345 lines
8.5 KiB
ReStructuredText
345 lines
8.5 KiB
ReStructuredText
=====================================================
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Memory Resource Controller(Memcg) Implementation Memo
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=====================================================
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Last Updated: 2010/2
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Base Kernel Version: based on 2.6.33-rc7-mm(candidate for 34).
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Because VM is getting complex (one of reasons is memcg...), memcg's behavior
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is complex. This is a document for memcg's internal behavior.
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Please note that implementation details can be changed.
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(*) Topics on API should be in Documentation/admin-guide/cgroup-v1/memory.rst)
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0. How to record usage ?
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========================
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2 objects are used.
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page_cgroup ....an object per page.
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Allocated at boot or memory hotplug. Freed at memory hot removal.
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swap_cgroup ... an entry per swp_entry.
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Allocated at swapon(). Freed at swapoff().
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The page_cgroup has USED bit and double count against a page_cgroup never
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occurs. swap_cgroup is used only when a charged page is swapped-out.
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1. Charge
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=========
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a page/swp_entry may be charged (usage += PAGE_SIZE) at
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mem_cgroup_try_charge()
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2. Uncharge
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===========
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a page/swp_entry may be uncharged (usage -= PAGE_SIZE) by
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mem_cgroup_uncharge()
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Called when a page's refcount goes down to 0.
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mem_cgroup_uncharge_swap()
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Called when swp_entry's refcnt goes down to 0. A charge against swap
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disappears.
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3. charge-commit-cancel
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=======================
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Memcg pages are charged in two steps:
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- mem_cgroup_try_charge()
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- mem_cgroup_commit_charge() or mem_cgroup_cancel_charge()
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At try_charge(), there are no flags to say "this page is charged".
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at this point, usage += PAGE_SIZE.
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At commit(), the page is associated with the memcg.
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At cancel(), simply usage -= PAGE_SIZE.
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Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.
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4. Anonymous
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============
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Anonymous page is newly allocated at
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- page fault into MAP_ANONYMOUS mapping.
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- Copy-On-Write.
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4.1 Swap-in.
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At swap-in, the page is taken from swap-cache. There are 2 cases.
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(a) If the SwapCache is newly allocated and read, it has no charges.
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(b) If the SwapCache has been mapped by processes, it has been
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charged already.
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4.2 Swap-out.
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At swap-out, typical state transition is below.
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(a) add to swap cache. (marked as SwapCache)
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swp_entry's refcnt += 1.
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(b) fully unmapped.
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swp_entry's refcnt += # of ptes.
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(c) write back to swap.
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(d) delete from swap cache. (remove from SwapCache)
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swp_entry's refcnt -= 1.
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Finally, at task exit,
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(e) zap_pte() is called and swp_entry's refcnt -=1 -> 0.
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5. Page Cache
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=============
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Page Cache is charged at
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- filemap_add_folio().
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The logic is very clear. (About migration, see below)
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Note:
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__remove_from_page_cache() is called by remove_from_page_cache()
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and __remove_mapping().
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6. Shmem(tmpfs) Page Cache
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===========================
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The best way to understand shmem's page state transition is to read
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mm/shmem.c.
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But brief explanation of the behavior of memcg around shmem will be
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helpful to understand the logic.
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Shmem's page (just leaf page, not direct/indirect block) can be on
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- radix-tree of shmem's inode.
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- SwapCache.
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- Both on radix-tree and SwapCache. This happens at swap-in
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and swap-out,
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It's charged when...
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- A new page is added to shmem's radix-tree.
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- A swp page is read. (move a charge from swap_cgroup to page_cgroup)
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7. Page Migration
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=================
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mem_cgroup_migrate()
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8. LRU
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======
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Each memcg has its own vector of LRUs (inactive anon, active anon,
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inactive file, active file, unevictable) of pages from each node,
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each LRU handled under a single lru_lock for that memcg and node.
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9. Typical Tests.
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=================
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Tests for racy cases.
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9.1 Small limit to memcg.
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-------------------------
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When you do test to do racy case, it's good test to set memcg's limit
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to be very small rather than GB. Many races found in the test under
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xKB or xxMB limits.
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(Memory behavior under GB and Memory behavior under MB shows very
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different situation.)
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9.2 Shmem
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---------
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Historically, memcg's shmem handling was poor and we saw some amount
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of troubles here. This is because shmem is page-cache but can be
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SwapCache. Test with shmem/tmpfs is always good test.
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9.3 Migration
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-------------
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For NUMA, migration is an another special case. To do easy test, cpuset
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is useful. Following is a sample script to do migration::
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mount -t cgroup -o cpuset none /opt/cpuset
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mkdir /opt/cpuset/01
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echo 1 > /opt/cpuset/01/cpuset.cpus
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echo 0 > /opt/cpuset/01/cpuset.mems
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echo 1 > /opt/cpuset/01/cpuset.memory_migrate
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mkdir /opt/cpuset/02
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echo 1 > /opt/cpuset/02/cpuset.cpus
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echo 1 > /opt/cpuset/02/cpuset.mems
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echo 1 > /opt/cpuset/02/cpuset.memory_migrate
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In above set, when you moves a task from 01 to 02, page migration to
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node 0 to node 1 will occur. Following is a script to migrate all
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under cpuset.::
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--
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move_task()
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{
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for pid in $1
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do
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/bin/echo $pid >$2/tasks 2>/dev/null
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echo -n $pid
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echo -n " "
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done
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echo END
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}
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G1_TASK=`cat ${G1}/tasks`
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G2_TASK=`cat ${G2}/tasks`
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move_task "${G1_TASK}" ${G2} &
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--
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9.4 Memory hotplug
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------------------
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memory hotplug test is one of good test.
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to offline memory, do following::
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# echo offline > /sys/devices/system/memory/memoryXXX/state
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(XXX is the place of memory)
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This is an easy way to test page migration, too.
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9.5 nested cgroups
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------------------
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Use tests like the following for testing nested cgroups::
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mkdir /opt/cgroup/01/child_a
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mkdir /opt/cgroup/01/child_b
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set limit to 01.
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add limit to 01/child_b
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run jobs under child_a and child_b
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create/delete following groups at random while jobs are running::
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/opt/cgroup/01/child_a/child_aa
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/opt/cgroup/01/child_b/child_bb
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/opt/cgroup/01/child_c
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running new jobs in new group is also good.
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9.6 Mount with other subsystems
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-------------------------------
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Mounting with other subsystems is a good test because there is a
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race and lock dependency with other cgroup subsystems.
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example::
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# mount -t cgroup none /cgroup -o cpuset,memory,cpu,devices
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and do task move, mkdir, rmdir etc...under this.
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9.7 swapoff
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-----------
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Besides management of swap is one of complicated parts of memcg,
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call path of swap-in at swapoff is not same as usual swap-in path..
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It's worth to be tested explicitly.
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For example, test like following is good:
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(Shell-A)::
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# mount -t cgroup none /cgroup -o memory
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# mkdir /cgroup/test
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# echo 40M > /cgroup/test/memory.limit_in_bytes
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# echo 0 > /cgroup/test/tasks
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Run malloc(100M) program under this. You'll see 60M of swaps.
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(Shell-B)::
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# move all tasks in /cgroup/test to /cgroup
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# /sbin/swapoff -a
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# rmdir /cgroup/test
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# kill malloc task.
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Of course, tmpfs v.s. swapoff test should be tested, too.
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9.8 OOM-Killer
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--------------
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Out-of-memory caused by memcg's limit will kill tasks under
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the memcg. When hierarchy is used, a task under hierarchy
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will be killed by the kernel.
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In this case, panic_on_oom shouldn't be invoked and tasks
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in other groups shouldn't be killed.
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It's not difficult to cause OOM under memcg as following.
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Case A) when you can swapoff::
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#swapoff -a
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#echo 50M > /memory.limit_in_bytes
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run 51M of malloc
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Case B) when you use mem+swap limitation::
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#echo 50M > memory.limit_in_bytes
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#echo 50M > memory.memsw.limit_in_bytes
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run 51M of malloc
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9.9 Move charges at task migration
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----------------------------------
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Charges associated with a task can be moved along with task migration.
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(Shell-A)::
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#mkdir /cgroup/A
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#echo $$ >/cgroup/A/tasks
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run some programs which uses some amount of memory in /cgroup/A.
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(Shell-B)::
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#mkdir /cgroup/B
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#echo 1 >/cgroup/B/memory.move_charge_at_immigrate
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#echo "pid of the program running in group A" >/cgroup/B/tasks
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You can see charges have been moved by reading ``*.usage_in_bytes`` or
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memory.stat of both A and B.
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See 8.2 of Documentation/admin-guide/cgroup-v1/memory.rst to see what value should
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be written to move_charge_at_immigrate.
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9.10 Memory thresholds
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----------------------
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Memory controller implements memory thresholds using cgroups notification
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API. You can use tools/cgroup/cgroup_event_listener.c to test it.
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(Shell-A) Create cgroup and run event listener::
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# mkdir /cgroup/A
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# ./cgroup_event_listener /cgroup/A/memory.usage_in_bytes 5M
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(Shell-B) Add task to cgroup and try to allocate and free memory::
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# echo $$ >/cgroup/A/tasks
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# a="$(dd if=/dev/zero bs=1M count=10)"
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# a=
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You will see message from cgroup_event_listener every time you cross
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the thresholds.
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Use /cgroup/A/memory.memsw.usage_in_bytes to test memsw thresholds.
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It's good idea to test root cgroup as well.
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