memcg: explain details and test document

Documentation for implementation details and how to test.

Just an example. feel free to modify, add, remove lines.
Signed-off-by: NKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/controllers/memcg_test.txt

+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.