memcg creates an unkillable task in 3.2-rc2

Eric W. Biederman ebiederm at
Mon Jul 29 10:21:59 UTC 2013

Michal Hocko <mhocko at> writes:

> On Mon 29-07-13 01:54:01, Eric W. Biederman wrote:
>> Michal Hocko <mhocko at> writes:
>> > On Sun 28-07-13 17:42:28, Eric W. Biederman wrote:
>> >> Tejun Heo <tj at> writes:
>> >> 
>> >> > Hello, Linus.
>> >> >
>> >> > This pull request contains two patches, both of which aren't fixes
>> >> > per-se but I think it'd be better to fast-track them.
>> >> >
>> >> Darn.  I was hoping to see a fix for the bug I just tripped over,
>> >> that results in a process stuck in short term disk wait.
>> >> 
>> >> Using the memory control group for it's designed function aka killing
>> >> processes that eats too much memory I just would up with an unkillable
>> >> process in 3.11-rc2.
>> >
>> > How many processes are in that group? Could you post stacks for all of
>> > them? Is the stack bellow stable?
>> Just this one, and yes the stack is stable.
>> And there was a pending sigkill.  Which is what is so bizarre.
> Strange indeed. We have a shortcut to skip the charge if the task has
> fatal_signals pending in __mem_cgroup_try_charge and
> mem_cgroup_handle_oom. With a single task in the group it always calls
> mem_cgroup_out_of_memory unless it is locked because of OOM from up the
> hierarchy (but as you are able to echo to oom_control then this means
> that you are under any hierarchy).
>> > Could you post dmesg output?
>> Nothing interesting was in dmesg.
> No OOM messages at all?

Not that I saw.  Perhaps I have something misconfigured, or perhaps I
just missed it.

>> I lost the original hang but I seem to be able to reproduce it fairly
>> easily.
> What are the steps to reproduce?

In  There is a test case
src/tests/ if you can get it to run it triggers all
kinds of cgroups nasties by default.  Right now the shell script that
starts it is broken.  I fixed the shell script and the cgroups started
falling down around my ears.

I just reproduced it again and this time something was able to delete the
memory control group with one process with 3 threads remaining inside.
All unkillable.  Rebooting to clear this kind of mess gets old very

>> echo 0 > memory.oom_control is enough to unstick it.  But that does not
>> explain why the process does not die when SIGKILL is sent.
> Interesting. This would mean that memcg_oom_recover woke up the task
> from the wait queue and so it realizes it should die. This would suggest
> a race when the task misses memcg_oom_recover resp. memcg_wakeup_oom but
> that doesn't match with your single task in the group description or is
> this just a final state and there were more tasks before OOM happened?

There was one process with I think originally with 4 threads (one per
cpu). Some of the tasks are getting killd off some of the time.

>> > You seem to have CONFIG_MEMCG_KMEM enabled. Have you set up kmem
>> > limit?
>> No kmem limits set.
>> >> I am really not certain what is going on although I haven't rebooted the
>> >> machine yet so I can look a bit further if someone has a good idea.
>> >> 
>> >> On the unkillable task I see.
>> >> 
>> >> /proc/<pid>/stack:
>> >> 
>> >> [<ffffffff8110342c>] mem_cgroup_iter+0x1e/0x1d2
>> >> [<ffffffff81105630>] __mem_cgroup_try_charge+0x779/0x8f9
>> >> [<ffffffff81070d46>] ktime_get_ts+0x36/0x74
>> >> [<ffffffff81104d84>] memcg_oom_wake_function+0x0/0x5a
>> >> [<ffffffff8110620c>] __mem_cgroup_try_charge_swapin+0x6c/0xac
>> >
>> > Hmm, mem_cgroup_handle_oom should be setting up the task for wait queue
>> > so the above is a bit confusing.
>> The mem_cgroup_iter looks like it is somethine stale on the stack.
> mem_cgroup_iter could be part of mem_cgroup_{,un}mark_under_oom

mem_cgroup_handle_oom calls mem_cgroup_iter a littler earlier in the
function and I believe that address is stale upon the stack.

>> The __mem_cgroup_try_charge is immediately after the schedule in
>> mem_cgroup_handle_oom.
> I am confused now mem_cgroup_handle_oom doesn't call
> __mem_cgroup_try_charge or have I just misunderstood what you are
> saying?

mem_cgroup_handle_oom is inlined in __mem_cgroup_try_charge.

>> I have played with it a little bit and added
>> 	if (!fatal_signal_pending(current))
>> 		schedule();
>> On the off chance that it was an ordering thing that was triggering
>> this.  And that does not seem to be the problem in this instance.
>> The missing test before the schedule still looks wrong.
> Shouldn't schedule take care of the pending singnals on its own and keep
> the task on the runqueue?

Certainly that is not the assumption the sane wait functions in wait.h
make.  To the best of my knowledge schedule just give something else a
chance to run.  Maybe there is a special case with signals but I have
not run into it.

>> > Anyway your group seems to be under OOM and the task is in the middle of
>> > mem_cgroup_handle_oom which tries to kill something. That something is
>> > probably not willing to die so this task will loop trying to charge the
>> > memory until something releases a charge or the limit for the group is
>> > increased.
>> And it is configured so that the manager process needs to send SIGKILL
>> instead of having the kernel pick a random process.
> Ahh, OK, so you are having memcg OOM disabled and a manager sits on the
> eventfd and sending SIGKILL to a task, right?

Yes.  And things are not dying when the SIGKILL is sent.

>> > It would be interesting to see what other tasks are doing. We are aware
>> > of certain deadlock situations where memcg OOM killer tries to kill a
>> > task which is blocked on a lock (e.g. i_mutex) which is held by a task
>> > which is trying to charge but failing due to oom.
>> The only other weird thing that I see going on is the manager process
>> tries to freeze the entire cgroup, kill the processes, and the unfreeze
>> the cgroup and the freeze is failing.  But looking at /proc/<pid>/status
>> there was a SIGKILL pending.
>> Given how easy it was to wake up the process when I reproduced this
>> I don't think there is anything particularly subtle going on.  But
>> somehow we are going to sleep having SIGKILL delivered and not waking
>> up.  The not waking up bugs me.
> OK, I guess this answers the most of my questions above.
> Isn't this a bug in freezer then? I am not familiar with the freezer
> much but memcg oom handling seems correct to me. The task is sleeping
> KILLABLE and fatal_signal_pending in mem_cgroup_handle_oom will tell us
> to bypass the charge and let the taks go away.

I am really not certain where what this is a bug.  The involvement of
the freezer makes adds another dimension.  I think I will have to
instrument up the code a little and see if I can figure out just what is
going on.

Sometimes I can get the test case to run for quite a while without
problems other times I shake things up a little and I get into a
weird and completely unexpected cgroup state.

However I was able to send SIGTERM after I had all of the annoying
management processes killed and the freezing disabled and SIGTERM showed
up as pending but nothing happened.  Sigh I guess that makes sense as
we are only in a killable sleep.  So wake up will only wake the thing
up if there is a signal that promises to kill the process.  Ugh.  So
maybe just dropping the original SIGKILL is sufficient.

Ugh nasy ick.  And now I had better sleep on it so I have the some grey
matter functioning so I can look into this tomorrow.


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