mm: make counting of list_lru_one::nr_items lockless

During the reclaiming slab of a memcg, shrink_slab iterates over all
registered shrinkers in the system, and tries to count and consume
objects related to the cgroup.  In case of memory pressure, this behaves
bad: I observe high system time and time spent in list_lru_count_one()
for many processes on RHEL7 kernel.

This patch makes list_lru_node::memcg_lrus rcu protected, that allows to
skip taking spinlock in list_lru_count_one().

Shakeel Butt with the patch observes significant perf graph change.  He
says:

========================================================================
Setup: running a fork-bomb in a memcg of 200MiB on a 8GiB and 4 vcpu
VM and recording the trace with 'perf record -g -a'.

The trace without the patch:

+  34.19%     fb.sh  [kernel.kallsyms]  [k] queued_spin_lock_slowpath
+  30.77%     fb.sh  [kernel.kallsyms]  [k] _raw_spin_lock
+   3.53%     fb.sh  [kernel.kallsyms]  [k] list_lru_count_one
+   2.26%     fb.sh  [kernel.kallsyms]  [k] super_cache_count
+   1.68%     fb.sh  [kernel.kallsyms]  [k] shrink_slab
+   0.59%     fb.sh  [kernel.kallsyms]  [k] down_read_trylock
+   0.48%     fb.sh  [kernel.kallsyms]  [k] _raw_spin_unlock_irqrestore
+   0.38%     fb.sh  [kernel.kallsyms]  [k] shrink_node_memcg
+   0.32%     fb.sh  [kernel.kallsyms]  [k] queue_work_on
+   0.26%     fb.sh  [kernel.kallsyms]  [k] count_shadow_nodes

With the patch:

+   0.16%     swapper  [kernel.kallsyms]    [k] default_idle
+   0.13%     oom_reaper  [kernel.kallsyms]    [k] mutex_spin_on_owner
+   0.05%     perf  [kernel.kallsyms]    [k] copy_user_generic_string
+   0.05%     init.real  [kernel.kallsyms]    [k] wait_consider_task
+   0.05%     kworker/0:0  [kernel.kallsyms]    [k] finish_task_switch
+   0.04%     kworker/2:1  [kernel.kallsyms]    [k] finish_task_switch
+   0.04%     kworker/3:1  [kernel.kallsyms]    [k] finish_task_switch
+   0.04%     kworker/1:0  [kernel.kallsyms]    [k] finish_task_switch
+   0.03%     binary  [kernel.kallsyms]    [k] copy_page
========================================================================

Thanks Shakeel for the testing.

[[email protected]: v2]
  Link: http://lkml.kernel.org/r/151203869520.3915.2587549826865799173.stgit@localhost.localdomain
Link: http://lkml.kernel.org/r/150583358557.26700.8490036563698102569.stgit@localhost.localdomain
Signed-off-by: Kirill Tkhai <[email protected]>
Tested-by: Shakeel Butt <[email protected]>
Acked-by: Vladimir Davydov <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Johannes Weiner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/list_lru.h

@@ -32,6 +32,7 @@ struct list_lru_one {
 };
 
 struct list_lru_memcg {
+	struct rcu_head		rcu;
 	/* array of per cgroup lists, indexed by memcg_cache_id */
 	struct list_lru_one	*lru[0];
 };
@@ -43,7 +44,7 @@ struct list_lru_node {
 	struct list_lru_one	lru;
 #if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
 	/* for cgroup aware lrus points to per cgroup lists, otherwise NULL */
-	struct list_lru_memcg	*memcg_lrus;
+	struct list_lru_memcg	__rcu *memcg_lrus;
 #endif
 	long nr_items;
 } ____cacheline_aligned_in_smp;

mm/list_lru.c

@@ -52,14 +52,15 @@ static inline bool list_lru_memcg_aware(struct list_lru *lru)
 static inline struct list_lru_one *
 list_lru_from_memcg_idx(struct list_lru_node *nlru, int idx)
 {
+	struct list_lru_memcg *memcg_lrus;
 	/*
-	 * The lock protects the array of per cgroup lists from relocation
-	 * (see memcg_update_list_lru_node).
+	 * Either lock or RCU protects the array of per cgroup lists
+	 * from relocation (see memcg_update_list_lru_node).
 	 */
-	lockdep_assert_held(&nlru->lock);
-	if (nlru->memcg_lrus && idx >= 0)
-		return nlru->memcg_lrus->lru[idx];
-
+	memcg_lrus = rcu_dereference_check(nlru->memcg_lrus,
+					   lockdep_is_held(&nlru->lock));
+	if (memcg_lrus && idx >= 0)
+		return memcg_lrus->lru[idx];
 	return &nlru->lru;
 }
 
@@ -168,10 +169,10 @@ static unsigned long __list_lru_count_one(struct list_lru *lru,
 	struct list_lru_one *l;
 	unsigned long count;
 
-	spin_lock(&nlru->lock);
+	rcu_read_lock();
 	l = list_lru_from_memcg_idx(nlru, memcg_idx);
 	count = l->nr_items;
-	spin_unlock(&nlru->lock);
+	rcu_read_unlock();
 
 	return count;
 }
@@ -324,24 +325,41 @@ static int __memcg_init_list_lru_node(struct list_lru_memcg *memcg_lrus,
 
 static int memcg_init_list_lru_node(struct list_lru_node *nlru)
 {
+	struct list_lru_memcg *memcg_lrus;
 	int size = memcg_nr_cache_ids;
 
-	nlru->memcg_lrus = kvmalloc(size * sizeof(void *), GFP_KERNEL);
-	if (!nlru->memcg_lrus)
+	memcg_lrus = kvmalloc(sizeof(*memcg_lrus) +
+			      size * sizeof(void *), GFP_KERNEL);
+	if (!memcg_lrus)
 		return -ENOMEM;
 
-	if (__memcg_init_list_lru_node(nlru->memcg_lrus, 0, size)) {
-		kvfree(nlru->memcg_lrus);
+	if (__memcg_init_list_lru_node(memcg_lrus, 0, size)) {
+		kvfree(memcg_lrus);
 		return -ENOMEM;
 	}
+	RCU_INIT_POINTER(nlru->memcg_lrus, memcg_lrus);
 
 	return 0;
 }
 
 static void memcg_destroy_list_lru_node(struct list_lru_node *nlru)
 {
-	__memcg_destroy_list_lru_node(nlru->memcg_lrus, 0, memcg_nr_cache_ids);
-	kvfree(nlru->memcg_lrus);
+	struct list_lru_memcg *memcg_lrus;
+	/*
+	 * This is called when shrinker has already been unregistered,
+	 * and nobody can use it. So, there is no need to use kvfree_rcu().
+	 */
+	memcg_lrus = rcu_dereference_protected(nlru->memcg_lrus, true);
+	__memcg_destroy_list_lru_node(memcg_lrus, 0, memcg_nr_cache_ids);
+	kvfree(memcg_lrus);
+}
+
+static void kvfree_rcu(struct rcu_head *head)
+{
+	struct list_lru_memcg *mlru;
+
+	mlru = container_of(head, struct list_lru_memcg, rcu);
+	kvfree(mlru);
 }
 
 static int memcg_update_list_lru_node(struct list_lru_node *nlru,
@@ -351,8 +369,9 @@ static int memcg_update_list_lru_node(struct list_lru_node *nlru,
 
 	BUG_ON(old_size > new_size);
 
-	old = nlru->memcg_lrus;
-	new = kvmalloc(new_size * sizeof(void *), GFP_KERNEL);
+	old = rcu_dereference_protected(nlru->memcg_lrus,
+					lockdep_is_held(&list_lrus_mutex));
+	new = kvmalloc(sizeof(*new) + new_size * sizeof(void *), GFP_KERNEL);
 	if (!new)
 		return -ENOMEM;
 
@@ -361,29 +380,33 @@ static int memcg_update_list_lru_node(struct list_lru_node *nlru,
 		return -ENOMEM;
 	}
 
-	memcpy(new, old, old_size * sizeof(void *));
+	memcpy(&new->lru, &old->lru, old_size * sizeof(void *));
 
 	/*
-	 * The lock guarantees that we won't race with a reader
-	 * (see list_lru_from_memcg_idx).
+	 * The locking below allows readers that hold nlru->lock avoid taking
+	 * rcu_read_lock (see list_lru_from_memcg_idx).
 	 *
 	 * Since list_lru_{add,del} may be called under an IRQ-safe lock,
 	 * we have to use IRQ-safe primitives here to avoid deadlock.
 	 */
 	spin_lock_irq(&nlru->lock);
-	nlru->memcg_lrus = new;
+	rcu_assign_pointer(nlru->memcg_lrus, new);
 	spin_unlock_irq(&nlru->lock);
 
-	kvfree(old);
+	call_rcu(&old->rcu, kvfree_rcu);
 	return 0;
 }
 
 static void memcg_cancel_update_list_lru_node(struct list_lru_node *nlru,
 					      int old_size, int new_size)
 {
+	struct list_lru_memcg *memcg_lrus;
+
+	memcg_lrus = rcu_dereference_protected(nlru->memcg_lrus,
+					       lockdep_is_held(&list_lrus_mutex));
 	/* do not bother shrinking the array back to the old size, because we
 	 * cannot handle allocation failures here */
-	__memcg_destroy_list_lru_node(nlru->memcg_lrus, old_size, new_size);
+	__memcg_destroy_list_lru_node(memcg_lrus, old_size, new_size);
 }
 
 static int memcg_init_list_lru(struct list_lru *lru, bool memcg_aware)