diff options
Diffstat (limited to 'Documentation/cgroups/memcg_test.txt')
| -rw-r--r-- | Documentation/cgroups/memcg_test.txt | 160 |
1 files changed, 11 insertions, 149 deletions
diff --git a/Documentation/cgroups/memcg_test.txt b/Documentation/cgroups/memcg_test.txt index 80ac454704b8..8870b0212150 100644 --- a/Documentation/cgroups/memcg_test.txt +++ b/Documentation/cgroups/memcg_test.txt @@ -24,64 +24,27 @@ Please note that implementation details can be changed. a page/swp_entry may be charged (usage += PAGE_SIZE) at - mem_cgroup_charge_anon() - 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_charge_file() - 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. + mem_cgroup_try_charge() 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() + 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. - 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. + 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 function checks the page should be charged or not - and set flags or avoid charging.(usage -= PAGE_SIZE) + At commit(), the page is associated with the memcg. At cancel(), simply usage -= PAGE_SIZE. @@ -91,18 +54,6 @@ Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y. 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. @@ -111,41 +62,6 @@ Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y. (b) If the SwapCache has been mapped by processes, it has been charged already. - This swap-in is one of the most complicated work. In do_swap_page(), - following events occur when pte is unchanged. - - (1) the page (SwapCache) is looked up. - (2) lock_page() - (3) try_charge_swapin() - (4) reuse_swap_page() (may call delete_swap_cache()) - (5) commit_charge_swapin() - (6) swap_free(). - - Considering following situation for example. - - (A) The page has not been charged before (2) and reuse_swap_page() - doesn't call delete_from_swap_cache(). - (B) The page has not been charged before (2) and reuse_swap_page() - calls delete_from_swap_cache(). - (C) The page has been charged before (2) and reuse_swap_page() doesn't - call delete_from_swap_cache(). - (D) The page has been charged before (2) and reuse_swap_page() calls - delete_from_swap_cache(). - - memory.usage/memsw.usage changes to this page/swp_entry will be - Case (A) (B) (C) (D) - Event - Before (2) 0/ 1 0/ 1 1/ 1 1/ 1 - =========================================== - (3) +1/+1 +1/+1 +1/+1 +1/+1 - (4) - 0/ 0 - -1/ 0 - (5) 0/-1 0/ 0 -1/-1 0/ 0 - (6) - 0/-1 - 0/-1 - =========================================== - Result 1/ 1 1/ 1 1/ 1 1/ 1 - - In any cases, charges to this page should be 1/ 1. - 4.2 Swap-out. At swap-out, typical state transition is below. @@ -158,28 +74,20 @@ Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y. 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. + 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. @@ -192,56 +100,10 @@ Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y. 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(). + + mem_cgroup_migrate() 8. LRU Each memcg has its own private LRU. Now, its handling is under global |