[Devel] [RFC] memory controller : backgorund reclaim and avoid excessive locking [3/5] throttling

[KAMEZAWA Hiroyuki]

Throttle memory reclaim interface for cgroup.

This patch adds..
 - a limit for simultaneous callers of try_to_free_mem_cgroup_pages().
 - interface for that. memory.shrinkers.

There are some reasons.
 - try_to_free... is very heavy and shoulnd't be called too much at once.
 - When the number of callers of try_to_free.. is big, we'll reclaim
   too much memory.

By this interface, a user can control the # of threads which can enter
try_to_free...

Default is 10240 ...a enough big number for unlimited. Maybe this should
be changed.


Changes from previous one.
  - Added an interface to control the limit.
  - don't call wake_up at uncharge()...it seems hevay..
    Instead of that, sleepers use schedule_timeout(HZ/100). (10ms)

Considerations:
  - Should we add this 'throttle' to global_lru at first ?
  - Do we need more knobs ?
  - Should default value to be automtically estimated value ?
    (I used '# of cpus/4' as default in previous version.)

    
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>


 mm/memcontrol.c |  235 ++++++++++++++++++++++++++++++--------------------------
 1 files changed, 127 insertions(+), 108 deletions(-)

Index: linux-2.6.24-mm1/mm/memcontrol.c
===================================================================
--- linux-2.6.24-mm1.orig/mm/memcontrol.c
+++ linux-2.6.24-mm1/mm/memcontrol.c
@@ -145,8 +145,17 @@ struct mem_cgroup {
 		wait_queue_head_t	waitq;
 		struct task_struct	*kthread;
 	} daemon;
+	/*
+	 * throttling params for reclaim.
+	 */
+	struct {
+		int		limit;
+		atomic_t	reclaimers;
+		wait_queue_head_t waitq;
+	} throttle;
 };
 
+
 /*
  * We use the lower bit of the page->page_cgroup pointer as a bit spin
  * lock. We need to ensure that page->page_cgroup is atleast two
@@ -520,6 +529,27 @@ static inline void mem_cgroup_schedule_d
 		wake_up_interruptible(&mem->daemon.waitq);
 }
 
+static inline void mem_cgroup_wait_reclaim(struct mem_cgroup *mem)
+{
+	DEFINE_WAIT(wait);
+	while (1) {
+		prepare_to_wait(&mem->throttle.waitq, &wait,
+						TASK_INTERRUPTIBLE);
+		if (res_counter_check_under_limit(&mem->res)) {
+			finish_wait(&mem->throttle.waitq, &wait);
+			break;
+		}
+		/* expect some progress in... */
+		schedule_timeout(HZ/50);
+		finish_wait(&mem->throttle.waitq, &wait);
+	}
+}
+
+static inline int mem_cgroup_throttle_reclaim(struct mem_cgroup *mem)
+{
+	return atomic_add_unless(&mem->throttle.reclaimers, 1,
+					mem->throttle.limit);
+}
 
 unsigned long mem_cgroup_isolate_pages(unsigned long nr_to_scan,
 					struct list_head *dst,
@@ -652,11 +682,22 @@ retry:
 	 * the cgroup limit.
 	 */
 	while (res_counter_charge(&mem->res, PAGE_SIZE)) {
+		int ret;
 		if (!(gfp_mask & __GFP_WAIT))
 			goto out;
 
-		if (try_to_free_mem_cgroup_pages(mem, gfp_mask))
+		if (((gfp_mask & (__GFP_FS|__GFP_IO)) != (__GFP_FS|__GFP_IO))
+		    || mem_cgroup_throttle_reclaim(mem)) {
+			ret = try_to_free_mem_cgroup_pages(mem, gfp_mask);
+			atomic_dec(&mem->throttle.reclaimers);
+			if (waitqueue_active(&mem->throttle.waitq))
+				wake_up_all(&mem->throttle.waitq);
+			if (ret)
+				continue;
+		} else {
+			mem_cgroup_wait_reclaim(mem);
 			continue;
+		}
 
 		/*
  		 * try_to_free_mem_cgroup_pages() might not give us a full
@@ -1054,6 +1095,19 @@ static ssize_t mem_force_empty_read(stru
 	return -EINVAL;
 }
 
+static int mem_throttle_write(struct cgroup *cont, struct cftype *cft, u64 val)
+{
+	struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
+	int limit = (int)val;
+	mem->throttle.limit = limit;
+	return 0;
+}
+
+static u64 mem_throttle_read(struct cgroup *cont, struct cftype *cft)
+{
+	struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
+	return (u64)mem->throttle.limit;
+}
 
 static const struct mem_cgroup_stat_desc {
 	const char *msg;
@@ -1146,6 +1200,11 @@ static struct cftype mem_cgroup_files[] 
 		.read = mem_force_empty_read,
 	},
 	{
+		.name = "shrinks",
+		.write_uint = mem_throttle_write,
+		.read_uint  = mem_throttle_read,
+	},
+	{
 		.name = "stat",
 		.open = mem_control_stat_open,
 	},
@@ -1215,6 +1274,11 @@ mem_cgroup_create(struct cgroup_subsys *
 			goto free_out;
 	init_waitqueue_head(&mem->daemon.waitq);
 	mem->daemon.kthread = NULL;
+
+	init_waitqueue_head(&mem->throttle.waitq);
+	mem->throttle.limit = 10240; /* maybe enough big for no throttle */
+	atomic_set(&mem->throttle.reclaimers, 0);
+
 	return &mem->css;
 free_out:
 	for_each_node_state(node, N_POSSIBLE)

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