Re: [patch 0/7] improve memcg oom killer robustness v2

[Michal Hocko <mhocko@suse.cz>]

On Thu 05-09-13 13:47:02, azurIt wrote:
> >On Thu 05-09-13 12:17:00, azurIt wrote:
> >> >[...]
> >> >> My script detected another freezed cgroup today, sending stacks. Is
> >> >> there anything interesting?
> >> >
> >> >3 tasks are sleeping and waiting for somebody to take an action to
> >> >resolve memcg OOM. The memcg oom killer is enabled for that group?  If
> >> >yes, which task has been selected to be killed? You can find that in oom
> >> >report in dmesg.
> >> >
> >> >I can see a way how this might happen. If the killed task happened to
> >> >allocate a memory while it is exiting then it would get to the oom
> >> >condition again without freeing any memory so nobody waiting on the
> >> >memcg_oom_waitq gets woken. We have a report like that: 
> >> >https://lkml.org/lkml/2013/7/31/94
> >> >
> >> >The issue got silent in the meantime so it is time to wake it up.
> >> >It would be definitely good to see what happened in your case though.
> >> >If any of the bellow tasks was the oom victim then it is very probable
> >> >this is the same issue.
> >> 
> >> Here it is:
> >> http://watchdog.sk/lkml/kern5.log
> >
> >$ grep "Killed process \<103[168]\>" kern5.log
> >$
> >
> >So none of the sleeping tasks has been killed previously.
> >
> >> Processes were killed by my script
> >
> >OK, I am really confused now. The log contains a lot of in-kernel memcg
> >oom killer messages:
> >$ grep "Memory cgroup out of memory:" kern5.log | wc -l
> >809
> >
> >This suggests that the oom killer is not disabled. What exactly has you
> >script done?
> >
> >> at about 11:05:35.
> >
> >There is an oom killer striking at 11:05:35:
> >Sep  5 11:05:35 server02 kernel: [1751856.433101] Task in /1066/uid killed as a result of limit of /1066
> >[...]
> >Sep  5 11:05:35 server02 kernel: [1751856.539356] [ pid ]   uid  tgid total_vm      rss cpu oom_adj oom_score_adj name
> >Sep  5 11:05:35 server02 kernel: [1751856.539745] [ 1046]  1066  1046   228537    95491   3       0             0 apache2
> >Sep  5 11:05:35 server02 kernel: [1751856.539894] [ 1047]  1066  1047   228604    95488   6       0             0 apache2
> >Sep  5 11:05:35 server02 kernel: [1751856.540043] [ 1050]  1066  1050   228470    95452   5       0             0 apache2
> >Sep  5 11:05:35 server02 kernel: [1751856.540191] [ 1051]  1066  1051   228592    95521   6       0             0 apache2
> >Sep  5 11:05:35 server02 kernel: [1751856.540340] [ 1052]  1066  1052   228594    95546   5       0             0 apache2
> >Sep  5 11:05:35 server02 kernel: [1751856.540489] [ 1054]  1066  1054   228470    95453   5       0             0 apache2
> >Sep  5 11:05:35 server02 kernel: [1751856.540646] Memory cgroup out of memory: Kill process 1046 (apache2) score 1000 or sacrifice child
> >
> >And this doesn't list any of the tasks sleeping and waiting for oom
> >resolving so they must have been created after this OOM. Is this the
> >same group?
> 
> cgroup was 1066. My script is doing this:
> 1.) It checks memory usage of all cgroups and is searching for those whos memory usage is >= 99% of their limit.
> 2.) If any are found, they are saved in an array of 'candidates for killing'.
> 3.) It sleep for 30 seconds.
> 4.) Do (1) and if any of found cgorups were also found in (2), it kills all processes inside it.
> 5.) Clear array of saved cgroups and continue.

This is racy and doesn't really tell you anything about any group being
frozen.

[...]
> But, of course, i cannot guarantee that the killed cgroup was really
> freezed (because of bug in linux kernel), there could be some false
> positives - for example, cgroup has 99% usage of memory, my script
> detected it, OOM successfully resolved the problem and, after 30
> seconds, the same cgroup has again 99% usage of it's memory and my
> script detected it again.

Exactly

> This is why i'm sending stacks here, i simply cannot tell if
> there was or wasn't a problem.

On the other hand if those processes would be stuck waiting for somebody
to resolve the OOM for a long time without any change then yes we have a
problem.

Just to be sure I got you right. You have killed all the processes from
the group you have sent stacks for, right? If that is the case I am
really curious about processes sitting in sleep_on_page_killable because
those are killable by definition.

> I can disable the script and wait until the problem really occurs but
> when it happens, our services will go down.

I definitely do not want to encourage you to let your services down...

> Hope i was clear enough - if not, i can post the source code of that
> script.

-- 
Michal Hocko
SUSE Labs