> > We could add the following API to resource counters
> > 
> > res_counter_set_low_watermark
> > res_counter_set_high_watermark
> > res_counter_below_low_watermark
> > res_counter_above_high_watermark
> > 
> > and add
> > 
> > low_watermark
> > high_watermark
> > 
> > members to the resource group. We could push out data
> > upto the low watermark from the cgroup.

i implemented something like that.  (and rebased to 2.6.24-rc2-mm1.)

what's the best way to expose watermarks to userland is an open question.
i took the simplest way for now.  do you have any suggestions?

YAMAMOTO Takashi


--- ./include/linux/res_counter.h.orig	2007-11-14 15:57:31.000000000 +0900
+++ ./include/linux/res_counter.h	2007-11-14 16:03:24.000000000 +0900
@@ -32,6 +32,13 @@ struct res_counter {
 	 * the number of unsuccessful attempts to consume the resource
 	 */
 	unsigned long long failcnt;
+
+	/*
+	 * watermarks
+	 */
+	unsigned long long high_watermark;
+	unsigned long long low_watermark;
+
 	/*
 	 * the lock to protect all of the above.
 	 * the routines below consider this to be IRQ-safe
@@ -66,6 +73,8 @@ enum {
 	RES_USAGE,
 	RES_LIMIT,
 	RES_FAILCNT,
+	RES_HIGH_WATERMARK,
+	RES_LOW_WATERMARK,
 };
 
 /*
@@ -124,4 +133,26 @@ static inline bool res_counter_check_und
 	return ret;
 }
 
+static inline bool res_counter_below_low_watermark(struct res_counter *cnt)
+{
+	bool ret;
+	unsigned long flags;
+
+	spin_lock_irqsave(&cnt->lock, flags);
+	ret = cnt->usage < cnt->low_watermark;
+	spin_unlock_irqrestore(&cnt->lock, flags);
+	return ret;
+}
+
+static inline bool res_counter_above_high_watermark(struct res_counter *cnt)
+{
+	bool ret;
+	unsigned long flags;
+
+	spin_lock_irqsave(&cnt->lock, flags);
+	ret = cnt->usage > cnt->high_watermark;
+	spin_unlock_irqrestore(&cnt->lock, flags);
+	return ret;
+}
+
 #endif
--- ./kernel/res_counter.c.orig	2007-11-14 15:57:31.000000000 +0900
+++ ./kernel/res_counter.c	2007-11-14 16:03:24.000000000 +0900
@@ -17,6 +17,8 @@ void res_counter_init(struct res_counter
 {
 	spin_lock_init(&counter->lock);
 	counter->limit = (unsigned long long)LLONG_MAX;
+	counter->high_watermark = (unsigned long long)LLONG_MAX;
+	counter->low_watermark = (unsigned long long)LLONG_MAX;
 }
 
 int res_counter_charge_locked(struct res_counter *counter, unsigned long val)
@@ -69,6 +71,10 @@ res_counter_member(struct res_counter *c
 		return &counter->limit;
 	case RES_FAILCNT:
 		return &counter->failcnt;
+	case RES_HIGH_WATERMARK:
+		return &counter->high_watermark;
+	case RES_LOW_WATERMARK:
+		return &counter->low_watermark;
 	};
 
 	BUG();
@@ -99,6 +105,7 @@ ssize_t res_counter_write(struct res_cou
 	int ret;
 	char *buf, *end;
 	unsigned long long tmp, *val;
+	unsigned long flags;
 
 	buf = kmalloc(nbytes + 1, GFP_KERNEL);
 	ret = -ENOMEM;
@@ -122,9 +129,29 @@ ssize_t res_counter_write(struct res_cou
 			goto out_free;
 	}
 
+	spin_lock_irqsave(&counter->lock, flags);
 	val = res_counter_member(counter, member);
+	/* ensure low_watermark <= high_watermark <= limit */
+	switch (member) {
+	case RES_LIMIT:
+		if (tmp < counter->high_watermark)
+			goto out_locked;
+		break;
+	case RES_HIGH_WATERMARK:
+		if (tmp > counter->limit || tmp < counter->low_watermark)
+			goto out_locked;
+		break;
+	case RES_LOW_WATERMARK:
+		if (tmp > counter->high_watermark)
+			goto out_locked;
+		break;
+	}
 	*val = tmp;
+	BUG_ON(counter->high_watermark > counter->limit);
+	BUG_ON(counter->low_watermark > counter->high_watermark);
 	ret = nbytes;
+out_locked:
+	spin_unlock_irqrestore(&counter->lock, flags);
 out_free:
 	kfree(buf);
 out:
--- ./mm/memcontrol.c.orig	2007-11-14 15:57:46.000000000 +0900
+++ ./mm/memcontrol.c	2007-11-14 16:03:53.000000000 +0900
@@ -28,6 +28,7 @@
 #include <linux/rcupdate.h>
 #include <linux/swap.h>
 #include <linux/spinlock.h>
+#include <linux/workqueue.h>
 #include <linux/fs.h>
 #include <linux/seq_file.h>
 
@@ -110,6 +111,10 @@ struct mem_cgroup {
 	 * statistics.
 	 */
 	struct mem_cgroup_stat stat;
+	/*
+	 * background reclamation.
+	 */
+	struct work_struct reclaim_work;
 };
 
 /*
@@ -168,6 +173,9 @@ static void mem_cgroup_charge_statistics
 
 static struct mem_cgroup init_mem_cgroup;
 
+static DEFINE_MUTEX(mem_cgroup_workqueue_init_lock);
+static struct workqueue_struct *mem_cgroup_workqueue;
+
 static inline
 struct mem_cgroup *mem_cgroup_from_cont(struct cgroup *cont)
 {
@@ -375,6 +383,50 @@ unsigned long mem_cgroup_isolate_pages(u
 	return nr_taken;
 }
 
+static void
+mem_cgroup_schedule_reclaim(struct mem_cgroup *mem)
+{
+
+	if (mem_cgroup_workqueue == NULL) {
+		BUG_ON(mem->css.cgroup->parent != NULL);
+		return;
+	}
+
+	if (work_pending(&mem->reclaim_work))
+		return;
+
+	css_get(&mem->css); /* XXX need some thoughts wrt cgroup removal. */
+	/*
+	 * XXX workqueue is not an ideal mechanism for our purpose.
+	 * revisit later.
+	 */
+	if (!queue_work(mem_cgroup_workqueue, &mem->reclaim_work))
+		css_put(&mem->css);
+}
+
+static void
+mem_cgroup_reclaim(struct work_struct *work)
+{
+	struct mem_cgroup * const mem =
+	    container_of(work, struct mem_cgroup, reclaim_work);
+	int batch_count = 128; /* XXX arbitrary */
+
+	for (; batch_count > 0; batch_count--) {
+		if (res_counter_below_low_watermark(&mem->res))
+			break;
+		/*
+		 * XXX try_to_free_foo is not a correct mechanism to
+		 * use here.  eg. ALLOCSTALL counter
+		 * revisit later.
+		 */
+		if (!try_to_free_mem_cgroup_pages(mem, GFP_KERNEL))
+			break;
+	}
+	if (batch_count == 0)
+		mem_cgroup_schedule_reclaim(mem);
+	css_put(&mem->css);
+}
+
 /*
  * Charge the memory controller for page usage.
  * Return
@@ -439,11 +491,18 @@ retry:
 	rcu_read_unlock();
 
 	/*
+	 * schedule background reclaim if we are above the high watermark.
+	 */
+	if (res_counter_above_high_watermark(&mem->res))
+		mem_cgroup_schedule_reclaim(mem);
+
+	/*
 	 * If we created the page_cgroup, we should free it on exceeding
 	 * the cgroup limit.
 	 */
 	while (res_counter_charge(&mem->res, PAGE_SIZE)) {
 		bool is_atomic = gfp_mask & GFP_ATOMIC;
+
 		/*
 		 * We cannot reclaim under GFP_ATOMIC, fail the charge
 		 */
@@ -861,6 +920,18 @@ static struct cftype mem_cgroup_files[] 
 		.read = mem_cgroup_read,
 	},
 	{
+		.name = "high_watermark_in_bytes",
+		.private = RES_HIGH_WATERMARK,
+		.write = mem_cgroup_write,
+		.read = mem_cgroup_read,
+	},
+	{
+		.name = "low_watermark_in_bytes",
+		.private = RES_LOW_WATERMARK,
+		.write = mem_cgroup_write,
+		.read = mem_cgroup_read,
+	},
+	{
 		.name = "control_type",
 		.write = mem_control_type_write,
 		.read = mem_control_type_read,
@@ -886,8 +957,18 @@ mem_cgroup_create(struct cgroup_subsys *
 	if (unlikely((cont->parent) == NULL)) {
 		mem = &init_mem_cgroup;
 		init_mm.mem_cgroup = mem;
-	} else
+	} else {
+		/* XXX too late for the top-level cgroup */
+		if (mem_cgroup_workqueue == NULL) {
+			mutex_lock(&mem_cgroup_workqueue_init_lock);
+			if (mem_cgroup_workqueue == NULL) {
+				mem_cgroup_workqueue =
+				    create_workqueue("mem_cgroup");
+			}
+			mutex_unlock(&mem_cgroup_workqueue_init_lock);
+		}
 		mem = kzalloc(sizeof(struct mem_cgroup), GFP_KERNEL);
+	}
 
 	if (mem == NULL)
 		return NULL;
@@ -897,6 +978,7 @@ mem_cgroup_create(struct cgroup_subsys *
 	INIT_LIST_HEAD(&mem->inactive_list);
 	spin_lock_init(&mem->lru_lock);
 	mem->control_type = MEM_CGROUP_TYPE_ALL;
+	INIT_WORK(&mem->reclaim_work, mem_cgroup_reclaim);
 	return &mem->css;
 }
 
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