Kernel-2.6.32-573.12.1.el6_memcg_test

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)

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  1. 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.
    

    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.

    (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.
  1. 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().

  2. 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.
  3. 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().

  4. 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.)

  1. Typical Tests.

    Tests for racy cases.

  2. 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.)

  3. 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.

  4. 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} &

  5. 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.

  6. 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.

  7. 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.

  8. 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 -t 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

  10. 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.