Documentation/cgroup-v1/memcg_test.txt

   1 Memory Resource Controller(Memcg)  Implementation Memo.
   2 Last Updated: 2010/2
   3 Base Kernel Version: based on 2.6.33-rc7-mm(candidate for 34).
   4
   5 Because VM is getting complex (one of reasons is memcg...), memcg's behavior
   6 is complex. This is a document for memcg's internal behavior.
   7 Please note that implementation details can be changed.
   8
   9 (*) Topics on API should be in Documentation/cgroup-v1/memory.txt)
  10
  11 0. How to record usage ?
  12    2 objects are used.
  13
  14    page_cgroup ....an object per page.
  15         Allocated at boot or memory hotplug. Freed at memory hot removal.
  16
  17    swap_cgroup ... an entry per swp_entry.
  18         Allocated at swapon(). Freed at swapoff().
  19
  20    The page_cgroup has USED bit and double count against a page_cgroup never
  21    occurs. swap_cgroup is used only when a charged page is swapped-out.
  22
  23 1. Charge
  24
  25    a page/swp_entry may be charged (usage += PAGE_SIZE) at
  26
  27         mem_cgroup_try_charge()
  28
  29 2. Uncharge
  30   a page/swp_entry may be uncharged (usage -= PAGE_SIZE) by
  31
  32         mem_cgroup_uncharge()
  33           Called when a page's refcount goes down to 0.
  34
  35         mem_cgroup_uncharge_swap()
  36           Called when swp_entry's refcnt goes down to 0. A charge against swap
  37           disappears.
  38
  39 3. charge-commit-cancel
  40         Memcg pages are charged in two steps:
  41                 mem_cgroup_try_charge()
  42                 mem_cgroup_commit_charge() or mem_cgroup_cancel_charge()
  43
  44         At try_charge(), there are no flags to say "this page is charged".
  45         at this point, usage += PAGE_SIZE.
  46
  47         At commit(), the page is associated with the memcg.
  48
  49         At cancel(), simply usage -= PAGE_SIZE.
  50
  51 Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.
  52
  53 4. Anonymous
  54         Anonymous page is newly allocated at
  55                   - page fault into MAP_ANONYMOUS mapping.
  56                   - Copy-On-Write.
  57
  58         4.1 Swap-in.
  59         At swap-in, the page is taken from swap-cache. There are 2 cases.
  60
  61         (a) If the SwapCache is newly allocated and read, it has no charges.
  62         (b) If the SwapCache has been mapped by processes, it has been
  63             charged already.
  64
  65         4.2 Swap-out.
  66         At swap-out, typical state transition is below.
  67
  68         (a) add to swap cache. (marked as SwapCache)
  69             swp_entry's refcnt += 1.
  70         (b) fully unmapped.
  71             swp_entry's refcnt += # of ptes.
  72         (c) write back to swap.
  73         (d) delete from swap cache. (remove from SwapCache)
  74             swp_entry's refcnt -= 1.
  75
  76
  77         Finally, at task exit,
  78         (e) zap_pte() is called and swp_entry's refcnt -=1 -> 0.
  79
  80 5. Page Cache
  81         Page Cache is charged at
  82         - add_to_page_cache_locked().
  83
  84         The logic is very clear. (About migration, see below)
  85         Note: __remove_from_page_cache() is called by remove_from_page_cache()
  86         and __remove_mapping().
  87
  88 6. Shmem(tmpfs) Page Cache
  89         The best way to understand shmem's page state transition is to read
  90         mm/shmem.c.
  91         But brief explanation of the behavior of memcg around shmem will be
  92         helpful to understand the logic.
  93
  94         Shmem's page (just leaf page, not direct/indirect block) can be on
  95                 - radix-tree of shmem's inode.
  96                 - SwapCache.
  97                 - Both on radix-tree and SwapCache. This happens at swap-in
  98                   and swap-out,
  99
 100         It's charged when...
 101         - A new page is added to shmem's radix-tree.
 102         - A swp page is read. (move a charge from swap_cgroup to page_cgroup)
 103
 104 7. Page Migration
 105
 106         mem_cgroup_migrate()
 107
 108 8. LRU
 109         Each memcg has its own private LRU. Now, its handling is under global
 110         VM's control (means that it's handled under global zone_lru_lock).
 111         Almost all routines around memcg's LRU is called by global LRU's
 112         list management functions under zone_lru_lock().
 113
 114         A special function is mem_cgroup_isolate_pages(). This scans
 115         memcg's private LRU and call __isolate_lru_page() to extract a page
 116         from LRU.
 117         (By __isolate_lru_page(), the page is removed from both of global and
 118          private LRU.)
 119
 120
 121 9. Typical Tests.
 122
 123  Tests for racy cases.
 124
 125  9.1 Small limit to memcg.
 126         When you do test to do racy case, it's good test to set memcg's limit
 127         to be very small rather than GB. Many races found in the test under
 128         xKB or xxMB limits.
 129         (Memory behavior under GB and Memory behavior under MB shows very
 130          different situation.)
 131
 132  9.2 Shmem
 133         Historically, memcg's shmem handling was poor and we saw some amount
 134         of troubles here. This is because shmem is page-cache but can be
 135         SwapCache. Test with shmem/tmpfs is always good test.
 136
 137  9.3 Migration
 138         For NUMA, migration is an another special case. To do easy test, cpuset
 139         is useful. Following is a sample script to do migration.
 140
 141         mount -t cgroup -o cpuset none /opt/cpuset
 142
 143         mkdir /opt/cpuset/01
 144         echo 1 > /opt/cpuset/01/cpuset.cpus
 145         echo 0 > /opt/cpuset/01/cpuset.mems
 146         echo 1 > /opt/cpuset/01/cpuset.memory_migrate
 147         mkdir /opt/cpuset/02
 148         echo 1 > /opt/cpuset/02/cpuset.cpus
 149         echo 1 > /opt/cpuset/02/cpuset.mems
 150         echo 1 > /opt/cpuset/02/cpuset.memory_migrate
 151
 152         In above set, when you moves a task from 01 to 02, page migration to
 153         node 0 to node 1 will occur. Following is a script to migrate all
 154         under cpuset.
 155         --
 156         move_task()
 157         {
 158         for pid in $1
 159         do
 160                 /bin/echo $pid >$2/tasks 2>/dev/null
 161                 echo -n $pid
 162                 echo -n " "
 163         done
 164         echo END
 165         }
 166
 167         G1_TASK=`cat ${G1}/tasks`
 168         G2_TASK=`cat ${G2}/tasks`
 169         move_task "${G1_TASK}" ${G2} &
 170         --
 171  9.4 Memory hotplug.
 172         memory hotplug test is one of good test.
 173         to offline memory, do following.
 174         # echo offline > /sys/devices/system/memory/memoryXXX/state
 175         (XXX is the place of memory)
 176         This is an easy way to test page migration, too.
 177
 178  9.5 mkdir/rmdir
 179         When using hierarchy, mkdir/rmdir test should be done.
 180         Use tests like the following.
 181
 182         echo 1 >/opt/cgroup/01/memory/use_hierarchy
 183         mkdir /opt/cgroup/01/child_a
 184         mkdir /opt/cgroup/01/child_b
 185
 186         set limit to 01.
 187         add limit to 01/child_b
 188         run jobs under child_a and child_b
 189
 190         create/delete following groups at random while jobs are running.
 191         /opt/cgroup/01/child_a/child_aa
 192         /opt/cgroup/01/child_b/child_bb
 193         /opt/cgroup/01/child_c
 194
 195         running new jobs in new group is also good.
 196
 197  9.6 Mount with other subsystems.
 198         Mounting with other subsystems is a good test because there is a
 199         race and lock dependency with other cgroup subsystems.
 200
 201         example)
 202         # mount -t cgroup none /cgroup -o cpuset,memory,cpu,devices
 203
 204         and do task move, mkdir, rmdir etc...under this.
 205
 206  9.7 swapoff.
 207         Besides management of swap is one of complicated parts of memcg,
 208         call path of swap-in at swapoff is not same as usual swap-in path..
 209         It's worth to be tested explicitly.
 210
 211         For example, test like following is good.
 212         (Shell-A)
 213         # mount -t cgroup none /cgroup -o memory
 214         # mkdir /cgroup/test
 215         # echo 40M > /cgroup/test/memory.limit_in_bytes
 216         # echo 0 > /cgroup/test/tasks
 217         Run malloc(100M) program under this. You'll see 60M of swaps.
 218         (Shell-B)
 219         # move all tasks in /cgroup/test to /cgroup
 220         # /sbin/swapoff -a
 221         # rmdir /cgroup/test
 222         # kill malloc task.
 223
 224         Of course, tmpfs v.s. swapoff test should be tested, too.
 225
 226  9.8 OOM-Killer
 227         Out-of-memory caused by memcg's limit will kill tasks under
 228         the memcg. When hierarchy is used, a task under hierarchy
 229         will be killed by the kernel.
 230         In this case, panic_on_oom shouldn't be invoked and tasks
 231         in other groups shouldn't be killed.
 232
 233         It's not difficult to cause OOM under memcg as following.
 234         Case A) when you can swapoff
 235         #swapoff -a
 236         #echo 50M > /memory.limit_in_bytes
 237         run 51M of malloc
 238
 239         Case B) when you use mem+swap limitation.
 240         #echo 50M > memory.limit_in_bytes
 241         #echo 50M > memory.memsw.limit_in_bytes
 242         run 51M of malloc
 243
 244  9.9 Move charges at task migration
 245         Charges associated with a task can be moved along with task migration.
 246
 247         (Shell-A)
 248         #mkdir /cgroup/A
 249         #echo $$ >/cgroup/A/tasks
 250         run some programs which uses some amount of memory in /cgroup/A.
 251
 252         (Shell-B)
 253         #mkdir /cgroup/B
 254         #echo 1 >/cgroup/B/memory.move_charge_at_immigrate
 255         #echo "pid of the program running in group A" >/cgroup/B/tasks
 256
 257         You can see charges have been moved by reading *.usage_in_bytes or
 258         memory.stat of both A and B.
 259         See 8.2 of Documentation/cgroup-v1/memory.txt to see what value should be
 260         written to move_charge_at_immigrate.
 261
 262  9.10 Memory thresholds
 263         Memory controller implements memory thresholds using cgroups notification
 264         API. You can use tools/cgroup/cgroup_event_listener.c to test it.
 265
 266         (Shell-A) Create cgroup and run event listener
 267         # mkdir /cgroup/A
 268         # ./cgroup_event_listener /cgroup/A/memory.usage_in_bytes 5M
 269
 270         (Shell-B) Add task to cgroup and try to allocate and free memory
 271         # echo $$ >/cgroup/A/tasks
 272         # a="$(dd if=/dev/zero bs=1M count=10)"
 273         # a=
 274
 275         You will see message from cgroup_event_listener every time you cross
 276         the thresholds.
 277
 278         Use /cgroup/A/memory.memsw.usage_in_bytes to test memsw thresholds.
 279
 280         It's good idea to test root cgroup as well.