1=====================================================
2Memory Resource Controller(Memcg) Implementation Memo
3=====================================================
4
5Last Updated: 2010/2
6
7Base Kernel Version: based on 2.6.33-rc7-mm(candidate for 34).
8
9Because VM is getting complex (one of reasons is memcg...), memcg's behavior
10is complex. This is a document for memcg's internal behavior.
11Please note that implementation details can be changed.
12
13(*) Topics on API should be in Documentation/admin-guide/cgroup-v1/memory.rst)
14
150. How to record usage ?
16========================
17
18   2 objects are used.
19
20   page_cgroup ....an object per page.
21
22	Allocated at boot or memory hotplug. Freed at memory hot removal.
23
24   swap_cgroup ... an entry per swp_entry.
25
26	Allocated at swapon(). Freed at swapoff().
27
28   The page_cgroup has USED bit and double count against a page_cgroup never
29   occurs. swap_cgroup is used only when a charged page is swapped-out.
30
311. Charge
32=========
33
34   a page/swp_entry may be charged (usage += PAGE_SIZE) at
35
36	mem_cgroup_try_charge()
37
382. Uncharge
39===========
40
41  a page/swp_entry may be uncharged (usage -= PAGE_SIZE) by
42
43	mem_cgroup_uncharge()
44	  Called when a page's refcount goes down to 0.
45
46	mem_cgroup_uncharge_swap()
47	  Called when swp_entry's refcnt goes down to 0. A charge against swap
48	  disappears.
49
503. charge-commit-cancel
51=======================
52
53	Memcg pages are charged in two steps:
54
55		- mem_cgroup_try_charge()
56		- mem_cgroup_commit_charge() or mem_cgroup_cancel_charge()
57
58	At try_charge(), there are no flags to say "this page is charged".
59	at this point, usage += PAGE_SIZE.
60
61	At commit(), the page is associated with the memcg.
62
63	At cancel(), simply usage -= PAGE_SIZE.
64
65Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.
66
674. Anonymous
68============
69
70	Anonymous page is newly allocated at
71		  - page fault into MAP_ANONYMOUS mapping.
72		  - Copy-On-Write.
73
74	4.1 Swap-in.
75	At swap-in, the page is taken from swap-cache. There are 2 cases.
76
77	(a) If the SwapCache is newly allocated and read, it has no charges.
78	(b) If the SwapCache has been mapped by processes, it has been
79	    charged already.
80
81	4.2 Swap-out.
82	At swap-out, typical state transition is below.
83
84	(a) add to swap cache. (marked as SwapCache)
85	    swp_entry's refcnt += 1.
86	(b) fully unmapped.
87	    swp_entry's refcnt += # of ptes.
88	(c) write back to swap.
89	(d) delete from swap cache. (remove from SwapCache)
90	    swp_entry's refcnt -= 1.
91
92
93	Finally, at task exit,
94	(e) zap_pte() is called and swp_entry's refcnt -=1 -> 0.
95
965. Page Cache
97=============
98
99	Page Cache is charged at
100	- filemap_add_folio().
101
102	The logic is very clear. (About migration, see below)
103
104	Note:
105	  __remove_from_page_cache() is called by remove_from_page_cache()
106	  and __remove_mapping().
107
1086. Shmem(tmpfs) Page Cache
109===========================
110
111	The best way to understand shmem's page state transition is to read
112	mm/shmem.c.
113
114	But brief explanation of the behavior of memcg around shmem will be
115	helpful to understand the logic.
116
117	Shmem's page (just leaf page, not direct/indirect block) can be on
118
119		- radix-tree of shmem's inode.
120		- SwapCache.
121		- Both on radix-tree and SwapCache. This happens at swap-in
122		  and swap-out,
123
124	It's charged when...
125
126	- A new page is added to shmem's radix-tree.
127	- A swp page is read. (move a charge from swap_cgroup to page_cgroup)
128
1297. Page Migration
130=================
131
132	mem_cgroup_migrate()
133
1348. LRU
135======
136	Each memcg has its own vector of LRUs (inactive anon, active anon,
137	inactive file, active file, unevictable) of pages from each node,
138	each LRU handled under a single lru_lock for that memcg and node.
139
1409. Typical Tests.
141=================
142
143 Tests for racy cases.
144
1459.1 Small limit to memcg.
146-------------------------
147
148	When you do test to do racy case, it's good test to set memcg's limit
149	to be very small rather than GB. Many races found in the test under
150	xKB or xxMB limits.
151
152	(Memory behavior under GB and Memory behavior under MB shows very
153	different situation.)
154
1559.2 Shmem
156---------
157
158	Historically, memcg's shmem handling was poor and we saw some amount
159	of troubles here. This is because shmem is page-cache but can be
160	SwapCache. Test with shmem/tmpfs is always good test.
161
1629.3 Migration
163-------------
164
165	For NUMA, migration is an another special case. To do easy test, cpuset
166	is useful. Following is a sample script to do migration::
167
168		mount -t cgroup -o cpuset none /opt/cpuset
169
170		mkdir /opt/cpuset/01
171		echo 1 > /opt/cpuset/01/cpuset.cpus
172		echo 0 > /opt/cpuset/01/cpuset.mems
173		echo 1 > /opt/cpuset/01/cpuset.memory_migrate
174		mkdir /opt/cpuset/02
175		echo 1 > /opt/cpuset/02/cpuset.cpus
176		echo 1 > /opt/cpuset/02/cpuset.mems
177		echo 1 > /opt/cpuset/02/cpuset.memory_migrate
178
179	In above set, when you moves a task from 01 to 02, page migration to
180	node 0 to node 1 will occur. Following is a script to migrate all
181	under cpuset.::
182
183		--
184		move_task()
185		{
186		for pid in $1
187		do
188			/bin/echo $pid >$2/tasks 2>/dev/null
189			echo -n $pid
190			echo -n " "
191		done
192		echo END
193		}
194
195		G1_TASK=`cat ${G1}/tasks`
196		G2_TASK=`cat ${G2}/tasks`
197		move_task "${G1_TASK}" ${G2} &
198		--
199
2009.4 Memory hotplug
201------------------
202
203	memory hotplug test is one of good test.
204
205	to offline memory, do following::
206
207		# echo offline > /sys/devices/system/memory/memoryXXX/state
208
209	(XXX is the place of memory)
210
211	This is an easy way to test page migration, too.
212
2139.5 nested cgroups
214------------------
215
216	Use tests like the following for testing nested cgroups::
217
218		mkdir /opt/cgroup/01/child_a
219		mkdir /opt/cgroup/01/child_b
220
221		set limit to 01.
222		add limit to 01/child_b
223		run jobs under child_a and child_b
224
225	create/delete following groups at random while jobs are running::
226
227		/opt/cgroup/01/child_a/child_aa
228		/opt/cgroup/01/child_b/child_bb
229		/opt/cgroup/01/child_c
230
231	running new jobs in new group is also good.
232
2339.6 Mount with other subsystems
234-------------------------------
235
236	Mounting with other subsystems is a good test because there is a
237	race and lock dependency with other cgroup subsystems.
238
239	example::
240
241		# mount -t cgroup none /cgroup -o cpuset,memory,cpu,devices
242
243	and do task move, mkdir, rmdir etc...under this.
244
2459.7 swapoff
246-----------
247
248	Besides management of swap is one of complicated parts of memcg,
249	call path of swap-in at swapoff is not same as usual swap-in path..
250	It's worth to be tested explicitly.
251
252	For example, test like following is good:
253
254	(Shell-A)::
255
256		# mount -t cgroup none /cgroup -o memory
257		# mkdir /cgroup/test
258		# echo 40M > /cgroup/test/memory.limit_in_bytes
259		# echo 0 > /cgroup/test/tasks
260
261	Run malloc(100M) program under this. You'll see 60M of swaps.
262
263	(Shell-B)::
264
265		# move all tasks in /cgroup/test to /cgroup
266		# /sbin/swapoff -a
267		# rmdir /cgroup/test
268		# kill malloc task.
269
270	Of course, tmpfs v.s. swapoff test should be tested, too.
271
2729.8 OOM-Killer
273--------------
274
275	Out-of-memory caused by memcg's limit will kill tasks under
276	the memcg. When hierarchy is used, a task under hierarchy
277	will be killed by the kernel.
278
279	In this case, panic_on_oom shouldn't be invoked and tasks
280	in other groups shouldn't be killed.
281
282	It's not difficult to cause OOM under memcg as following.
283
284	Case A) when you can swapoff::
285
286		#swapoff -a
287		#echo 50M > /memory.limit_in_bytes
288
289	run 51M of malloc
290
291	Case B) when you use mem+swap limitation::
292
293		#echo 50M > memory.limit_in_bytes
294		#echo 50M > memory.memsw.limit_in_bytes
295
296	run 51M of malloc
297
2989.9 Move charges at task migration
299----------------------------------
300
301	Charges associated with a task can be moved along with task migration.
302
303	(Shell-A)::
304
305		#mkdir /cgroup/A
306		#echo $$ >/cgroup/A/tasks
307
308	run some programs which uses some amount of memory in /cgroup/A.
309
310	(Shell-B)::
311
312		#mkdir /cgroup/B
313		#echo 1 >/cgroup/B/memory.move_charge_at_immigrate
314		#echo "pid of the program running in group A" >/cgroup/B/tasks
315
316	You can see charges have been moved by reading ``*.usage_in_bytes`` or
317	memory.stat of both A and B.
318
319	See 8.2 of Documentation/admin-guide/cgroup-v1/memory.rst to see what value should
320	be written to move_charge_at_immigrate.
321
3229.10 Memory thresholds
323----------------------
324
325	Memory controller implements memory thresholds using cgroups notification
326	API. You can use tools/cgroup/cgroup_event_listener.c to test it.
327
328	(Shell-A) Create cgroup and run event listener::
329
330		# mkdir /cgroup/A
331		# ./cgroup_event_listener /cgroup/A/memory.usage_in_bytes 5M
332
333	(Shell-B) Add task to cgroup and try to allocate and free memory::
334
335		# echo $$ >/cgroup/A/tasks
336		# a="$(dd if=/dev/zero bs=1M count=10)"
337		# a=
338
339	You will see message from cgroup_event_listener every time you cross
340	the thresholds.
341
342	Use /cgroup/A/memory.memsw.usage_in_bytes to test memsw thresholds.
343
344	It's good idea to test root cgroup as well.
345