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Date: Wed, 20 Jul 2016 15:39:03 +0200
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: oom-reaper choosing wrong processes.
Message-ID: <20160720133903.GO11249@dhcp22.suse.cz>
References: <20160718231850.GA23178@codemonkey.org.uk>
 <20160719090857.GB9490@dhcp22.suse.cz>
 <20160719153335.GA11863@codemonkey.org.uk>
 <20160720070923.GC11249@dhcp22.suse.cz>
 <20160720132304.GA11434@codemonkey.org.uk>
 <20160720133337.GA12457@codemonkey.org.uk>
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To: Dave Jones <davej@codemonkey.org.uk>
Cc: linux-mm@kvack.org

On Wed 20-07-16 09:33:37, Dave Jones wrote:
> On Wed, Jul 20, 2016 at 09:23:04AM -0400, Dave Jones wrote:
>  
>  >  > so this task has been already oom reaped and so oom_badness will ignore
>  >  > it (it simply doesn't make any sense to select this task because it
>  >  > has been already killed or exiting and oom reaped as well). Others might
>  >  > be in a similar position or they might have passed exit_mm->tsk->mm = NULL
>  >  > so they are ignored by the oom killer as well.
>  > 
>  > I feel like I'm still missing something.  Why isn't "wait for the already reaped trinity tasks to exit"
>  > the right thing to do here (as my diff forced it to do), instead of "pick even more victims even
>  > though we've already got some reaped processes that haven't exited"
>  > 
>  > Not killing systemd-journald allowed the machine to keep running just fine.
>  > If I hadn't have patched that out, it would have been killed unnecessarily.
> 
> nm, I figured it out. As Tetsuo pointed out, I was leaking a task struct,
> so those already reaped trinity processes would never truly 'exit'.

Leaked task_struct would leak some memory but they shouldn't have any
effect on the task visibility to the oom killer. Tasks are basically
visible until they are unhashed from the task_list. But the leak could
indeed have some other side effects - like pinning a lot of memory and
so the OOM kill wouldn't be sufficient to make a forward progress.

-- 
Michal Hocko
SUSE Labs

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