Re: [PATCH] oom, memcg: handle sysctl oom_kill_allocating_task whilememcg oom happening

From: Sha Zhengju
Date: Thu Oct 18 2012 - 09:51:44 EST

On 10/18/2012 07:56 PM, Michal Hocko wrote:
On Wed 17-10-12 01:14:48, Sha Zhengju wrote:
On Tuesday, October 16, 2012, Michal Hocko<mhocko@xxxxxxx> wrote:
Could you be more specific about the motivation for this patch? Is it
"let's be consistent with the global oom" or you have a real use case
for this knob.

In our environment(rhel6), we encounter a memcg oom 'deadlock'
problem. Simply speaking, suppose process A is selected to be killed
by memcg oom killer, but A is uninterruptible sleeping on a page
lock. What's worse, the exact page lock is holding by another memcg
process B which is trapped in mem_croup_oom_lock(proves to be a
Hmm, this is strange. How can you get down that road with the page lock
held? Is it possible this is related to the issue fixed by: 1d65f86d
(mm: preallocate page before lock_page() at filemap COW)?

No, it has nothing with the cow page. By checking stack of the process A
selected to be killed(uninterruptible sleeping), it was stuck at:
__do_fault->filemap_fault->__lock_page_or_retry->wait_on_page_bit--(D state).
The person B holding the exactly page lock is on the following path:
->add_to_page_cache_locked ---- >(in memcg oom and cannot exit)
In mpage_readpages, B tends to read a dozen of pages in: for each of page will do
locking, charging, and then send out a big bio. And A is waiting for one of the pages
and stuck.

As I said, 37b23e05 has made pagefault killable by changing uninterruptible sleeping
to killable sleeping. So A can be woke up to exit successfully and free the memory which
can in turn help B pass memcg charging period.

(By the way, it seems commit 37b23e05 and 7d9fdac need to be backported to --stable tree
to deliver RHEL users. ;-) )

Then A can not exit successfully to free the memory and both of them
can not moving on.
Indeed, we should dig into these locks to find the solution and
in fact the 37b23e05 (x86, mm: make pagefault killable) and
7d9fdac(Memcg: make oom_lock 0 and 1 based other than counter) have
already solved the problem, but if oom_killing_allocating_task is
memcg aware, enabling this suicide oom behavior will be a simpler
workaround. What's more, enabling the sysctl can avoid other potential
oom problems to some extent.
As I said, I am not against this but I really want to see a valid use
case first. So far I haven't seen any because what you mention above is
a clear bug which should be fixed. I can imagine the huge number of
tasks in the group could be a problem as well but I would like to see
what are those problems first.

In view of consistent with global oom and performance benefit, I suggest
we may as well open it in memcg oom as there's no obvious harm.
As refer to the bug I mentioned, obviously the key solution is the above two
patchset, but considing other *potential* memcg oom bugs, the sysctl may
be a role of temporary workaround to some extent... but it's just a workaround.


The primary motivation for oom_kill_allocating_tas AFAIU was to reduce
search over huge tasklists and reduce task_lock holding times. I am not
sure whether the original concern is still valid since 6b0c81b (mm,
oom: reduce dependency on tasklist_lock) as the tasklist_lock usage has
been reduced conciderably in favor of RCU read locks is taken but maybe
even that can be too disruptive?

On the other hand, from the semantic meaning of oom_kill_allocating_task,
it implies to allow suicide-like oom, which has no obvious relationship
with performance problems(such as huge task lists or task_lock holding
I guess that suicide-like oom in fact means "kill the poor soul that
happened to charge the last". I do not see any use case for this from
top of my head (appart from the performance benefits of course).

So make the sysctl be consistent with global oom will be better or set
an individual option for memcg oom just as panic_on_oom does.

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