memcg: explain details and test document

Memory Resource Controller(Memcg)  Implementation Memo.

Last Updated: 2008/12/10

Base Kernel Version: based on 2.6.28-rc7-mm.

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/controllers/memory.txt)

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_newpage_charge()

	  Called at new page fault and Copy-On-Write.

	mem_cgroup_try_charge_swapin()

	  Called at do_swap_page() (page fault on swap entry) and swapoff.

	  Followed by charge-commit-cancel protocol. (With swap accounting)

	  At commit, a charge recorded in swap_cgroup is removed.

	mem_cgroup_cache_charge()

	  Called at add_to_page_cache()

	mem_cgroup_cache_charge_swapin()

	  Called at shmem's swapin.

	mem_cgroup_prepare_migration()

	  Called before migration. "extra" charge is done and followed by

	  charge-commit-cancel protocol.

	  At commit, charge against oldpage or newpage will be committed.

2. Uncharge

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

	mem_cgroup_uncharge_page()

	  Called when an anonymous page is fully unmapped. I.e., mapcount goes

	  to 0. If the page is SwapCache, uncharge is delayed until

	  mem_cgroup_uncharge_swapcache().

	mem_cgroup_uncharge_cache_page()

	  Called when a page-cache is deleted from radix-tree. If the page is

	  SwapCache, uncharge is delayed until mem_cgroup_uncharge_swapcache().

	mem_cgroup_uncharge_swapcache()

	  Called when SwapCache is removed from radix-tree. The charge itself

	  is moved to swap_cgroup. (If mem+swap controller is disabled, no

	  charge to swap occurs.)

	mem_cgroup_uncharge_swap()

	  Called when swp_entry's refcnt goes down to 0. A charge against swap

	  disappears.

	mem_cgroup_end_migration(old, new)

	At success of migration old is uncharged (if necessary), a charge

	to new page is committed. At failure, charge to old page is committed.

3. charge-commit-cancel

	In some case, we can't know this "charge" is valid or not at charging

	(because of races).

	To handle such case, there are charge-commit-cancel functions.

		mem_cgroup_try_charge_XXX

		mem_cgroup_commit_charge_XXX

		mem_cgroup_cancel_charge_XXX

	these are used in swap-in and migration.

	At try_charge(), there are no flags to say "this page is charged".

	at this point, usage += PAGE_SIZE.

	At commit(), the function checks the page should be charged or not

	and set flags or avoid charging.(usage -= PAGE_SIZE)

	At cancel(), simply usage -= PAGE_SIZE.

Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.

4. Anonymous

	Anonymous page is newly allocated at

		  - page fault into MAP_ANONYMOUS mapping.

		  - Copy-On-Write.

 	It is charged right after it's allocated before doing any page table

	related operations. Of course, it's uncharged when another page is used

	for the fault address.

	At freeing anonymous page (by exit() or munmap()), zap_pte() is called

	and pages for ptes are freed one by one.(see mm/memory.c). Uncharges

	are done at page_remove_rmap() when page_mapcount() goes down to 0.

	Another page freeing is by page-reclaim (vmscan.c) and anonymous

	pages are swapped out. In this case, the page is marked as

	PageSwapCache(). uncharge() routine doesn't uncharge the page marked

	as SwapCache(). It's delayed until __delete_from_swap_cache().

	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.

	In case (a), we charge it. In case (b), we don't charge it.

	(But racy state between (a) and (b) exists. We do check it.)

	At charging, a charge recorded in swap_cgroup is moved to page_cgroup.

	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.

	At (b), the page is marked as SwapCache and not uncharged.

	At (d), the page is removed from SwapCache and a charge in page_cgroup

	is moved to swap_cgroup.

	Finally, at task exit,

	(e) zap_pte() is called and swp_entry's refcnt -=1 -> 0.

	Here, a charge in swap_cgroup disappears.

5. Page Cache

   	Page Cache is charged at

	- add_to_page_cache_locked().

	uncharged at

	- __remove_from_page_cache().

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

	Note: __remove_from_page_cache() is called by remove_from_page_cache()

	and __remove_mapping().

6. Shmem(tmpfs) Page Cache

	Memcg's charge/uncharge have special handlers of shmem. 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)

	It's uncharged when

	- A page is removed from radix-tree and not SwapCache.

	- When SwapCache is removed, a charge is moved to swap_cgroup.

	- When swp_entry's refcnt goes down to 0, a charge in swap_cgroup

	  disappears.

7. Page Migration

   	One of the most complicated functions is page-migration-handler.

	Memcg has 2 routines. Assume that we are migrating a page's contents

	from OLDPAGE to NEWPAGE.

	Usual migration logic is..

	(a) remove the page from LRU.

	(b) allocate NEWPAGE (migration target)

	(c) lock by lock_page().

	(d) unmap all mappings.

	(e-1) If necessary, replace entry in radix-tree.

	(e-2) move contents of a page.

	(f) map all mappings again.

	(g) pushback the page to LRU.

	(-) OLDPAGE will be freed.

	Before (g), memcg should complete all necessary charge/uncharge to

	NEWPAGE/OLDPAGE.

	The point is....

	- If OLDPAGE is anonymous, all charges will be dropped at (d) because

          try_to_unmap() drops all mapcount and the page will not be

	  SwapCache.

	- If OLDPAGE is SwapCache, charges will be kept at (g) because

	  __delete_from_swap_cache() isn't called at (e-1)

	- If OLDPAGE is page-cache, charges will be kept at (g) because

	  remove_from_swap_cache() isn't called at (e-1)

	memcg provides following hooks.

	- mem_cgroup_prepare_migration(OLDPAGE)

	  Called after (b) to account a charge (usage += PAGE_SIZE) against

	  memcg which OLDPAGE belongs to.

        - mem_cgroup_end_migration(OLDPAGE, NEWPAGE)

	  Called after (f) before (g).

	  If OLDPAGE is used, commit OLDPAGE again. If OLDPAGE is already

	  charged, a charge by prepare_migration() is automatically canceled.

	  If NEWPAGE is used, commit NEWPAGE and uncharge OLDPAGE.

	  But zap_pte() (by exit or munmap) can be called while migration,

	  we have to check if OLDPAGE/NEWPAGE is a valid page after commit().

8. LRU

        Each memcg has its own private LRU. Now, it's handling is under global

	VM's control (means that it's handled under global zone->lru_lock).

	Almost all routines around memcg's LRU is called by global LRU's

	list management functions under zone->lru_lock().

	A special function is mem_cgroup_isolate_pages(). This scans

	memcg's private LRU and call __isolate_lru_page() to extract a page

	from LRU.

	(By __isolate_lru_page(), the page is removed from both of global and

	 private LRU.)

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 mkdir/rmdir

	When using hierarchy, mkdir/rmdir test should be done.

	Use tests like the following.

	echo 1 >/opt/cgroup/01/memory/use_hierarchy

	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 -t cpuset,memory,cpu,devices

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