[PATCH v2 08/11] memcg: disable kmem code when not in use.

[Glauber Costa <glommer@parallels.com>]

We can use jump labels to patch the code in or out when not used.

Because the assignment: memcg->kmem_accounted = true is done after the
jump labels increment, we guarantee that the root memcg will always be
selected until all call sites are patched (see memcg_kmem_enabled).
This guarantees that no mischarges are applied.

Jump label decrement happens when the last reference count from the
memcg dies. This will only happen when the caches are all dead.

Signed-off-by: Glauber Costa <glommer@parallels.com>
Acked-by: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
CC: Christoph Lameter <cl@linux.com>
CC: Pekka Enberg <penberg@cs.helsinki.fi>
CC: Michal Hocko <mhocko@suse.cz>
CC: Johannes Weiner <hannes@cmpxchg.org>
CC: Suleiman Souhlal <suleiman@google.com>
---
 include/linux/memcontrol.h |  5 ++++-
 mm/memcontrol.c            | 50 ++++++++++++++++++++++++++++++++++++----------
 2 files changed, 44 insertions(+), 11 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 75b247e..f39d933 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -22,6 +22,7 @@
 #include <linux/cgroup.h>
 #include <linux/vm_event_item.h>
 #include <linux/hardirq.h>
+#include <linux/jump_label.h>
 
 struct mem_cgroup;
 struct page_cgroup;
@@ -401,7 +402,9 @@ struct sock;
 void sock_update_memcg(struct sock *sk);
 void sock_release_memcg(struct sock *sk);
 
-#define memcg_kmem_on 1
+extern struct static_key memcg_kmem_enabled_key;
+#define memcg_kmem_on static_key_false(&memcg_kmem_enabled_key)
+
 bool __memcg_kmem_new_page(gfp_t gfp, void *handle, int order);
 void __memcg_kmem_commit_page(struct page *page, void *handle, int order);
 void __memcg_kmem_free_page(struct page *page, int order);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e9824c1..3216292 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -437,6 +437,10 @@ struct mem_cgroup *mem_cgroup_from_css(struct cgroup_subsys_state *s)
 #include <net/sock.h>
 #include <net/ip.h>
 
+struct static_key memcg_kmem_enabled_key;
+/* so modules can inline the checks */
+EXPORT_SYMBOL(memcg_kmem_enabled_key);
+
 static bool mem_cgroup_is_root(struct mem_cgroup *memcg);
 static int memcg_charge_kmem(struct mem_cgroup *memcg, gfp_t gfp, s64 delta);
 static void memcg_uncharge_kmem(struct mem_cgroup *memcg, s64 delta);
@@ -607,6 +611,16 @@ void __memcg_kmem_free_page(struct page *page, int order)
 	mem_cgroup_put(memcg);
 }
 EXPORT_SYMBOL(__memcg_kmem_free_page);
+
+static void disarm_kmem_keys(struct mem_cgroup *memcg)
+{
+	if (memcg->kmem_accounted)
+		static_key_slow_dec(&memcg_kmem_enabled_key);
+}
+#else
+static void disarm_kmem_keys(struct mem_cgroup *memcg)
+{
+}
 #endif /* CONFIG_MEMCG_KMEM */
 
 #if defined(CONFIG_INET) && defined(CONFIG_MEMCG_KMEM)
@@ -622,6 +636,12 @@ static void disarm_sock_keys(struct mem_cgroup *memcg)
 }
 #endif
 
+static void disarm_static_keys(struct mem_cgroup *memcg)
+{
+	disarm_sock_keys(memcg);
+	disarm_kmem_keys(memcg);
+}
+
 static void drain_all_stock_async(struct mem_cgroup *memcg);
 
 static struct mem_cgroup_per_zone *
@@ -4147,6 +4167,24 @@ static ssize_t mem_cgroup_read(struct cgroup *cont, struct cftype *cft,
 	len = scnprintf(str, sizeof(str), "%llu\n", (unsigned long long)val);
 	return simple_read_from_buffer(buf, nbytes, ppos, str, len);
 }
+
+static void memcg_update_kmem_limit(struct mem_cgroup *memcg, u64 val)
+{
+#ifdef CONFIG_MEMCG_KMEM
+	/*
+	 * Once enabled, can't be disabled. We could in theory disable it if we
+	 * haven't yet created any caches, or if we can shrink them all to
+	 * death. But it is not worth the trouble.
+	 */
+	mutex_lock(&set_limit_mutex);
+	if (!memcg->kmem_accounted && val != RESOURCE_MAX) {
+		static_key_slow_inc(&memcg_kmem_enabled_key);
+		memcg->kmem_accounted = true;
+	}
+	mutex_unlock(&set_limit_mutex);
+#endif
+}
+
 /*
  * The user of this function is...
  * RES_LIMIT.
@@ -4184,15 +4222,7 @@ static int mem_cgroup_write(struct cgroup *cont, struct cftype *cft,
 			ret = res_counter_set_limit(&memcg->kmem, val);
 			if (ret)
 				break;
-			/*
-			 * Once enabled, can't be disabled. We could in theory
-			 * disable it if we haven't yet created any caches, or
-			 * if we can shrink them all to death.
-			 *
-			 * But it is not worth the trouble
-			 */
-			if (!memcg->kmem_accounted && val != RESOURCE_MAX)
-				memcg->kmem_accounted = true;
+			memcg_update_kmem_limit(memcg, val);
 		} else
 			return -EINVAL;
 		break;
@@ -5054,7 +5084,7 @@ static void free_work(struct work_struct *work)
 	 * to move this code around, and make sure it is outside
 	 * the cgroup_lock.
 	 */
-	disarm_sock_keys(memcg);
+	disarm_static_keys(memcg);
 	if (size < PAGE_SIZE)
 		kfree(memcg);
 	else
-- 
1.7.11.2

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