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    Subject[PATCH 1/1] Memory usage limit notification addition to memcg
    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 <>
    Signed-off-by: Vladislav Buzov <>
    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 <>
    +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 <>
    + * Memory Limit Notification update
    + * Copyright 2009 CE Linux Forum and Embedded Alley Solutions, Inc.
    + * Author: Dan Malek <>
    + *
    * 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);
    +#endif /* CONFIG_CGROUP_MEM_NOTIFY */

    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

     \ /
      Last update: 2009-07-07 22:29    [W:0.046 / U:14.588 seconds]
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