachep);
}
diff --git a/mm/slab.h b/mm/slab.h
index c036e520d2cf..041260197984 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -138,7 +138,7 @@ static inline unsigned long kmem_cache_flags(unsigned long object_size,
#define CACHE_CREATE_MASK (SLAB_CORE_FLAGS | SLAB_DEBUG_FLAGS | SLAB_CACHE_FLAGS)

int __kmem_cache_shutdown(struct kmem_cache *);
-void __kmem_cache_shrink(struct kmem_cache *);
+void __kmem_cache_shrink(struct kmem_cache *, bool);
void slab_kmem_cache_release(struct kmem_cache *);

struct seq_file;
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 6803639fdff0..472ab7fcffd4 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -549,10 +549,13 @@ void memcg_deactivate_kmem_caches(struct mem_cgroup *memcg)
{
int idx;
struct memcg_cache_array *arr;
- struct kmem_cache *s;
+ struct kmem_cache *s, *c;

idx = memcg_cache_id(memcg);

+ get_online_cpus();
+ get_online_mems();
+
mutex_lock(&slab_mutex);
list_for_each_entry(s, &slab_caches, list) {
if (!is_root_cache(s))
@@ -560,9 +563,17 @@ void memcg_deactivate_kmem_caches(struct mem_cgroup *memcg)

arr = rcu_dereference_protected(s->memcg_params.memcg_caches,
lockdep_is_held(&slab_mutex));
+ c = arr->entries[idx];
+ if (!c)
+ continue;
+
+ __kmem_cache_shrink(c, true);
arr->entries[idx] = NULL;
}
mutex_unlock(&slab_mutex);
+
+ put_online_mems();
+ put_online_cpus();
}

void memcg_destroy_kmem_caches(struct mem_cgroup *memcg)
@@ -646,7 +657,7 @@ void kmem_cache_shrink(struct kmem_cache *cachep)
{
get_online_cpus();
get_online_mems();
- __kmem_cache_shrink(cachep);
+ __kmem_cache_shrink(cachep, false);
put_online_mems();
put_online_cpus();
}
diff --git a/mm/slob.c b/mm/slob.c
index 043a14b6ccbe..e63ff9d926dc 100644
--- a/mm/slob.c
+++ b/mm/slob.c
@@ -618,7 +618,7 @@ int __kmem_cache_shutdown(struct kmem_cache *c)
return 0;
}

-void __kmem_cache_shrink(struct kmem_cache *c)
+void __kmem_cache_shrink(struct kmem_cache *c, bool deactivate)
{
}

diff --git a/mm/slub.c b/mm/slub.c
index c09d93dde40e..6f57824af019 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2007,6 +2007,7 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
int pages;
int pobjects;

+ preempt_disable();
do {
pages = 0;
pobjects = 0;
@@ -2040,6 +2041,14 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)

} while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page)
!= oldpage);
+ if (unlikely(!s->cpu_partial)) {
+ unsigned long flags;
+
+ local_irq_save(flags);
+ unfreeze_partials(s, this_cpu_ptr(s->cpu_slab));
+ local_irq_restore(flags);
+ }
+ preempt_enable();
#endif
}

@@ -3368,7 +3377,7 @@ EXPORT_SYMBOL(kfree);
* being allocated from last increasing the chance that the last objects
* are freed in them.
*/
-void __kmem_cache_shrink(struct kmem_cache *s)
+void __kmem_cache_shrink(struct kmem_cache *s, bool deactivate)
{
int node;
int i;
@@ -3381,6 +3390,15 @@ void __kmem_cache_shrink(struct kmem_cache *s)
kmalloc(sizeof(struct list_head) * objects, GFP_KERNEL);
unsigned long flags;

+ if (deactivate) {
+ /*
+ * Disable empty slabs caching. Used to avoid pinning offline
+ * memory cgroups by freeable kmem pages.
+ */
+ s->cpu_partial = 0;
+ s->min_partial = 0;
+ }
+
if (!slabs_by_inuse) {
/*
* Do not abort if we failed to allocate a temporary array.
@@ -3392,9 +3410,6 @@ void __kmem_cache_shrink(struct kmem_cache *s)

flush_all(s);
for_each_kmem_cache_node(s, node, n) {
- if (!n->nr_partial)
- continue;
-
for (i = 0; i < objects; i++)
INIT_LIST_HEAD(slabs_by_inuse + i);

@@ -3438,7 +3453,7 @@ static int slab_mem_going_offline_callback(void *arg)

mutex_lock(&slab_mutex);
list_for_each_entry(s, &slab_caches, list)
- __kmem_cache_shrink(s);
+ __kmem_cache_shrink(s, false);
mutex_unlock(&slab_mutex);

return 0;
--
1.7.10.4

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