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.