linux-kernel - Re: [PATCH v2] mm,oom: exclude oom_task_origin processes if they are OOM-unkillable.

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Date:	Wed, 24 Feb 2016 11:05:20 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	akpm@...ux-foundation.org, mgorman@...e.de, oleg@...hat.com,
	torvalds@...ux-foundation.org, hughd@...gle.com, andrea@...nel.org,
	riel@...hat.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm,oom: exclude oom_task_origin processes if they are
 OOM-unkillable.

On Tue 23-02-16 14:33:01, David Rientjes wrote:
> On Tue, 23 Feb 2016, Michal Hocko wrote:
> 
> > > oom_badness() ranges from 0 (don't kill) to 1000 (please kill).  It 
> > > factors in the setting of /proc/self/oom_score_adj to change that value.  
> > > That is where OOM_SCORE_ADJ_MIN is enforced. 
> > 
> > The question is whether the current placement of OOM_SCORE_ADJ_MIN
> > is appropriate. Wouldn't it make more sense to check it in oom_unkillable_task
> > instead?
> 
> oom_unkillable_task() deals with the type of task it is (init or kthread) 
> or being ineligible due to the memcg and cpuset placement.

Yes and OOM disabled is yet another condition.

> We want to 
> exclude them from consideration and also suppress them from the task dump 
> in the kernel log.  We don't want to suppress oom disabled processes, we 
> really want to know their rss, for example.

Hmm, is it really helpful though? What would you deduce from seeing a
large rss an OOM_SCORE_ADJ_MIN task? Misconfigured system? There must
have been a reason to mark the task that way in the first place so you
can hardly do anything about it. Moreover you can deduce the same from
the available information.

I would even argue that displaying OOM_SCORE_ADJ_MIN might be a bit
counterproductive because you have to filter them out when looking at
the listing.

> It could be renamed is_ineligible_task().

That wouldn't really help imho because OOM_SCORE_ADJ_MIN is an
uneligible task.

> > Sure, checking oom_score_adj under task_lock inside oom_badness will
> > prevent from races but the question I raised previously was whether we
> > actually care about those races? When would it matter? Is it really
> > likely that the update happen during the oom killing? And if yes what
> > prevents from the update happening _after_ the check?
> > 
> 
> It's not necessarily to take task_lock(), but find_lock_task_mm() is the 
> means we have to iterate threads to find any with memory attached.  We 
> need that logic in oom_badness() to avoid racing with threads that have 
> entered exit_mm().  It's possible for a thread to have a non-NULL ->mm in 
> oom_scan_process_thread(), the thread enters exit_mm() without kill, and 
> oom_badness() can still find it to be eligible because other threads have 
> not exited.  We still want to issue a kill to this process and task_lock() 
> protects the setting of task->mm to NULL: don't consider it to be a race 
> in setting oom_score_adj, consider it to be a race in unmapping (but not 
> freeing) memory in th exit path.

I am confused now. This all is true but it is independent on OOM_SCORE_ADJ_MIN
check? The check is per signal_struct so checking all the threads will
not change anything.

-- 
Michal Hocko
SUSE Labs