=====================================================
Memory Resource Controller(Memcg) Implementation Memo
=====================================================

Last Updated: 2010/2

Base Kernel Version: based on 2.6.33-rc7-mm(candidate for 34).

Because VM is getting complex (one of reasons is memcg...), memcg's behavior
is complex. This is a document for memcg's internal behavior.
Please note that implementation details can be changed.

(*) Topics on API should be in Documentation/admin-guide/cgroup-v1/memory.rst)

0. How to record usage ?
========================

   2 objects are used.

   page_cgroup ....an object per page.

	Allocated at boot or memory hotplug. Freed at memory hot removal.

   swap_cgroup ... an entry per swp_entry.

	Allocated at swapon(). Freed at swapoff().

   The page_cgroup has USED bit and double count against a page_cgroup never
   occurs. swap_cgroup is used only when a charged page is swapped-out.

1. Charge
=========

   a page/swp_entry may be charged (usage += PAGE_SIZE) at

	mem_cgroup_try_charge()

2. Uncharge
===========

  a page/swp_entry may be uncharged (usage -= PAGE_SIZE) by

	mem_cgroup_uncharge()
	  Called when a page's refcount goes down to 0.

	mem_cgroup_uncharge_swap()
	  Called when swp_entry's refcnt goes down to 0. A charge against swap
	  disappears.

3. charge-commit
=======================

	Memcg pages are charged in two steps:

		- mem_cgroup_try_charge()
		- commit_charge()

	At try_charge(), there are no flags to say "this page is charged".
	at this point, usage += PAGE_SIZE.

	At commit(), the page is associated with the memcg.

Under below explanation, we assume CONFIG_SWAP=y.

4. Anonymous
============

	Anonymous page is newly allocated at
		  - page fault into MAP_ANONYMOUS mapping.
		  - Copy-On-Write.

	4.1 Swap-in.
	At swap-in, the page is taken from swap-cache. There are 2 cases.

	(a) If the SwapCache is newly allocated and read, it has no charges.
	(b) If the SwapCache has been mapped by processes, it has been
	    charged already.

	4.2 Swap-out.
	At swap-out, typical state transition is below.

	(a) add to swap cache. (marked as SwapCache)
	    swp_entry's refcnt += 1.
	(b) fully unmapped.
	    swp_entry's refcnt += # of ptes.
	(c) write back to swap.
	(d) delete from swap cache. (remove from SwapCache)
	    swp_entry's refcnt -= 1.


	Finally, at task exit,
	(e) zap_pte() is called and swp_entry's refcnt -=1 -> 0.

5. Page Cache
=============

	Page Cache is charged at
	- filemap_add_folio().

	The logic is very clear. (About migration, see below)

	Note:
	  __filemap_remove_folio() is called by filemap_remove_folio()
	  and __remove_mapping().

6. Shmem(tmpfs) Page Cache
===========================

	The best way to understand shmem's page state transition is to read
	mm/shmem.c.

	But brief explanation of the behavior of memcg around shmem will be
	helpful to understand the logic.

	Shmem's page (just leaf page, not direct/indirect block) can be on

		- radix-tree of shmem's inode.
		- SwapCache.
		- Both on radix-tree and SwapCache. This happens at swap-in
		  and swap-out,

	It's charged when...

	- A new page is added to shmem's radix-tree.
	- A swp page is read. (move a charge from swap_cgroup to page_cgroup)

7. Page Migration
=================

	mem_cgroup_migrate()

8. LRU
======
	Each memcg has its own vector of LRUs (inactive anon, active anon,
	inactive file, active file, unevictable) of pages from each node,
	each LRU handled under a single lru_lock for that memcg and node.

9. Typical Tests.
=================

 Tests for racy cases.

9.1 Small limit to memcg.
-------------------------

	When you do test to do racy case, it's good test to set memcg's limit
	to be very small rather than GB. Many races found in the test under
	xKB or xxMB limits.

	(Memory behavior under GB and Memory behavior under MB shows very
	different situation.)

9.2 Shmem
---------

	Historically, memcg's shmem handling was poor and we saw some amount
	of troubles here. This is because shmem is page-cache but can be
	SwapCache. Test with shmem/tmpfs is always good test.

9.3 Migration
-------------

	For NUMA, migration is an another special case. To do easy test, cpuset
	is useful. Following is a sample script to do migration::

		mount -t cgroup -o cpuset none /opt/cpuset

		mkdir /opt/cpuset/01
		echo 1 > /opt/cpuset/01/cpuset.cpus
		echo 0 > /opt/cpuset/01/cpuset.mems
		echo 1 > /opt/cpuset/01/cpuset.memory_migrate
		mkdir /opt/cpuset/02
		echo 1 > /opt/cpuset/02/cpuset.cpus
		echo 1 > /opt/cpuset/02/cpuset.mems
		echo 1 > /opt/cpuset/02/cpuset.memory_migrate

	In above set, when you moves a task from 01 to 02, page migration to
	node 0 to node 1 will occur. Following is a script to migrate all
	under cpuset.::

		--
		move_task()
		{
		for pid in $1
		do
			/bin/echo $pid >$2/tasks 2>/dev/null
			echo -n $pid
			echo -n " "
		done
		echo END
		}

		G1_TASK=`cat ${G1}/tasks`
		G2_TASK=`cat ${G2}/tasks`
		move_task "${G1_TASK}" ${G2} &
		--

9.4 Memory hotplug
------------------

	memory hotplug test is one of good test.

	to offline memory, do following::

		# echo offline > /sys/devices/system/memory/memoryXXX/state

	(XXX is the place of memory)

	This is an easy way to test page migration, too.

9.5 nested cgroups
------------------

	Use tests like the following for testing nested cgroups::

		mkdir /opt/cgroup/01/child_a
		mkdir /opt/cgroup/01/child_b

		set limit to 01.
		add limit to 01/child_b
		run jobs under child_a and child_b

	create/delete following groups at random while jobs are running::

		/opt/cgroup/01/child_a/child_aa
		/opt/cgroup/01/child_b/child_bb
		/opt/cgroup/01/child_c

	running new jobs in new group is also good.

9.6 Mount with other subsystems
-------------------------------

	Mounting with other subsystems is a good test because there is a
	race and lock dependency with other cgroup subsystems.

	example::

		# mount -t cgroup none /cgroup -o cpuset,memory,cpu,devices

	and do task move, mkdir, rmdir etc...under this.

9.7 swapoff
-----------

	Besides management of swap is one of complicated parts of memcg,
	call path of swap-in at swapoff is not same as usual swap-in path..
	It's worth to be tested explicitly.

	For example, test like following is good:

	(Shell-A)::

		# mount -t cgroup none /cgroup -o memory
		# mkdir /cgroup/test
		# echo 40M > /cgroup/test/memory.limit_in_bytes
		# echo 0 > /cgroup/test/tasks

	Run malloc(100M) program under this. You'll see 60M of swaps.

	(Shell-B)::

		# move all tasks in /cgroup/test to /cgroup
		# /sbin/swapoff -a
		# rmdir /cgroup/test
		# kill malloc task.

	Of course, tmpfs v.s. swapoff test should be tested, too.

9.8 OOM-Killer
--------------

	Out-of-memory caused by memcg's limit will kill tasks under
	the memcg. When hierarchy is used, a task under hierarchy
	will be killed by the kernel.

	In this case, panic_on_oom shouldn't be invoked and tasks
	in other groups shouldn't be killed.

	It's not difficult to cause OOM under memcg as following.

	Case A) when you can swapoff::

		#swapoff -a
		#echo 50M > /memory.limit_in_bytes

	run 51M of malloc

	Case B) when you use mem+swap limitation::

		#echo 50M > memory.limit_in_bytes
		#echo 50M > memory.memsw.limit_in_bytes

	run 51M of malloc

9.9 Move charges at task migration
----------------------------------

	Charges associated with a task can be moved along with task migration.

	(Shell-A)::

		#mkdir /cgroup/A
		#echo $$ >/cgroup/A/tasks

	run some programs which uses some amount of memory in /cgroup/A.

	(Shell-B)::

		#mkdir /cgroup/B
		#echo 1 >/cgroup/B/memory.move_charge_at_immigrate
		#echo "pid of the program running in group A" >/cgroup/B/tasks

	You can see charges have been moved by reading ``*.usage_in_bytes`` or
	memory.stat of both A and B.

	See 8.2 of Documentation/admin-guide/cgroup-v1/memory.rst to see what value should
	be written to move_charge_at_immigrate.

9.10 Memory thresholds
----------------------

	Memory controller implements memory thresholds using cgroups notification
	API. You can use samples/cgroup/cgroup_event_listener.c to test it.

	(Shell-A) Create cgroup and run event listener::

		# mkdir /cgroup/A
		# ./cgroup_event_listener /cgroup/A/memory.usage_in_bytes 5M

	(Shell-B) Add task to cgroup and try to allocate and free memory::

		# echo $$ >/cgroup/A/tasks
		# a="$(dd if=/dev/zero bs=1M count=10)"
		# a=

	You will see message from cgroup_event_listener every time you cross
	the thresholds.

	Use /cgroup/A/memory.memsw.usage_in_bytes to test memsw thresholds.

	It's good idea to test root cgroup as well.
