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

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

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 ?

----

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

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