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

["Luis Claudio R. Goncalves" <lclaudio@uudg.org>]

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.

I don't know what is the system-wide latency effect of a memcg OOM, if any,
or if it would affect an RT task running on another memcg. That is the case
where a more careful priority selection could be necessary.
 
| On the other hand, normal processes don't have a requirement of RT.
| But it isn't a big problem that it lost little time slice, I think. 
| So how about raising max normal priority? 
| but I am not sure this is right solution.
| Let's listen other's opinion. 
| I believe Peter have a good idea.

Thanks again for helping to discuss this idea.

Luis
-- 
[ Luis Claudio R. Goncalves                    Bass - Gospel - RT ]
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