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Subject[PATCH 2/2] Memory usage limit notification addition to memcg (v3)
This patch updates the Memory Controller Control Group to add a
configurable memory usage limit notification. The feature was
presented at the April 2009 Embedded Linux Conference.

Signed-off-by: Vladislav Buzov <>
Signed-off-by: Dan Malek <>
Documentation/cgroups/mem_notify.txt | 140 ++++++++++++++++++++++++++++++++++
mm/memcontrol.c | 100 ++++++++++++++++++++++++-
2 files changed, 239 insertions(+), 1 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..94be3f3
--- /dev/null
+++ b/Documentation/cgroups/mem_notify.txt
@@ -0,0 +1,140 @@
+Memory Limit Notification
+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 contemporary implementation, and this is the result.
+The memory limit notification is an extension to the existing Memory
+Resource Controller. Please read memory.txt in this directory to
+understand its operation before continuing here.
+1. Operation
+When the Memory Controller cgroup file system is mounted, the following
+files will appear:
+ memory.notify_threshold_in_bytes
+ memory.notify_threshold_lowait
+The notification is based upon reaching a threshold below the Memory
+Resource 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 computed at any time as a
+difference between the memory.limit_in_bytes and memory.usage_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_usage_in_bytes" ..) or
+fopen("memory.notify_threshold_lowait" ...), then either poll the former
+files or block read on the latter file using fread() or fscanf() as desired.
+Subtracting the value returned from either of these read function from the
+value obtained by reading memory.limit_in_bytes and further comparing it with
+the threshold 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. Writing a value higher
+than the Memory Controller limit will cause an error while setting the limit
+lower than the threshold will cause setting the threshold to zero.
+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 Resource Controller utilizes the Resource Counter to track and manage
+the memory of the Control Group. The Resource Counter was extended to support
+the resource usage threshold, which is the minimal difference between the
+resource limit and usage causing the notification. For the Memory Controller
+cgroup it means a number of bytes of the memory not in use so the cgroup
+parameters may continue to be dynamically modified without the need to modify
+the notification 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 notification
+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 <>
+Vladislav Buzov <>
+Embedded Alley Solutions, Inc.
+10 July 2009
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e2fa20d..3b49fd4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6,6 +6,11 @@
* Copyright 2007 OpenVZ SWsoft Inc
* Author: Pavel Emelianov <>
+ * Memory Limit Notification update
+ * Copyright 2009 CE Linux Forum and Embedded Alley Solutions, Inc.
+ * Author: Dan Malek <>
+ * Author: Vladislav Buzov <>
+ *
* 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 +185,9 @@ struct mem_cgroup {
/* set when res.limit == memsw.limit */
bool memsw_is_minimum;

+ /* tasks waiting for memory usage threshold notification */
+ wait_queue_head_t notify_threshold_wait;
* statistics. This must be placed at the end of memcg.
@@ -2052,7 +2060,7 @@ static u64 mem_cgroup_read(struct cgroup *cont, struct cftype *cft)
* The user of this function is...
static int mem_cgroup_write(struct cgroup *cont, struct cftype *cft,
const char *buffer)
@@ -2075,6 +2083,17 @@ static int mem_cgroup_write(struct cgroup *cont, struct cftype *cft,
ret = mem_cgroup_resize_memsw_limit(memcg, val);
+ /* This function does all necessary parse...reuse it */
+ ret = res_counter_memparse_write_strategy(buffer, &val);
+ if (ret)
+ break;
+ /* For memsw threshold is not implemented */
+ if (type == _MEM)
+ ret = res_counter_set_threshold(&memcg->res, val);
+ else
+ ret = -EINVAL;
+ break;
ret = -EINVAL; /* should be BUG() ? */
@@ -2308,6 +2327,68 @@ static int mem_cgroup_swappiness_write(struct cgroup *cgrp, struct cftype *cft,
return 0;

+ * This is a blocking read operation forcing a reader to sleep unless
+ * a low memory condition occurs, someone intentionaly writes to
+ * "memory.notify_threshold_lowait" or cgroup state is changed. E.g.
+ * the cgroup is destroyed or task is moved to another cgroup.
+ */
+static u64 mem_cgroup_notify_threshold_lowait(struct cgroup *cgrp,
+ struct cftype *cft)
+ struct mem_cgroup *mem = mem_cgroup_from_cont(cgrp);
+ 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.
+ */
+ if (likely(mem->res.usage != 0)) {
+ prepare_to_wait(&mem->notify_threshold_wait, &notify_lowait,
+ if (res_counter_check_under_threshold(&mem->res))
+ schedule();
+ finish_wait(&mem->notify_threshold_wait, &notify_lowait);
+ }
+ return res_counter_read_u64(&mem->res, RES_USAGE);
+ * Memory usage threshold notification callback. Called under disabled
+ * interrupts by the memory resource counter when low memory condition
+ * occurs.
+ */
+static void mem_cgroup_res_threshold_notifier(struct res_counter *cnt)
+ struct mem_cgroup *memcg;
+ memcg = mem_cgroup_from_res_counter(cnt, res);
+ if (waitqueue_active(&memcg->notify_threshold_wait))
+ wake_up_locked(&memcg->notify_threshold_wait);
+ * 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 *memcg;
+ memcg = mem_cgroup_from_cont(cgrp);
+ if (waitqueue_active(&memcg->notify_threshold_wait))
+ wake_up(&memcg->notify_threshold_wait);
+ return 0;

static struct cftype mem_cgroup_files[] = {
@@ -2351,6 +2432,17 @@ 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_write,
+ .read_u64 = mem_cgroup_read,
+ },
+ {
+ .name = "notify_threshold_lowait",
+ .trigger = mem_cgroup_notify_threshold_wake_em_up,
+ .read_u64 = mem_cgroup_notify_threshold_lowait,
+ },

@@ -2554,6 +2646,9 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
mem->last_scanned_child = 0;

+ init_waitqueue_head(&mem->notify_threshold_wait);
+ mem->res.threshold_notifier = mem_cgroup_res_threshold_notifier;
if (parent)
mem->swappiness = get_swappiness(parent);
atomic_set(&mem->refcnt, 1);
@@ -2568,6 +2663,7 @@ static int mem_cgroup_pre_destroy(struct cgroup_subsys *ss,
struct mem_cgroup *mem = mem_cgroup_from_cont(cont);

+ mem_cgroup_notify_threshold_wake_em_up(cont, 0);
return mem_cgroup_force_empty(mem, false);

@@ -2597,6 +2693,8 @@ static void mem_cgroup_move_task(struct cgroup_subsys *ss,
struct cgroup *old_cont,
struct task_struct *p)
+ mem_cgroup_notify_threshold_wake_em_up(old_cont, 0);
* FIXME: It's better to move charges of this process from old

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