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Subject[PATCH v4 13/19] mm: memcg/slab: simplify memcg cache creation
Date
Because the number of non-root kmem_caches doesn't depend on the
number of memory cgroups anymore and is generally not very big,
there is no more need for a dedicated workqueue.

Also, as there is no more need to pass any arguments to the
memcg_create_kmem_cache() except the root kmem_cache, it's
possible to just embed the work structure into the kmem_cache
and avoid the dynamic allocation of the work structure.

This will also simplify the synchronization: for each root kmem_cache
there is only one work. So there will be no more concurrent attempts
to create a non-root kmem_cache for a root kmem_cache: the second and
all following attempts to queue the work will fail.

On the kmem_cache destruction path there is no more need to call the
expensive flush_workqueue() and wait for all pending works to be
finished. Instead, cancel_work_sync() can be used to cancel/wait for
only one work.

Signed-off-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
---
include/linux/memcontrol.h | 1 -
mm/memcontrol.c | 48 +-------------------------------------
mm/slab.h | 2 ++
mm/slab_common.c | 22 +++++++++--------
4 files changed, 15 insertions(+), 58 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 4f5cf4d621f6..18ef6c85de5b 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1420,7 +1420,6 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size);
void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size);

extern struct static_key_false memcg_kmem_enabled_key;
-extern struct workqueue_struct *memcg_kmem_cache_wq;

extern int memcg_nr_cache_ids;
void memcg_get_cache_ids(void);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 7674c8a6c84d..883969cdfb5f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -399,8 +399,6 @@ void memcg_put_cache_ids(void)
*/
DEFINE_STATIC_KEY_FALSE(memcg_kmem_enabled_key);
EXPORT_SYMBOL(memcg_kmem_enabled_key);
-
-struct workqueue_struct *memcg_kmem_cache_wq;
#endif

static int memcg_shrinker_map_size;
@@ -2852,39 +2850,6 @@ static void memcg_free_cache_id(int id)
ida_simple_remove(&memcg_cache_ida, id);
}

-struct memcg_kmem_cache_create_work {
- struct kmem_cache *cachep;
- struct work_struct work;
-};
-
-static void memcg_kmem_cache_create_func(struct work_struct *w)
-{
- struct memcg_kmem_cache_create_work *cw =
- container_of(w, struct memcg_kmem_cache_create_work, work);
- struct kmem_cache *cachep = cw->cachep;
-
- memcg_create_kmem_cache(cachep);
-
- kfree(cw);
-}
-
-/*
- * Enqueue the creation of a per-memcg kmem_cache.
- */
-static void memcg_schedule_kmem_cache_create(struct kmem_cache *cachep)
-{
- struct memcg_kmem_cache_create_work *cw;
-
- cw = kmalloc(sizeof(*cw), GFP_NOWAIT | __GFP_NOWARN);
- if (!cw)
- return;
-
- cw->cachep = cachep;
- INIT_WORK(&cw->work, memcg_kmem_cache_create_func);
-
- queue_work(memcg_kmem_cache_wq, &cw->work);
-}
-
/**
* memcg_kmem_get_cache: select memcg or root cache for allocation
* @cachep: the original global kmem cache
@@ -2901,7 +2866,7 @@ struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep)

memcg_cachep = READ_ONCE(cachep->memcg_params.memcg_cache);
if (unlikely(!memcg_cachep)) {
- memcg_schedule_kmem_cache_create(cachep);
+ queue_work(system_wq, &cachep->memcg_params.work);
return cachep;
}

@@ -7000,17 +6965,6 @@ static int __init mem_cgroup_init(void)
{
int cpu, node;

-#ifdef CONFIG_MEMCG_KMEM
- /*
- * Kmem cache creation is mostly done with the slab_mutex held,
- * so use a workqueue with limited concurrency to avoid stalling
- * all worker threads in case lots of cgroups are created and
- * destroyed simultaneously.
- */
- memcg_kmem_cache_wq = alloc_workqueue("memcg_kmem_cache", 0, 1);
- BUG_ON(!memcg_kmem_cache_wq);
-#endif
-
cpuhp_setup_state_nocalls(CPUHP_MM_MEMCQ_DEAD, "mm/memctrl:dead", NULL,
memcg_hotplug_cpu_dead);

diff --git a/mm/slab.h b/mm/slab.h
index 31a54e22b3fb..a730cbe1db66 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -45,12 +45,14 @@ struct kmem_cache {
* @memcg_cache: pointer to memcg kmem cache, used by all non-root memory
* cgroups.
* @root_caches_node: list node for slab_root_caches list.
+ * @work: work struct used to create the non-root cache.
*/
struct memcg_cache_params {
struct kmem_cache *root_cache;

struct kmem_cache *memcg_cache;
struct list_head __root_caches_node;
+ struct work_struct work;
};
#endif /* CONFIG_SLOB */

diff --git a/mm/slab_common.c b/mm/slab_common.c
index e9deaafddbb6..10aa2acb84ca 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -132,10 +132,18 @@ int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t nr,

LIST_HEAD(slab_root_caches);

+static void memcg_kmem_cache_create_func(struct work_struct *work)
+{
+ struct kmem_cache *cachep = container_of(work, struct kmem_cache,
+ memcg_params.work);
+ memcg_create_kmem_cache(cachep);
+}
+
void slab_init_memcg_params(struct kmem_cache *s)
{
s->memcg_params.root_cache = NULL;
s->memcg_params.memcg_cache = NULL;
+ INIT_WORK(&s->memcg_params.work, memcg_kmem_cache_create_func);
}

static void init_memcg_params(struct kmem_cache *s,
@@ -584,15 +592,9 @@ static int shutdown_memcg_caches(struct kmem_cache *s)
return 0;
}

-static void flush_memcg_workqueue(struct kmem_cache *s)
+static void cancel_memcg_cache_creation(struct kmem_cache *s)
{
- /*
- * SLAB and SLUB create memcg kmem_caches through workqueue and SLUB
- * deactivates the memcg kmem_caches through workqueue. Make sure all
- * previous workitems on workqueue are processed.
- */
- if (likely(memcg_kmem_cache_wq))
- flush_workqueue(memcg_kmem_cache_wq);
+ cancel_work_sync(&s->memcg_params.work);
}
#else
static inline int shutdown_memcg_caches(struct kmem_cache *s)
@@ -600,7 +602,7 @@ static inline int shutdown_memcg_caches(struct kmem_cache *s)
return 0;
}

-static inline void flush_memcg_workqueue(struct kmem_cache *s)
+static inline void cancel_memcg_cache_creation(struct kmem_cache *s)
{
}
#endif /* CONFIG_MEMCG_KMEM */
@@ -619,7 +621,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
if (unlikely(!s))
return;

- flush_memcg_workqueue(s);
+ cancel_memcg_cache_creation(s);

get_online_cpus();
get_online_mems();
--
2.25.4
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