Re: [PATCH] mm, oom: make the calculation of oom badness more accurate

[Michal Hocko <mhocko@kernel.org>]

On Wed 08-07-20 23:11:43, Yafang Shao wrote:
> On Wed, Jul 8, 2020 at 10:28 PM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Wed 08-07-20 09:24:09, Yafang Shao wrote:
> > > Recently we found an issue on our production environment that when memcg
> > > oom is triggered the oom killer doesn't chose the process with largest
> > > resident memory but chose the first scanned process. Note that all
> > > processes in this memcg have the same oom_score_adj, so the oom killer
> > > should chose the process with largest resident memory.
> > >
> > > Bellow is part of the oom info, which is enough to analyze this issue.
> > > [7516987.983223] memory: usage 16777216kB, limit 16777216kB, failcnt 52843037
> > > [7516987.983224] memory+swap: usage 16777216kB, limit 9007199254740988kB, failcnt 0
> > > [7516987.983225] kmem: usage 301464kB, limit 9007199254740988kB, failcnt 0
> > > [...]
> > > [7516987.983293] [ pid ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name
> > > [7516987.983510] [ 5740]     0  5740      257        1    32768        0          -998 pause
> > > [7516987.983574] [58804]     0 58804     4594      771    81920        0          -998 entry_point.bas
> > > [7516987.983577] [58908]     0 58908     7089      689    98304        0          -998 cron
> > > [7516987.983580] [58910]     0 58910    16235     5576   163840        0          -998 supervisord
> > > [7516987.983590] [59620]     0 59620    18074     1395   188416        0          -998 sshd
> > > [7516987.983594] [59622]     0 59622    18680     6679   188416        0          -998 python
> > > [7516987.983598] [59624]     0 59624  1859266     5161   548864        0          -998 odin-agent
> > > [7516987.983600] [59625]     0 59625   707223     9248   983040        0          -998 filebeat
> > > [7516987.983604] [59627]     0 59627   416433    64239   774144        0          -998 odin-log-agent
> > > [7516987.983607] [59631]     0 59631   180671    15012   385024        0          -998 python3
> > > [7516987.983612] [61396]     0 61396   791287     3189   352256        0          -998 client
> > > [7516987.983615] [61641]     0 61641  1844642    29089   946176        0          -998 client
> > > [7516987.983765] [ 9236]     0  9236     2642      467    53248        0          -998 php_scanner
> > > [7516987.983911] [42898]     0 42898    15543      838   167936        0          -998 su
> > > [7516987.983915] [42900]  1000 42900     3673      867    77824        0          -998 exec_script_vr2
> > > [7516987.983918] [42925]  1000 42925    36475    19033   335872        0          -998 python
> > > [7516987.983921] [57146]  1000 57146     3673      848    73728        0          -998 exec_script_J2p
> > > [7516987.983925] [57195]  1000 57195   186359    22958   491520        0          -998 python2
> > > [7516987.983928] [58376]  1000 58376   275764    14402   290816        0          -998 rosmaster
> > > [7516987.983931] [58395]  1000 58395   155166     4449   245760        0          -998 rosout
> > > [7516987.983935] [58406]  1000 58406 18285584  3967322 37101568        0          -998 data_sim
> > > [7516987.984221] oom-kill:constraint=CONSTRAINT_MEMCG,nodemask=(null),cpuset=3aa16c9482ae3a6f6b78bda68a55d32c87c99b985e0f11331cddf05af6c4d753,mems_allowed=0-1,oom_memcg=/kubepods/podf1c273d3-9b36-11ea-b3df-246e9693c184,task_memcg=/kubepods/podf1c273d3-9b36-11ea-b3df-246e9693c184/1f246a3eeea8f70bf91141eeaf1805346a666e225f823906485ea0b6c37dfc3d,task=pause,pid=5740,uid=0
> > > [7516987.984254] Memory cgroup out of memory: Killed process 5740 (pause) total-vm:1028kB, anon-rss:4kB, file-rss:0kB, shmem-rss:0kB
> > > [7516988.092344] oom_reaper: reaped process 5740 (pause), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
> > >
> > > We can find that the first scanned process 5740 (pause) was killed, but its
> > > rss is only one page. That is because, when we calculate the oom badness in
> > > oom_badness(), we always ignore the negtive point and convert all of these
> > > negtive points to 1. Now as oom_score_adj of all the processes in this
> > > targeted memcg have the same value -998, the points of these processes are
> > > all negtive value. As a result, the first scanned process will be killed.
> >
> > Such a large bias can skew results quite considerably.
> >
> 
> Right.
> Pls. refer the kubernetes doc[1] for more information about this large bias .
> 
> [1]. https://kubernetes.io/docs/tasks/administer-cluster/out-of-resource/
> 
> > > The oom_socre_adj (-998) in this memcg is set by kubelet, because it is a
> > > a Guaranteed pod, which has higher priority to prevent from being killed by
> > > system oom.
> >
> > This is really interesting! I assume that the oom_score_adj is set to
> > protect from the global oom situation right?
> 
> Right. See also the kubernetes doc.
> 
> > I am struggling to
> > understand what is the expected behavior when the oom is internal for
> > such a group though. Does killing a single task from such a group is a
> > sensible choice? I am not really familiar with kubelet but can it cope
> > with data_sim going away from under it while the rest would still run?
> > Wouldn't it make more sense to simply tear down the whole thing?
> >
> 
> There are two containers in one kubernetes pod, one of which is a
> pause-container, which has only one process - the pause, which is
> managing the netns, and the other is the docker-init-container, in
> which all other processes are running.
> Once the pause process is killed, the kubelet will rebuild all the
> containers in this pod, while if one of the processes in the
> docker-init-container is killed, the kubelet will try to re-run it.
> So tearing down the whole thing is more costly than only trying to
> re-running one process.
> I'm not familiar with kubernetes as well, that is my understanding.

Thanks for the clarification!
 
[...]
> > oom_score has a very coarse units because it maps all the consumed
> > memory into 0 - 1000 scale so effectively per-mille of the usable
> > memory. oom_score_adj acts on top of that as a bias. This is
> > exported to the userspace and I do not think we can change that (see
> > Documentation/filesystems/proc.rst) unfortunately.
> 
> In this doc, I only find the oom_score and oom_score_adj is exposed to
> the userspace.
> While this patch only changes the oom_control->chosen_points, which is
> only for oom internally use.
> So I don't think we can't change oom_control->chosen_points.

Unless I am misreading the patch you are allowing negative values to be
returned from proc_oom_score and that is used by proc_oom_score which is
exported to the userspace.
-- 
Michal Hocko
SUSE Labs