[PATCH 1/1] Memory usage limit notification addition to memcg

Vladislav Buzov vbuzov at embeddedalley.com
Tue Jul 7 13:25:10 PDT 2009

This patch updates the Memory Controller cgroup to add
a configurable memory usage limit notification. The feature
was presented at the April 2009 Embedded Linux Conference.

Signed-off-by: Dan Malek <dan at embeddedalley.com>
Signed-off-by: Vladislav Buzov <vbuzov at embeddedalley.com>
 Documentation/cgroups/mem_notify.txt |  140 ++++++++++++++++++++++++++
 include/linux/memcontrol.h           |   21 ++++
 init/Kconfig                         |    9 ++
 mm/memcontrol.c                      |  178 ++++++++++++++++++++++++++++++++++
 4 files changed, 348 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/cgroups/mem_notify.txt

diff --git a/Documentation/cgroups/mem_notify.txt b/Documentation/cgroups/mem_notify.txt
new file mode 100644
index 0000000..b4f20d0
--- /dev/null
+++ b/Documentation/cgroups/mem_notify.txt
@@ -0,0 +1,140 @@
+Memory Limit Notificiation
+Attempts have been made in the past to provide a mechanism for
+the notification to processes (task, an address space) when memory
+usage is approaching a high limit.  The intention is that it gives
+the application an opportunity to release some memory and continue
+operation rather than be OOM killed.  The CE Linux Forum requested
+a more comtemporary implementation, and this is the result.
+The memory threshold notification is a configurable extension to the
+existing Memory Resource Controller.  Please read memory.txt in this
+directory to understand its operation before continuing here.
+1. Operation
+When a kernel is configured with CGROUP_MEM_NOTIFY, three additional
+files will appear in the memory resource controller:
+	memory.notify_threshold_in_bytes
+	memory.notify_available_in_bytes
+	memory.notify_threshold_lowait
+The notification is based upon reaching a threshold below the memory
+resouce controller limit (memory.limit_in_bytes). The threshold
+represents the minimal number of bytes that should be available under
+the limit. When the controller group is created, the threshold is set
+to zero which triggers notification when the memory resource controller
+limit is reached.
+The threshold may be set by writing to memory.notify_threshold_in_bytes,
+such as:
+	echo 10M > memory.notify_threshold_in_bytes
+The current number of available bytes may be read at any time from
+the memory.notify_available_in_bytes
+The memory.notify_threshold_lowait is a blocking read file.  The read will
+block until one of four conditions occurs:
+    - The amount of available memory is equal or less than the threshold
+      defined in memory.notify_threshold_in_bytes
+    - The memory.notify_threshold_lowait file is written with any value (debug)
+    - A thread is moved to another controller group
+    - The cgroup is destroyed or forced empty (memory.force_empty)
+1.1 Example Usage
+An application must be designed to properly take advantage of this
+memory threshold notification feature.  It is a powerful management component
+of some operating systems and embedded devices that must provide
+highly available and reliable computing services.  The application works
+in conjunction with information provided by the operating system to
+control limited resource usage.  Since many programmers still think
+memory is infinite and never check the return value from malloc(), it
+may come as a surprise that such mechanisms have been utilized long ago.
+A typical application will be multithreaded, with one thread either
+polling or waiting for the notification event.  When the event occurs,
+the thread will take whatever action is appropriate within the application
+design.  This could be actually running a garbage collection algorithm
+or to simply signal other processing threads they must do something to
+reduce their memory usage.  The notification thread will then be required
+to poll the actual usage until the low limit of its choosing is met,
+at which time the reclaim of memory can stop and the notification thread
+will wait for the next event.
+Internally, the application only needs to
+fopen("memory.notify_available_in_bytes" ..) or
+fopen("memory.notify_threshold_lowait" ...), then either poll the former
+file or block read on the latter file using fread() or fscanf() as desired.
+Comparing the value returned from either of these read function with the
+value obtained by reading memory.notify_threshold_in_bytes will be an
+indication of the amount of memory used over the threshold limit.
+2. Configuration
+Follow the instructions in memory.txt for the configuration and usage of
+the Memory Resource Controller cgroup.  Once this is created and tasks
+assigned, use the memory threshold notification as described here.
+The only action that is needed outside of the application waiting or polling
+is to set the memory.notify_threshold_in_bytes.  To set a notification to occur
+when memory usage of the cgroup reaches or exceeds 1 MByte below the limit
+can be simply done:
+	echo 1M > memory.notify_threshold_in_bytes
+This value may be read or changed at any time.  Writing a higher value once
+the Memory Resource Controller is in operation may trigger immediate
+notification if the usage is above the new threshold.
+3. Debug and Testing
+The design of cgroups makes it easier to perform some debugging or
+monitoring tasks without modification to the application.  For example,
+a write of any value to memory.notify_threshold_lowait will wake up all
+threads waiting for notifications regardless of current memory usage.
+Collecting performance data about the cgroup is also simplified, as
+no application modifications are necessary.  A separate task can be
+created that will open and monitor any necessary files of the cgroup
+(such as current limits, usage and usage percentages and even when
+notification occurs).  This task can also operate outside of the cgroup,
+so its memory usage is not charged to the cgroup.
+4. Design
+The memory threshold notification is a configurable extension to the
+existing Memory Resource Controller, which operates as described to
+track and manage the memory of the Control Group. The Memory Resource
+Controller will still continue to reclaim memory under pressure
+of the limits, and may OOM kill tasks within the cgroup according to
+the OOM Killer configuration.
+The memory notification threshold was chosen as a number of bytes of the
+memory not in use so the cgroup paramaters may continue to be dynamically
+modified without the need to modify the notificaton parameters.
+Otherwise, the notification threshold would have to also be computed
+and modified on any Memory Resource Controller operating parameter change.
+The cgroup file semantics are not well suited for this type of notificaton
+mechanism.  While applications may choose to simply poll the current
+usage at their convenience, it was also desired to have a notification
+event that would trigger when the usage attained the threshold.  The
+blocking read() was chosen, as it is the only current useful method.
+This presented the problems of "out of band" notification, when you want
+to return some exceptional status other than reaching the notification
+threshold.  In the cases listed above, the read() on the
+memory.notify_threshold_lowait file will not block and return "0" for
+the remaining size.  When this occurs, the thread must determine if the task
+has moved to a new cgroup or if the cgroup has been destroyed.  Due to
+the usage model of this cgroup, neither is likely to happen during normal
+operation of a product.
+Dan Malek <dan at embeddedalley.com>
+Embedded Alley Solutions, Inc.
+6 July 2009
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index e46a073..78205a3 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -118,6 +118,27 @@ static inline bool mem_cgroup_disabled(void)
 extern bool mem_cgroup_oom_called(struct task_struct *task);
 void mem_cgroup_update_mapped_file_stat(struct page *page, int val);
+void mem_cgroup_notify_test_and_wakeup(struct mem_cgroup *mcg,
+			unsigned long long usage, unsigned long long limit);
+void mem_cgroup_notify_new_limit(struct mem_cgroup *mcg,
+						unsigned long long newlimit);
+void mem_cgroup_notify_move_task(struct cgroup *old_cont);
+static inline void mem_cgroup_notify_test_and_wakeup(struct mem_cgroup *mcg,
+			unsigned long long usage, unsigned long long limit)
+static inline void mem_cgroup_notify_new_limit(struct mem_cgroup *mcg,
+						unsigned long long newlimit)
+static inline void mem_cgroup_notify_move_task(struct cgroup *old_cont)
 struct mem_cgroup;
diff --git a/init/Kconfig b/init/Kconfig
index 1ce05a4..fb2f7d5 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -594,6 +594,15 @@ config CGROUP_MEM_RES_CTLR
 	  This config option also selects MM_OWNER config option, which
 	  could in turn add some fork/exit overhead.
+	bool "Memory Usage Limit Notification"
+	depends on CGROUP_MEM_RES_CTLR
+	help
+	  Provides a memory notification when usage reaches a preset limit.
+	  It is an extenstion to the memory resource controller, since it
+	  uses the memory usage accounting of the cgroup to test against
+	  the notification limit.  (See Documentation/cgroups/mem_notify.txt)
 	bool "Memory Resource Controller Swap Extension(EXPERIMENTAL)"
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e2fa20d..cf04279 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6,6 +6,10 @@
  * Copyright 2007 OpenVZ SWsoft Inc
  * Author: Pavel Emelianov <xemul at openvz.org>
+ * Memory Limit Notification update
+ * Copyright 2009 CE Linux Forum and Embedded Alley Solutions, Inc.
+ * Author: Dan Malek <dan at embeddedalley.com>
+ *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
  * the Free Software Foundation; either version 2 of the License, or
@@ -180,6 +184,11 @@ struct mem_cgroup {
 	/* set when res.limit == memsw.limit */
 	bool		memsw_is_minimum;
+	unsigned long long notify_threshold_bytes;
+	wait_queue_head_t notify_threshold_wait;
 	 * statistics. This must be placed at the end of memcg.
@@ -995,6 +1004,13 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
+	/*
+	 * We check on the way in so we don't have to duplicate code
+	 * in both the normal and error exit path.
+	 */
+	mem_cgroup_notify_test_and_wakeup(mem, mem->res.usage + PAGE_SIZE,
+							mem->res.limit);
 	while (1) {
 		int ret;
 		bool noswap = false;
@@ -1744,6 +1760,12 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
 	u64 curusage, oldusage;
+	 * Test and notify ahead of the necessity to free pages, as
+	 * applications giving up pages may help this reclaim procedure.
+	 */
+	mem_cgroup_notify_new_limit(memcg, val);
+	/*
 	 * For keeping hierarchical_reclaim simple, how long we should retry
 	 * is depends on callers. We set our retry-count to be function
 	 * of # of children which we should visit in this loop.
@@ -2308,6 +2330,139 @@ static int mem_cgroup_swappiness_write(struct cgroup *cgrp, struct cftype *cft,
 	return 0;
+ * Check if a task exceeded notification threshold set for a memory cgroup.
+ * Wake up waiting notification threads, if any.
+ */
+void mem_cgroup_notify_test_and_wakeup(struct mem_cgroup *mcg,
+				       unsigned long long usage,
+				       unsigned long long limit)
+	if (unlikely(usage == RESOURCE_MAX))
+		return;
+	if ((limit - usage <= mcg->notify_threshold_bytes) &&
+	    waitqueue_active(&mcg->notify_threshold_wait))
+		wake_up(&mcg->notify_threshold_wait);
+ * Check if current notification threshold exceeds new memory usage
+ * limit set for a memory cgroup. If so, set threshold to zero to
+ * notify tasks in the group when maximal memory usage is achieved.
+ */
+void mem_cgroup_notify_new_limit(struct mem_cgroup *mcg,
+				 unsigned long long newlimit)
+	if (newlimit <= mcg->notify_threshold_bytes)
+		mcg->notify_threshold_bytes = 0;
+	mem_cgroup_notify_test_and_wakeup(mcg, mcg->res.usage, newlimit);
+static u64 mem_cgroup_notify_threshold_read(struct cgroup *cgrp,
+					    struct cftype *cft)
+	struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp);
+	return memcg->notify_threshold_bytes;
+static int mem_cgroup_notify_threshold_write(struct cgroup *cgrp,
+					     struct cftype *cft,
+					     const char *buffer)
+	struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp);
+	unsigned long long val;
+	int ret;
+	/* This function does all necessary parse...reuse it */
+	ret = res_counter_memparse_write_strategy(buffer, &val);
+	if (ret)
+		return ret;
+	/* Threshold must be lower than usage limit */
+	if (val >= memcg->res.limit)
+		return -EINVAL;
+	memcg->notify_threshold_bytes = val;
+	/* Check to see if the new threshold should cause notification */
+	mem_cgroup_notify_test_and_wakeup(memcg, memcg->res.usage,
+							memcg->res.limit);
+	return 0;
+static u64 mem_cgroup_notify_available_read(struct cgroup *cgrp,
+					    struct cftype *cft)
+	struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp);
+	return memcg->res.limit - memcg->res.usage;
+static u64 mem_cgroup_notify_threshold_lowait(struct cgroup *cgrp,
+					      struct cftype *cft)
+	struct mem_cgroup *mem = mem_cgroup_from_cont(cgrp);
+	unsigned long long available_bytes;
+	DEFINE_WAIT(notify_lowait);
+	/*
+	 * A memory resource usage of zero is a special case that
+	 * causes us not to sleep.  It normally happens when the
+	 * cgroup is about to be destroyed, and we don't want someone
+	 * trying to sleep on a queue that is about to go away.  This
+	 * condition can also be forced as part of testing.
+	 */
+	available_bytes = mem->res.limit - mem->res.usage;
+	if (likely(mem->res.usage != 0)) {
+		prepare_to_wait(&mem->notify_threshold_wait, &notify_lowait,
+		if (available_bytes > mem->notify_threshold_bytes)
+			schedule();
+		available_bytes = mem->res.limit - mem->res.usage;
+		finish_wait(&mem->notify_threshold_wait, &notify_lowait);
+	}
+	return available_bytes;
+ * This is used to wake up all threads that may be hanging
+ * out waiting for a low memory condition prior to that happening.
+ * Useful for triggering the event to assist with debug of applications.
+ */
+static int mem_cgroup_notify_threshold_wake_em_up(struct cgroup *cgrp,
+						  unsigned int event)
+	struct mem_cgroup *mem;
+	mem = mem_cgroup_from_cont(cgrp);
+	wake_up(&mem->notify_threshold_wait);
+	return 0;
+ * We wake up all notification threads any time a migration takes
+ * place.  They will have to check to see if a move is needed to
+ * a new cgroup file to wait for notification.
+ * This isn't so much a task move as it is an attach.  A thread not
+ * a child of an existing task won't have a valid parent, which
+ * is necessary to test because it won't have a valid mem_cgroup
+ * either.  Which further means it won't have a proper wait queue
+ * and we can't do a wakeup.
+ */
+void mem_cgroup_notify_move_task(struct cgroup *old_cont)
+	if (old_cont->parent != NULL)
+		mem_cgroup_notify_threshold_wake_em_up(old_cont, 0);
 static struct cftype mem_cgroup_files[] = {
@@ -2351,6 +2506,22 @@ static struct cftype mem_cgroup_files[] = {
 		.read_u64 = mem_cgroup_swappiness_read,
 		.write_u64 = mem_cgroup_swappiness_write,
+	{
+		.name = "notify_threshold_in_bytes",
+		.write_string = mem_cgroup_notify_threshold_write,
+		.read_u64 = mem_cgroup_notify_threshold_read,
+	},
+	{
+		.name = "notify_available_in_bytes",
+		.read_u64 = mem_cgroup_notify_available_read,
+	},
+	{
+		.name = "notify_threshold_lowait",
+		.trigger = mem_cgroup_notify_threshold_wake_em_up,
+		.read_u64 = mem_cgroup_notify_threshold_lowait,
+	},
@@ -2554,6 +2725,11 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
 	mem->last_scanned_child = 0;
+	init_waitqueue_head(&mem->notify_threshold_wait);
+	mem->notify_threshold_bytes = 0;
 	if (parent)
 		mem->swappiness = get_swappiness(parent);
 	atomic_set(&mem->refcnt, 1);
@@ -2597,6 +2773,8 @@ static void mem_cgroup_move_task(struct cgroup_subsys *ss,
 				struct cgroup *old_cont,
 				struct task_struct *p)
+	mem_cgroup_notify_move_task(old_cont);
 	 * FIXME: It's better to move charges of this process from old

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