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SubjectRe: [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

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