mm, slab: extend slab/shrink to shrink all memcg caches

Currently, a value of '1" is written to /sys/kernel/slab/<slab>/shrink
file to shrink the slab by flushing out all the per-cpu slabs and free
slabs in partial lists.  This can be useful to squeeze out a bit more
memory under extreme condition as well as making the active object counts
in /proc/slabinfo more accurate.

This usually applies only to the root caches, as the SLUB_MEMCG_SYSFS_ON
option is usually not enabled and "slub_memcg_sysfs=1" not set.  Even if
memcg sysfs is turned on, it is too cumbersome and impractical to manage
all those per-memcg sysfs files in a real production system.

So there is no practical way to shrink memcg caches.  Fix this by enabling
a proper write to the shrink sysfs file of the root cache to scan all the
available memcg caches and shrink them as well.  For a non-root memcg
cache (when SLUB_MEMCG_SYSFS_ON or slub_memcg_sysfs is on), only that
cache will be shrunk when written.

On a 2-socket 64-core 256-thread arm64 system with 64k page after
a parallel kernel build, the the amount of memory occupied by slabs
before shrinking slabs were:

 # grep task_struct /proc/slabinfo
 task_struct        53137  53192   4288   61    4 : tunables    0    0
 0 : slabdata    872    872      0
 # grep "^S[lRU]" /proc/meminfo
 Slab:            3936832 kB
 SReclaimable:     399104 kB
 SUnreclaim:      3537728 kB

After shrinking slabs (by echoing "1" to all shrink files):

 # grep "^S[lRU]" /proc/meminfo
 Slab:            1356288 kB
 SReclaimable:     263296 kB
 SUnreclaim:      1092992 kB
 # grep task_struct /proc/slabinfo
 task_struct         2764   6832   4288   61    4 : tunables    0    0
 0 : slabdata    112    112      0

Link: http://lkml.kernel.org/r/20190723151445.7385-1-longman@redhat.com
Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: 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: Michal Hocko <mhocko@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/ABI/testing/sysfs-kernel-slab

@@ -429,10 +429,15 @@ KernelVersion:	2.6.22
 Contact:	Pekka Enberg <penberg@cs.helsinki.fi>,
 		Christoph Lameter <cl@linux-foundation.org>
 Description:
-		The shrink file is written when memory should be reclaimed from
-		a cache.  Empty partial slabs are freed and the partial list is
-		sorted so the slabs with the fewest available objects are used
-		first.
+		The shrink file is used to reclaim unused slab cache
+		memory from a cache.  Empty per-cpu or partial slabs
+		are freed and the partial list is sorted so the slabs
+		with the fewest available objects are used first.
+		It only accepts a value of "1" on write for shrinking
+		the cache. Other input values are considered invalid.
+		Shrinking slab caches might be expensive and can
+		adversely impact other running applications.  So it
+		should be used with care.
 
 What:		/sys/kernel/slab/cache/slab_size
 Date:		May 2007

mm/slab.h

@@ -174,6 +174,7 @@ int __kmem_cache_shrink(struct kmem_cache *);
 void __kmemcg_cache_deactivate(struct kmem_cache *s);
 void __kmemcg_cache_deactivate_after_rcu(struct kmem_cache *s);
 void slab_kmem_cache_release(struct kmem_cache *);
+void kmem_cache_shrink_all(struct kmem_cache *s);
 
 struct seq_file;
 struct file;

mm/slab_common.c

@@ -981,6 +981,43 @@ int kmem_cache_shrink(struct kmem_cache *cachep)
 }
 EXPORT_SYMBOL(kmem_cache_shrink);
 
+/**
+ * kmem_cache_shrink_all - shrink a cache and all memcg caches for root cache
+ * @s: The cache pointer
+ */
+void kmem_cache_shrink_all(struct kmem_cache *s)
+{
+	struct kmem_cache *c;
+
+	if (!IS_ENABLED(CONFIG_MEMCG_KMEM) || !is_root_cache(s)) {
+		kmem_cache_shrink(s);
+		return;
+	}
+
+	get_online_cpus();
+	get_online_mems();
+	kasan_cache_shrink(s);
+	__kmem_cache_shrink(s);
+
+	/*
+	 * We have to take the slab_mutex to protect from the memcg list
+	 * modification.
+	 */
+	mutex_lock(&slab_mutex);
+	for_each_memcg_cache(c, s) {
+		/*
+		 * Don't need to shrink deactivated memcg caches.
+		 */
+		if (s->flags & SLAB_DEACTIVATED)
+			continue;
+		kasan_cache_shrink(c);
+		__kmem_cache_shrink(c);
+	}
+	mutex_unlock(&slab_mutex);
+	put_online_mems();
+	put_online_cpus();
+}
+
 bool slab_is_available(void)
 {
 	return slab_state >= UP;

mm/slub.c

@@ -5298,7 +5298,7 @@ static ssize_t shrink_store(struct kmem_cache *s,
 			const char *buf, size_t length)
 {
 	if (buf[0] == '1')
-		kmem_cache_shrink(s);
+		kmem_cache_shrink_all(s);
 	else
 		return -EINVAL;
 	return length;