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

[Minchan Kim]

On Wed, 5 Aug 2009 15:03:23 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:

> On Wed, 5 Aug 2009 14:55:16 +0900
> Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
> 
> > On Wed,  5 Aug 2009 11:51:31 +0900 (JST)
> > KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
> > 
> > > > On Wed,  5 Aug 2009 11:29:34 +0900 (JST)
> > > > KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> 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 ?
> > 
> keeping this in "task" struct is troublesome.
> It may not livelock but near-to-livelock state, in bad case.

Hmm. I can't understand why it is troublesome. 
I think it's related to moving oom_adj to singal_struct. 
Unfortunately, I can't understand why we have to put oom_adj 
in singal_struct?

That's why I have a question to Kosaki a while ago. 
I can't understand it still. :-(

Could you elaborate it ?

> After applying Kosaki's , oom_kill will use
> "for_each_process()" instead of "do_each_thread", I think it's a way to go.

I didn't review kosaki's approach entirely. 
After reviewing, let's discuss it, again. 

> But, yes, your "scale_down" idea itself is interesitng.
> Then, hmm, merging two of yours ?

If it is possible, I will do so. 

Thnaks for good comment, kame.

> Thanks,
> -Kame
> 
> 
> 
> > ----
> > 
> > This is based on 2.6.30 which is kernel before applying David Patch. 
> > 
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index b4c38bc..6e195f7 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1150,6 +1150,11 @@ struct task_struct {
> >          */
> >         unsigned char fpu_counter;
> >         s8 oomkilladj; /* OOM kill score adjustment (bit shift). */
> > +       /*
> > +        * If OOM kill happens at one process repeately, 
> > +        * oom_sacle_down will be increased to prevent OOM live lock 
> > +        */
> > +       unsigned int oom_scale_down;
> >  #ifdef CONFIG_BLK_DEV_IO_TRACE
> >         unsigned int btrace_seq;
> >  #endif
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index a7b2460..3592786 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -159,6 +159,11 @@ unsigned long badness(struct task_struct *p, unsigned long uptime)
> >                         points >>= -(p->oomkilladj);
> >         }
> >  
> > +       /*
> > +        * adjust the score by number of OOM kill retrial
> > +        */
> > +       points >>= p->oom_scale_down;
> > +
> >  #ifdef DEBUG
> >         printk(KERN_DEBUG "OOMkill: task %d (%s) got %lu points\n",
> >         p->pid, p->comm, points);
> > @@ -367,8 +372,10 @@ static int oom_kill_task(struct task_struct *p)
> >          * Don't kill the process if any threads are set to OOM_DISABLE
> >          */
> >         do_each_thread(g, q) {
> > -               if (q->mm == mm && q->oomkilladj == OOM_DISABLE)
> > +               if (q->mm == mm && q->oomkilladj == OOM_DISABLE) {
> > +                       p->oom_scale_down++;
> >                         return 1;
> > +               }
> >         } while_each_thread(g, q);
> >  
> >         __oom_kill_task(p, 1);
> > 
> > 
> > 
> > -- 
> > Kind regards,
> > Minchan Kim
> > 
> 


-- 
Kind regards,
Minchan Kim
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