281 строка
8.3 KiB
Plaintext
281 строка
8.3 KiB
Plaintext
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.
|
|
|
|
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 -o cpuset,memory,cpu,devices
|
|
|
|
and do task move, mkdir, rmdir etc...under this.
|
|
|
|
9.7 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.
|
|
|
|
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
|
|
|
|
9.9 Move charges at task migration
|
|
Charges associated with a task can be moved along with task migration.
|
|
|
|
(Shell-A)
|
|
#mkdir /cgroup/A
|
|
#echo $$ >/cgroup/A/tasks
|
|
run some programs which uses some amount of memory in /cgroup/A.
|
|
|
|
(Shell-B)
|
|
#mkdir /cgroup/B
|
|
#echo 1 >/cgroup/B/memory.move_charge_at_immigrate
|
|
#echo "pid of the program running in group A" >/cgroup/B/tasks
|
|
|
|
You can see charges have been moved by reading *.usage_in_bytes or
|
|
memory.stat of both A and B.
|
|
See 8.2 of Documentation/cgroup-v1/memory.txt to see what value should be
|
|
written to move_charge_at_immigrate.
|
|
|
|
9.10 Memory thresholds
|
|
Memory controller implements memory thresholds using cgroups notification
|
|
API. You can use tools/cgroup/cgroup_event_listener.c to test it.
|
|
|
|
(Shell-A) Create cgroup and run event listener
|
|
# mkdir /cgroup/A
|
|
# ./cgroup_event_listener /cgroup/A/memory.usage_in_bytes 5M
|
|
|
|
(Shell-B) Add task to cgroup and try to allocate and free memory
|
|
# echo $$ >/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.
|