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Date:	Mon, 09 Nov 2009 13:50:28 +0100
From:	Mike Galbraith <efault@....de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>, mingo@...e.hu,
	linux-kernel@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: Kernel oops in resched_task() with 2.6.31.5

On Mon, 2009-11-09 at 13:45 +0100, Peter Zijlstra wrote:
> On Mon, 2009-11-09 at 21:31 +0900, Kenji Kaneshige wrote:
> > Hi,
> > 
> > I frequently encounter the kernel oops attached below in resched_task()
> > with 2.6.31.5. This kernel oops happens also with 2.6.32-rc5. I don't
> > know about other kernel.
> > 
> > Here is my analysis:
> > 
> > The immediate cause of this kernel oops is that NULL was passed to
> > resched_task() from resched_cpu(). From my investigation, this was
> > caused as follows:
> > 
> > - trigger_load_balance() caluculated cpu number of idle load balancer
> >   using find_new_ilb(), and find_new_ilb() returned *offline* CPU
> >   number (16 in my case). Note that I didn't do any CPU hotplug
> >   operation. On my system, present, online and offline under
> >   /sys/devices/system/cpu/ are
> > 
> >     [kanesige@...alhost ~]$ cat /sys/devices/system/cpu/present
> >     0-15
> >     [kanesige@...alhost ~]$ cat /sys/devices/system/cpu/online
> >     0-15
> >     [kanesige@...alhost ~]$ cat /sys/devices/system/cpu/offline
> >     16-255
> > 
> >   And nr_cpu_ids is 256.
> > 
> > - resched_cpu() calculated current task by cpu_curr() with offline CPU
> >   number.
> > 
> > So this kernel oops seems to be caused by invalid CPU number returned
> > from find_new_ilb(). I don't know the find_new_ilb() implementation,
> > but I suspect the initialization of cpumasks used by find_new_ilb().
> > The patch attached below seems to fix the problem (With this patch,
> > the kernel oops doesn't happen). But I don't know if this is the
> > correct fix.
> 
> Please send patches against -tip.
> 
> You might find that Rusty has already fixed a similar issue there in
> commit: 49557e620339cb134127b5bfbcfecc06b77d0232.
> 
> Now, Rusty's patch does not clear the ilb mask, so maybe it doesn't
> fully cover your issue, please test.

Doesn't 31 need this too?  (for me it did)

diff --git a/kernel/sched.c b/kernel/sched.c
index 1b59e26..6e71932 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4032,7 +4049,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 	unsigned long flags;
 	struct cpumask *cpus = __get_cpu_var(load_balance_tmpmask);
 
-	cpumask_setall(cpus);
+	cpumask_copy(cpus, cpu_online_mask);
 
 	/*
 	 * When power savings policy is enabled for the parent domain, idle
@@ -4195,7 +4212,7 @@ load_balance_newidle(int this_cpu, struct rq *this_rq, struct sched_domain *sd)
 	int all_pinned = 0;
 	struct cpumask *cpus = __get_cpu_var(load_balance_tmpmask);
 
-	cpumask_setall(cpus);
+	cpumask_copy(cpus, cpu_online_mask);
 
 	/*
 	 * When power savings policy is enabled for the parent domain, idle


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