linux-kernel - Re: [PATCH for 3.2.34] memcg: do not trigger OOM if PF_NO_MEMCG

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Message-Id: <20130210174619.24F20488@pobox.sk>
Date:	Sun, 10 Feb 2013 17:46:19 +0100
From:	"azurIt" <azurit@...ox.sk>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	cgroups mailinglist <cgroups@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH for 3.2.34] memcg: do not trigger OOM if PF_NO_MEMCG_OOM is set

>stuck in the ptrace code.

But this happens _after_ the cgroup was freezed and i tried to strace one of it's processes (to see what's happening):

Feb  8 01:29:46 server01 kernel: [ 1187.540672] grsec: From 178.40.250.111: process /usr/lib/apache2/mpm-itk/apache2(apache2:18211) attached to via ptrace by /usr/bin/strace[strace:18258] uid/euid:0/0 gid/egid:0/0, parent /usr/bin/htop[htop:2901] uid/euid:0/0 gid/egid:0/0

>> Why are all PIDs waiting on 'mem_cgroup_handle_oom' and there is no
>> OOM message in the log?
>
>I am not sure what you mean here but there are
>$ grep "Memory cgroup out of memory:" kern2.collected.log | wc -l
>16
>
>OOM killer events during the time you were gathering memcg-bug-4 data.
>
>>  Data in memcg-bug-4.tar.gz are only for 2
>> minutes but i let it run for about 15-20 minutes, no single process
>> killed by OOM.
>
>I can see
>$ grep "Memory cgroup out of memory:" kern2.after.log | wc -l
>57
>
>killed after 02:38:47 when you stopped gathering data for memcg-bug-4

I meant no single process was killed inside cgroup 1258 (data from this cgroup are in memcg-bug-4.tar.gz).

Just get data from memcg-bug-4.tar.gz which were taken from cgroup 1258. Almost all processes are in 'mem_cgroup_handle_oom' so cgroup is under OOM. I assume that this is suppose to take only few seconds while kernel finds any process and kill it (and maybe do it again until enough of memory is freed). I was gathering the data for about 2 and a half minutes and NO SINGLE process was killed (just compate list of PIDs from the first and the last directory inside memcg-bug-4.tar.gz). Even more, no single process was killed in cgroup 1258 also after i stopped gathering the data. You can also take the list od PID from memcg-bug-4.tar.gz and you will find only 18211 and 8102 (which are the two stucked processes).

So my question is: Why no process was killed inside cgroup 1258 while it was under OOM? It was under OOM for at least 2 and a half of minutes while i was gathering the data (then i let it run for additional, cca, 10 minutes and then killed processes by hand but i cannot proof this). Why kernel didn't kill any process for so long and ends the OOM?

Btw, processes in cgroup 1258 (memcg-bug-4.tar.gz) are looping in this two tasks (i pasted only first line of stack):
mem_cgroup_handle_oom+0x241/0x3b0
0xffffffffffffffff

Some of them are in 'poll_schedule_timeout' and then they start to loop as above. Is this correct behavior?

For example, do (first line of stack from process 7710 from all timestamps):
for i in */7710/stack; do head -n1 $i; done
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