Kernel-4.18.0-80.el8_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/cgroup-v1/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_try_charge()

2. Uncharge
   a page/swp_entry may be uncharged (usage -= PAGE_SIZE) by

   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.

3. charge-commit-cancel
   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 page is associated with the memcg.

   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.
   

   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.
   

   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.
   

Finally, at task exit,
(e) zap_pte() is called and swp_entry's refcnt -=1 -> 0.

5. Page Cache

   Page Cache is charged at
   

   • add_to_page_cache_locked().

     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
   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)

7. Page Migration

   mem_cgroup_migrate()

8. LRU

    Each memcg has its own private LRU. Now, its 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.

10. 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.)

11. 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.

12. 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} &

13. 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.

14. 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.

15. 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 -o cpuset,memory,cpu,devices

    and do task move, mkdir, rmdir etc…under this.

16. 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 -o 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.

17. 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

18. 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/cgroup-v1/memory.txt to see what value should be
    written to move_charge_at_immigrate.

19. 10 Memory thresholds
    Memory controller implements memory thresholds using cgroups notification
    API. You can use tools/cgroup/cgroup_event_listener.c to test it.

    (Shell-A) Create cgroup and run event listener

    mkdir /cgroup/A

    ./cgroup_event_listener /cgroup/A/memory.usage_in_bytes 5M

    (Shell-B) Add task to cgroup and try to allocate and free memory

    echo $$ >/cgroup/A/tasks

    a=”$(dd if=/dev/zero bs=1M count=10)”

    a=

    You will see message from cgroup_event_listener every time you cross
    the thresholds.

    Use /cgroup/A/memory.memsw.usage_in_bytes to test memsw thresholds.

    It’s good idea to test root cgroup as well.