Re: [RFC] oom-kill: give the dying task a higher priority

[Minchan Kim <minchan.kim@gmail.com>]

On Fri, May 28, 2010 at 12:28:42PM -0300, Luis Claudio R. Goncalves wrote:
> On Sat, May 29, 2010 at 12:12:49AM +0900, Minchan Kim wrote:
> | On Fri, May 28, 2010 at 11:36:17AM -0300, Luis Claudio R. Goncalves wrote:
> | > On Fri, May 28, 2010 at 11:06:23PM +0900, Minchan Kim wrote:
> | > | On Fri, May 28, 2010 at 09:53:05AM -0300, Luis Claudio R. Goncalves wrote:
> | > | > On Fri, May 28, 2010 at 02:59:02PM +0900, KOSAKI Motohiro wrote:
> | > ...
> | > | > | As far as my observation, RT-function always have some syscall. because pure
> | > | > | calculation doesn't need deterministic guarantee. But _if_ you are really
> | > | > | using such priority design. I'm ok maximum NonRT priority instead maximum
> | > | > | RT priority too.
> | > | > 
> | > | > I confess I failed to distinguish memcg OOM and system OOM and used "in
> | > | > case of OOM kill the selected task the faster you can" as the guideline.
> | > | > If the exit code path is short that shouldn't be a problem.
> | > | > 
> | > | > Maybe the right way to go would be giving the dying task the biggest
> | > | > priority inside that memcg to be sure that it will be the next process from
> | > | > that memcg to be scheduled. Would that be reasonable?
> | > | 
> | > | Hmm. I can't understand your point. 
> | > | What do you mean failing distinguish memcg and system OOM?
> | > | 
> | > | We already have been distinguish it by mem_cgroup_out_of_memory.
> | > | (but we have to enable CONFIG_CGROUP_MEM_RES_CTLR). 
> | > | So task selected in select_bad_process is one out of memcg's tasks when 
> | > | memcg have a memory pressure. 
> | > 
> | > The approach of giving the highest priority to the dying task makes sense
> | > in a system wide OOM situation. I though that would also be good for the
> | > memcg OOM case.
> | > 
> | > After Balbir Singh's comment, I understand that in a memcg OOM the dying
> | > task should have a priority just above the priority of the main task of
> | > that memcg, in order to avoid interfering in the rest of the system.
> | > 
> | > That is the point where I failed to distinguish between memcg and system OOM.
> | > 
> | > Should I pursue that new idea of looking for the right priority inside the
> | > memcg or is it overkill? I really don't have a clear view of the impact of
> | > a memcg OOM on system performance - don't know if it is better to solve the
> | > issue sooner (highest RT priority) or leave it to be solved later (highest
> | > prio on the memcg). I have the impression the general case points to the
> | > simpler solution.
> | 
> | I think highest RT proirity ins't good solution.
> | As I mentiond, Some RT functions don't want to be preempted by other processes
> | which cause memory pressure. It makes RT task broken.
> 
> For the RT case, if you reached a system OOM situation, your determinism has
> already been hurt. If the memcg OOM happens on the same memcg your RT task
> is - what will probably be the case most of time - again, the determinism
> has deteriorated. For both these cases, giving the dying task SCHED_FIFO
> MAX_RT_PRIO-1 means a faster recovery.

What I want to say is that determinisic has no relation with OOM. 
Why is some RT task affected by other process's OOM?

Of course, if system has no memory, it is likely to slow down RT task. 
But it's just only thought. If some task scheduled just is exit, we don't need
to raise OOMed task's priority.

But raising min rt priority on your patch was what I want.
It doesn't preempt any RT task.

So until now, I have made noise about your patch.
Really, sorry for that. 
I don't have any objection on raising priority part from now on. 

Thanks, Luis.
-- 
Kind regards,
Minchan Kim

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