--- /dev/null
+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/cgroups/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_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-cancel
+ Memcg pages are charged in two steps:
+ mem_cgroup_try_charge()
+ mem_cgroup_commit_charge() or mem_cgroup_cancel_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.
+
+ 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.
+
+ 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
+ - add_to_page_cache_locked().
+
+ 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
+ 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 private LRU. Now, its 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 -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 $$ >/cgroup/A/tasks
+ run some programs which uses some amount of memory in /cgroup/A.
+
+ (Shell-B)
+ #mkdir /cgroup/B
+ #echo 1 >/cgroup/B/memory.move_charge_at_immigrate
+ #echo "pid of the program running in group A" >/cgroup/B/tasks
+
+ You can see charges have been moved by reading *.usage_in_bytes or
+ memory.stat of both A and B.
+ See 8.2 of Documentation/cgroups/memory.txt to see what value should be
+ written to move_charge_at_immigrate.
+
+ 9.10 Memory thresholds
+ Memory controller implements memory thresholds using cgroups notification
+ API. You can use tools/cgroup/cgroup_event_listener.c to test it.
+
+ (Shell-A) Create cgroup and run event listener
+ # mkdir /cgroup/A
+ # ./cgroup_event_listener /cgroup/A/memory.usage_in_bytes 5M
+
+ (Shell-B) Add task to cgroup and try to allocate and free memory
+ # echo $$ >/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.
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