memcg: update documentation

Memory Resource Controller

Memory Resource Controller

NOTE: The Memory Resource Controller has been generically been referred

NOTE: The Memory Resource Controller has been generically been referred

to as the memory controller in this document. Do not confuse memory controller

      to as the memory controller in this document. Do not confuse memory

used here with the memory controller that is used in hardware.

      controller used here with the memory controller that is used in hardware.

Salient features

(For editors)

In this document:

a. Enable control of Anonymous, Page Cache (mapped and unmapped) and

      When we mention a cgroup (cgroupfs's directory) with memory controller,

   Swap Cache memory pages.

      we call it "memory cgroup". When you see git-log and source code, you'll

b. The infrastructure allows easy addition of other types of memory to control

      see patch's title and function names tend to use "memcg".

c. Provides *zero overhead* for non memory controller users

      In this document, we avoid using it.

d. Provides a double LRU: global memory pressure causes reclaim from the

   global LRU; a cgroup on hitting a limit, reclaims from the per

   cgroup LRU

Benefits and Purpose of the memory controller

Benefits and Purpose of the memory controller

e. There are several other use cases, find one or use the controller just

e. There are several other use cases, find one or use the controller just

   for fun (to learn and hack on the VM subsystem).

   for fun (to learn and hack on the VM subsystem).

Current Status: linux-2.6.34-mmotm(development version of 2010/April)

Features:

 - accounting anonymous pages, file caches, swap caches usage and limiting them.

 - private LRU and reclaim routine. (system's global LRU and private LRU

   work independently from each other)

 - optionally, memory+swap usage can be accounted and limited.

 - hierarchical accounting

 - soft limit

 - moving(recharging) account at moving a task is selectable.

 - usage threshold notifier

 - oom-killer disable knob and oom-notifier

 - Root cgroup has no limit controls.

 Kernel memory and Hugepages are not under control yet. We just manage

 pages on LRU. To add more controls, we have to take care of performance.

Brief summary of control files.

 tasks				 # attach a task(thread) and show list of threads

 cgroup.procs			 # show list of processes

 cgroup.event_control		 # an interface for event_fd()

 memory.usage_in_bytes		 # show current memory(RSS+Cache) usage.

 memory.memsw.usage_in_bytes	 # show current memory+Swap usage

 memory.limit_in_bytes		 # set/show limit of memory usage

 memory.memsw.limit_in_bytes	 # set/show limit of memory+Swap usage

 memory.failcnt			 # show the number of memory usage hits limits

 memory.memsw.failcnt		 # show the number of memory+Swap hits limits

 memory.max_usage_in_bytes	 # show max memory usage recorded

 memory.memsw.usage_in_bytes	 # show max memory+Swap usage recorded

 memory.soft_limit_in_bytes	 # set/show soft limit of memory usage

 memory.stat			 # show various statistics

 memory.use_hierarchy		 # set/show hierarchical account enabled

 memory.force_empty		 # trigger forced move charge to parent

 memory.swappiness		 # set/show swappiness parameter of vmscan

				 (See sysctl's vm.swappiness)

 memory.move_charge_at_immigrate # set/show controls of moving charges

 memory.oom_control		 # set/show oom controls.

1. History

1. History

The memory controller has a long history. A request for comments for the memory

The memory controller has a long history. A request for comments for the memory

is over its limit. If it is then reclaim is invoked on the cgroup.

is over its limit. If it is then reclaim is invoked on the cgroup.

More details can be found in the reclaim section of this document.

More details can be found in the reclaim section of this document.

If everything goes well, a page meta-data-structure called page_cgroup is

If everything goes well, a page meta-data-structure called page_cgroup is

allocated and associated with the page.  This routine also adds the page to

updated. page_cgroup has its own LRU on cgroup.

the per cgroup LRU.

(*) page_cgroup structure is allocated at boot/memory-hotplug time.

2.2.1 Accounting details

2.2.1 Accounting details

All mapped anon pages (RSS) and cache pages (Page Cache) are accounted.

All mapped anon pages (RSS) and cache pages (Page Cache) are accounted.

(some pages which never be reclaimable and will not be on global LRU

Some pages which are never reclaimable and will not be on the global LRU

 are not accounted. we just accounts pages under usual vm management.)

are not accounted. We just account pages under usual VM management.

RSS pages are accounted at page_fault unless they've already been accounted

RSS pages are accounted at page_fault unless they've already been accounted

for earlier. A file page will be accounted for as Page Cache when it's

for earlier. A file page will be accounted for as Page Cache when it's

processes, duplicate accounting is carefully avoided.

processes, duplicate accounting is carefully avoided.

A RSS page is unaccounted when it's fully unmapped. A PageCache page is

A RSS page is unaccounted when it's fully unmapped. A PageCache page is

unaccounted when it's removed from radix-tree.

unaccounted when it's removed from radix-tree. Even if RSS pages are fully

unmapped (by kswapd), they may exist as SwapCache in the system until they

are really freed. Such SwapCaches also also accounted.

A swapped-in page is not accounted until it's mapped.

Note: The kernel does swapin-readahead and read multiple swaps at once.

This means swapped-in pages may contain pages for other tasks than a task

causing page fault. So, we avoid accounting at swap-in I/O.

At page migration, accounting information is kept.

At page migration, accounting information is kept.

Note: we just account pages-on-lru because our purpose is to control amount

Note: we just account pages-on-LRU because our purpose is to control amount

of used pages. not-on-lru pages are tend to be out-of-control from vm view.

of used pages; not-on-LRU pages tend to be out-of-control from VM view.

2.3 Shared Page Accounting

2.3 Shared Page Accounting

2.4 Swap Extension (CONFIG_CGROUP_MEM_RES_CTLR_SWAP)

2.4 Swap Extension (CONFIG_CGROUP_MEM_RES_CTLR_SWAP)

Swap Extension allows you to record charge for swap. A swapped-in page is

Swap Extension allows you to record charge for swap. A swapped-in page is

charged back to original page allocator if possible.

charged back to original page allocator if possible.

 - memory.memsw.usage_in_bytes.

 - memory.memsw.usage_in_bytes.

 - memory.memsw.limit_in_bytes.

 - memory.memsw.limit_in_bytes.

usage of mem+swap is limited by memsw.limit_in_bytes.

memsw means memory+swap. Usage of memory+swap is limited by

memsw.limit_in_bytes.

Example: Assume a system with 4G of swap. A task which allocates 6G of memory

(by mistake) under 2G memory limitation will use all swap.

In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap.

By using memsw limit, you can avoid system OOM which can be caused by swap

shortage.

* why 'mem+swap' rather than swap.

* why 'memory+swap' rather than swap.

The global LRU(kswapd) can swap out arbitrary pages. Swap-out means

The global LRU(kswapd) can swap out arbitrary pages. Swap-out means

to move account from memory to swap...there is no change in usage of

to move account from memory to swap...there is no change in usage of

mem+swap. In other words, when we want to limit the usage of swap without

memory+swap. In other words, when we want to limit the usage of swap without

affecting global LRU, mem+swap limit is better than just limiting swap from

affecting global LRU, memory+swap limit is better than just limiting swap from

OS point of view.

OS point of view.

* What happens when a cgroup hits memory.memsw.limit_in_bytes

* What happens when a cgroup hits memory.memsw.limit_in_bytes

2.5 Reclaim

2.5 Reclaim

Each cgroup maintains a per cgroup LRU that consists of an active

Each cgroup maintains a per cgroup LRU which has the same structure as

and inactive list. When a cgroup goes over its limit, we first try

global VM. When a cgroup goes over its limit, we first try

to reclaim memory from the cgroup so as to make space for the new

to reclaim memory from the cgroup so as to make space for the new

pages that the cgroup has touched. If the reclaim is unsuccessful,

pages that the cgroup has touched. If the reclaim is unsuccessful,

an OOM routine is invoked to select and kill the bulkiest task in the

an OOM routine is invoked to select and kill the bulkiest task in the

cgroup.

cgroup. (See 10. OOM Control below.)

The reclaim algorithm has not been modified for cgroups, except that

The reclaim algorithm has not been modified for cgroups, except that

pages that are selected for reclaiming come from the per cgroup LRU

pages that are selected for reclaiming come from the per cgroup LRU

When oom event notifier is registered, event will be delivered.

When oom event notifier is registered, event will be delivered.

(See oom_control section)

(See oom_control section)

2. Locking

2.6 Locking

The memory controller uses the following hierarchy

   lock_page_cgroup()/unlock_page_cgroup() should not be called under

   mapping->tree_lock.

1. zone->lru_lock is used for selecting pages to be isolated

   Other lock order is following:

2. mem->per_zone->lru_lock protects the per cgroup LRU (per zone)

   PG_locked.

3. lock_page_cgroup() is used to protect page->page_cgroup

   mm->page_table_lock

       zone->lru_lock

	  lock_page_cgroup.

  In many cases, just lock_page_cgroup() is called.

  per-zone-per-cgroup LRU (cgroup's private LRU) is just guarded by

  zone->lru_lock, it has no lock of its own.

3. User Interface

3. User Interface

a. Enable CONFIG_CGROUPS

a. Enable CONFIG_CGROUPS

b. Enable CONFIG_RESOURCE_COUNTERS

b. Enable CONFIG_RESOURCE_COUNTERS

c. Enable CONFIG_CGROUP_MEM_RES_CTLR

c. Enable CONFIG_CGROUP_MEM_RES_CTLR

d. Enable CONFIG_CGROUP_MEM_RES_CTLR_SWAP (to use swap extension)

1. Prepare the cgroups

1. Prepare the cgroups

# mkdir -p /cgroups

# mkdir -p /cgroups

# mkdir /cgroups/0

# mkdir /cgroups/0

# echo $$ > /cgroups/0/tasks

# echo $$ > /cgroups/0/tasks

Since now we're in the 0 cgroup,

Since now we're in the 0 cgroup, we can alter the memory limit:

We can alter the memory limit:

# echo 4M > /cgroups/0/memory.limit_in_bytes

# echo 4M > /cgroups/0/memory.limit_in_bytes

NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,

NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,

mega or gigabytes.

mega or gigabytes. (Here, Kilo, Mega, Giga are Kibibytes, Mebibytes, Gibibytes.)

NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited).

NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited).

NOTE: We cannot set limits on the root cgroup any more.

NOTE: We cannot set limits on the root cgroup any more.

# cat /cgroups/0/memory.limit_in_bytes

# cat /cgroups/0/memory.limit_in_bytes

4194304

4194304

NOTE: The interface has now changed to display the usage in bytes

instead of pages

We can check the usage:

We can check the usage:

# cat /cgroups/0/memory.usage_in_bytes

# cat /cgroups/0/memory.usage_in_bytes

1216512

1216512

4. Testing

4. Testing

Balbir posted lmbench, AIM9, LTP and vmmstress results [10] and [11].

For testing features and implementation, see memcg_test.txt.

Apart from that v6 has been tested with several applications and regular

daily use. The controller has also been tested on the PPC64, x86_64 and

Performance test is also important. To see pure memory controller's overhead,

UML platforms.

testing on tmpfs will give you good numbers of small overheads.

Example: do kernel make on tmpfs.

Page-fault scalability is also important. At measuring parallel

page fault test, multi-process test may be better than multi-thread

test because it has noise of shared objects/status.

But the above two are testing extreme situations.

Trying usual test under memory controller is always helpful.

4.1 Troubleshooting

4.1 Troubleshooting

Sometimes a user might find that the application under a cgroup is

Sometimes a user might find that the application under a cgroup is

terminated. There are several causes for this:

terminated by OOM killer. There are several causes for this:

1. The cgroup limit is too low (just too low to do anything useful)

1. The cgroup limit is too low (just too low to do anything useful)

2. The user is using anonymous memory and swap is turned off or too low

2. The user is using anonymous memory and swap is turned off or too low

A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of

A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of

some of the pages cached in the cgroup (page cache pages).

some of the pages cached in the cgroup (page cache pages).

To know what happens, disable OOM_Kill by 10. OOM Control(see below) and

seeing what happens will be helpful.

4.2 Task migration

4.2 Task migration

When a task migrates from one cgroup to another, its charge is not

When a task migrates from one cgroup to another, its charge is not

remain charged to it, the charge is dropped when the page is freed or

remain charged to it, the charge is dropped when the page is freed or

reclaimed.

reclaimed.

Note: You can move charges of a task along with task migration. See 8.

You can move charges of a task along with task migration.

See 8. "Move charges at task migration"

4.3 Removing a cgroup

4.3 Removing a cgroup

A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a

A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a

cgroup might have some charge associated with it, even though all

cgroup might have some charge associated with it, even though all

tasks have migrated away from it.

tasks have migrated away from it. (because we charge against pages, not

Such charges are freed(at default) or moved to its parent. When moved,

against tasks.)

both of RSS and CACHES are moved to parent.

If both of them are busy, rmdir() returns -EBUSY. See 5.1 Also.

Such charges are freed or moved to their parent. At moving, both of RSS

and CACHES are moved to parent.

rmdir() may return -EBUSY if freeing/moving fails. See 5.1 also.

Charges recorded in swap information is not updated at removal of cgroup.

Charges recorded in swap information is not updated at removal of cgroup.

Recorded information is discarded and a cgroup which uses swap (swapcache)

Recorded information is discarded and a cgroup which uses swap (swapcache)

  # echo 0 > memory.force_empty

  # echo 0 > memory.force_empty

  Almost all pages tracked by this memcg will be unmapped and freed. Some of

  Almost all pages tracked by this memory cgroup will be unmapped and freed.

  pages cannot be freed because it's locked or in-use. Such pages are moved

  Some pages cannot be freed because they are locked or in-use. Such pages are

  to parent and this cgroup will be empty. But this may return -EBUSY in

  moved to parent and this cgroup will be empty. This may return -EBUSY if

  some too busy case.

  VM is too busy to free/move all pages immediately.

  Typical use case of this interface is that calling this before rmdir().

  Typical use case of this interface is that calling this before rmdir().

  Because rmdir() moves all pages to parent, some out-of-use page caches can be

  Because rmdir() moves all pages to parent, some out-of-use page caches can be

memory.stat file includes following statistics

memory.stat file includes following statistics

# per-memory cgroup local status

cache		- # of bytes of page cache memory.

cache		- # of bytes of page cache memory.

rss		- # of bytes of anonymous and swap cache memory.

rss		- # of bytes of anonymous and swap cache memory.

mapped_file	- # of bytes of mapped file (includes tmpfs/shmem)

pgpgin		- # of pages paged in (equivalent to # of charging events).

pgpgin		- # of pages paged in (equivalent to # of charging events).

pgpgout		- # of pages paged out (equivalent to # of uncharging events).

pgpgout		- # of pages paged out (equivalent to # of uncharging events).

active_anon	- # of bytes of anonymous and  swap cache memory on active

swap		- # of bytes of swap usage

		  lru list.

inactive_anon	- # of bytes of anonymous memory and swap cache memory on

inactive_anon	- # of bytes of anonymous memory and swap cache memory on

		  inactive lru list.

		LRU list.

active_file	- # of bytes of file-backed memory on active lru list.

active_anon	- # of bytes of anonymous and swap cache memory on active

inactive_file	- # of bytes of file-backed memory on inactive lru list.

		inactive LRU list.

inactive_file	- # of bytes of file-backed memory on inactive LRU list.

active_file	- # of bytes of file-backed memory on active LRU list.

unevictable	- # of bytes of memory that cannot be reclaimed (mlocked etc).

unevictable	- # of bytes of memory that cannot be reclaimed (mlocked etc).

The following additional stats are dependent on CONFIG_DEBUG_VM.

# status considering hierarchy (see memory.use_hierarchy settings)

hierarchical_memory_limit - # of bytes of memory limit with regard to hierarchy

			under which the memory cgroup is

hierarchical_memsw_limit - # of bytes of memory+swap limit with regard to

			hierarchy under which memory cgroup is.

total_cache		- sum of all children's "cache"

total_rss		- sum of all children's "rss"

total_mapped_file	- sum of all children's "cache"

total_pgpgin		- sum of all children's "pgpgin"

total_pgpgout		- sum of all children's "pgpgout"

total_swap		- sum of all children's "swap"

total_inactive_anon	- sum of all children's "inactive_anon"

total_active_anon	- sum of all children's "active_anon"

total_inactive_file	- sum of all children's "inactive_file"

total_active_file	- sum of all children's "active_file"

total_unevictable	- sum of all children's "unevictable"

# The following additional stats are dependent on CONFIG_DEBUG_VM.

inactive_ratio		- VM internal parameter. (see mm/page_alloc.c)

inactive_ratio		- VM internal parameter. (see mm/page_alloc.c)

recent_rotated_anon	- VM internal parameter. (see mm/vmscan.c)

recent_rotated_anon	- VM internal parameter. (see mm/vmscan.c)

recent_scanned_file	- VM internal parameter. (see mm/vmscan.c)

recent_scanned_file	- VM internal parameter. (see mm/vmscan.c)

Memo:

Memo:

	recent_rotated means recent frequency of lru rotation.

	recent_rotated means recent frequency of LRU rotation.

	recent_scanned means recent # of scans to lru.

	recent_scanned means recent # of scans to LRU.

	showing for better debug please see the code for meanings.

	showing for better debug please see the code for meanings.

Note:

Note:

	Only anonymous and swap cache memory is listed as part of 'rss' stat.

	Only anonymous and swap cache memory is listed as part of 'rss' stat.

	This should not be confused with the true 'resident set size' or the

	This should not be confused with the true 'resident set size' or the

	amount of physical memory used by the cgroup. Per-cgroup rss

	amount of physical memory used by the cgroup.

	accounting is not done yet.

	'rss + file_mapped" will give you resident set size of cgroup.

	(Note: file and shmem may be shared among other cgroups. In that case,

	 file_mapped is accounted only when the memory cgroup is owner of page

	 cache.)

5.3 swappiness

5.3 swappiness

Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.

Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.

Following cgroups' swappiness can't be changed.

Following cgroups' swappiness can't be changed.

- root cgroup (uses /proc/sys/vm/swappiness).

- root cgroup (uses /proc/sys/vm/swappiness).

  - a cgroup which uses hierarchy and it has child cgroup.

- a cgroup which uses hierarchy and it has other cgroup(s) below it.

- a cgroup which uses hierarchy and not the root of hierarchy.

- a cgroup which uses hierarchy and not the root of hierarchy.

5.4 failcnt

A memory cgroup provides memory.failcnt and memory.memsw.failcnt files.

This failcnt(== failure count) shows the number of times that a usage counter

hit its limit. When a memory cgroup hits a limit, failcnt increases and

memory under it will be reclaimed.

You can reset failcnt by writing 0 to failcnt file.

# echo 0 > .../memory.failcnt

6. Hierarchy support

6. Hierarchy support

6.1 Enabling hierarchical accounting and reclaim

6.1 Enabling hierarchical accounting and reclaim

The memory controller by default disables the hierarchy feature. Support

A memory cgroup by default disables the hierarchy feature. Support

can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup

can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup

# echo 1 > memory.use_hierarchy

# echo 1 > memory.use_hierarchy

       cgroups created below it.

       cgroups created below it.

NOTE2: When panic_on_oom is set to "2", the whole system will panic in

NOTE2: When panic_on_oom is set to "2", the whole system will panic in

case of an oom event in any cgroup.

       case of an OOM event in any cgroup.

7. Soft limits

7. Soft limits

a. There is no memory contention

a. There is no memory contention

b. They do not exceed their hard limit

b. They do not exceed their hard limit

When the system detects memory contention or low memory control groups

When the system detects memory contention or low memory, control groups

are pushed back to their soft limits. If the soft limit of each control

are pushed back to their soft limits. If the soft limit of each control

group is very high, they are pushed back as much as possible to make

group is very high, they are pushed back as much as possible to make

sure that one control group does not starve the others of memory.

sure that one control group does not starve the others of memory.

7.1 Interface

7.1 Interface

Soft limits can be setup by using the following commands (in this example we

Soft limits can be setup by using the following commands (in this example we

assume a soft limit of 256 megabytes)

assume a soft limit of 256 MiB)

# echo 256M > memory.soft_limit_in_bytes

# echo 256M > memory.soft_limit_in_bytes

Note: If we cannot find enough space for the task in the destination cgroup, we

Note: If we cannot find enough space for the task in the destination cgroup, we

      try to make space by reclaiming memory. Task migration may fail if we

      try to make space by reclaiming memory. Task migration may fail if we

      cannot make enough space.

      cannot make enough space.

Note: It can take several seconds if you move charges in giga bytes order.

Note: It can take several seconds if you move charges much.

And if you want disable it again:

And if you want disable it again:

      | enable Swap Extension(see 2.4) to enable move of swap charges.

      | enable Swap Extension(see 2.4) to enable move of swap charges.

 -----+------------------------------------------------------------------------

 -----+------------------------------------------------------------------------

   1  | A charge of file pages(normal file, tmpfs file(e.g. ipc shared memory)

   1  | A charge of file pages(normal file, tmpfs file(e.g. ipc shared memory)

      | and swaps of tmpfs file) mmaped by the target task. Unlike the case of

      | and swaps of tmpfs file) mmapped by the target task. Unlike the case of

      | anonymous pages, file pages(and swaps) in the range mmapped by the task

      | anonymous pages, file pages(and swaps) in the range mmapped by the task

      | will be moved even if the task hasn't done page fault, i.e. they might

      | will be moved even if the task hasn't done page fault, i.e. they might

      | not be the task's "RSS", but other task's "RSS" that maps the same file.

      | not be the task's "RSS", but other task's "RSS" that maps the same file.

9. Memory thresholds

9. Memory thresholds

Memory controler implements memory thresholds using cgroups notification

Memory cgroup implements memory thresholds using cgroups notification

API (see cgroups.txt). It allows to register multiple memory and memsw

API (see cgroups.txt). It allows to register multiple memory and memsw

thresholds and gets notifications when it crosses.

thresholds and gets notifications when it crosses.

To register a threshold application need:

To register a threshold application need:

- create an eventfd using eventfd(2);

- create an eventfd using eventfd(2);

- open memory.usage_in_bytes or memory.memsw.usage_in_bytes;

- open memory.usage_in_bytes or memory.memsw.usage_in_bytes;

 - write string like "<event_fd> <memory.usage_in_bytes> <threshold>" to

- write string like "<event_fd> <fd of memory.usage_in_bytes> <threshold>" to

  cgroup.event_control.

  cgroup.event_control.

Application will be notified through eventfd when memory usage crosses

Application will be notified through eventfd when memory usage crosses

memory.oom_control file is for OOM notification and other controls.

memory.oom_control file is for OOM notification and other controls.

Memory controler implements oom notifier using cgroup notification

Memory cgroup implements OOM notifier using cgroup notification

API (See cgroups.txt). It allows to register multiple oom notification

API (See cgroups.txt). It allows to register multiple OOM notification

delivery and gets notification when oom happens.

delivery and gets notification when OOM happens.

To register a notifier, application need:

To register a notifier, application need:

 - create an eventfd using eventfd(2)

 - create an eventfd using eventfd(2)

 - open memory.oom_control file

 - open memory.oom_control file

 - write string like "<event_fd> <memory.oom_control>" to cgroup.event_control

 - write string like "<event_fd> <fd of memory.oom_control>" to

   cgroup.event_control

Application will be notifier through eventfd when oom happens.

Application will be notified through eventfd when OOM happens.

OOM notification doesn't work for root cgroup.

OOM notification doesn't work for root cgroup.

You can disable oom-killer by writing "1" to memory.oom_control file.

You can disable OOM-killer by writing "1" to memory.oom_control file, as:

As.

	#echo 1 > memory.oom_control

	#echo 1 > memory.oom_control

This operation is only allowed to the top cgroup of subhierarchy.

This operation is only allowed to the top cgroup of sub-hierarchy.

If oom-killer is disabled, tasks under cgroup will hang/sleep

If OOM-killer is disabled, tasks under cgroup will hang/sleep

in memcg's oom-waitq when they request accountable memory.

in memory cgroup's OOM-waitqueue when they request accountable memory.

For running them, you have to relax the memcg's oom sitaution by

For running them, you have to relax the memory cgroup's OOM status by

	* enlarge limit or reduce usage.

	* enlarge limit or reduce usage.

To reduce usage,

To reduce usage,

	* kill some tasks.

	* kill some tasks.

At reading, current status of OOM is shown.

At reading, current status of OOM is shown.

	oom_kill_disable 0 or 1 (if 1, oom-killer is disabled)

	oom_kill_disable 0 or 1 (if 1, oom-killer is disabled)

	under_oom	 0 or 1 (if 1, the memcg is under OOM,tasks may

	under_oom	 0 or 1 (if 1, the memory cgroup is under OOM, tasks may

				 be stopped.)

				 be stopped.)

11. TODO

11. TODO