Re: [PATCH 1/4] oom: move oom_adj to signal_struct

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

On Wed,  5 Aug 2009 15:04:48 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

> > On Wed,  5 Aug 2009 11:51:31 +0900 (JST)
> > KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
> > 
> > > > On Wed,  5 Aug 2009 11:29:34 +0900 (JST)
> > > > KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
> > > > 
> > > > > Hi
> > > > > 
> > > > > > Hi, Kosaki. 
> > > > > > 
> > > > > > I am so late to invole this thread. 
> > > > > > But let me have a question. 
> > > > > > 
> > > > > > What's advantage of placing oom_adj in singal rather than task ?
> > > > > > I mean task->oom_adj and task->signal->oom_adj ?
> > > > > > 
> > > > > > I am sorry if you already discussed it at last threads. 
> > > > > 
> > > > > Not sorry. that's very good question.
> > > > > 
> > > > > I'm trying to explain the detailed intention of commit 2ff05b2b4eac
> > > > > (move oom_adj to mm_struct).
> > > > > 
> > > > > In 2.6.30, OOM logic callflow is here.
> > > > > 
> > > > > __out_of_memory
> > > > > 	select_bad_process		for each task
> > > > > 		badness			calculate badness of one task
> > > > > 	oom_kill_process		search child
> > > > > 		oom_kill_task		kill target task and mm shared tasks with it
> > > > > 
> > > > > example, process-A have two thread, thread-A and thread-B and it 
> > > > > have very fat memory.
> > > > > And, each thread have following likes oom property.
> > > > > 
> > > > > 	thread-A: oom_adj = OOM_DISABLE, oom_score = 0
> > > > > 	thread-B: oom_adj = 0,           oom_score = very-high
> > > > > 
> > > > > Then, select_bad_process() select thread-B, but oom_kill_task refuse
> > > > > kill the task because thread-A have OOM_DISABLE.
> > > > > __out_of_memory() call select_bad_process() again. but select_bad_process()
> > > > > select the same task. It mean kernel fall in the livelock.
> > > > > 
> > > > > The fact is, select_bad_process() must select killable task. otherwise
> > > > > OOM logic go into livelock.
> > > > > 
> > > > > Is this enough explanation? thanks.
> > > > > 
> > 
> > The problem resulted from David patch.
> > It can solve live lock problem but make a new problem like vfork problem. 
> > I think both can be solved by different approach. 
> > 
> > It's just RFC. 
> > 
> > If some process is selected by OOM killer but it have a child of OOM immune,
> > We just decrease point of process. It can affect selection of bad process. 
> > After some trial, at last bad score is drastically low and another process is 
> > selected by OOM killer. So I think Live lock don't happen. 
> > 
> > New variable adding in task struct is rather high cost. 
> > But i think we can union it with oomkilladj 
> > since oomkilladj is used to present just -17 ~ 15. 
> > 
> > What do you think about this approach ?
> 
> I can ack this. but please re-initialize oom_scale_down at fork and
> exec time.
> currently oom_scale_down makes too big affect.


Thanks for carefult review. 
In fact, I didn't care of it 
since it just is RFC for making sure my idea. :)

> and, May I ask which you hate my approach? 
> 

Not at all. I never hate your approach. 
This problem resulted form David's original patch.
I thought if we will fix live lock with different approach, we can remove much pain.

-- 
Kind regards,
Minchan Kim

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