Based on kernel version 4.3. Page generated on 2015-11-02 12:44 EST.
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/cgroups/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 $$ >/cgroup/A/tasks 250 run some programs which uses some amount of memory in /cgroup/A. 251 252 (Shell-B) 253 #mkdir /cgroup/B 254 #echo 1 >/cgroup/B/memory.move_charge_at_immigrate 255 #echo "pid of the program running in group A" >/cgroup/B/tasks 256 257 You can see charges have been moved by reading *.usage_in_bytes or 258 memory.stat of both A and B. 259 See 8.2 of Documentation/cgroups/memory.txt to see what value should be 260 written to move_charge_at_immigrate. 261 262 9.10 Memory thresholds 263 Memory controller implements memory thresholds using cgroups notification 264 API. You can use tools/cgroup/cgroup_event_listener.c to test it. 265 266 (Shell-A) Create cgroup and run event listener 267 # mkdir /cgroup/A 268 # ./cgroup_event_listener /cgroup/A/memory.usage_in_bytes 5M 269 270 (Shell-B) Add task to cgroup and try to allocate and free memory 271 # echo $$ >/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.