Re: PROBLEM: 3.0-rc kernels unbootable since -rc3

[Konrad Rzeszutek Wilk]

> > http://darnok.org/xen/cpu1.log
> 
> OK, a fair amount of variety, then lots and lots of task_waking_fair(),
> so I still feel good about asking you for the following.
.. snup..
> Hmmm...  Given that this is persisting for many many seconds, it might
> be better to check for at least 10,000,000 passes.  In contrast, 1000
> passes might elapse just waiting for a cache miss to complete.

Changed it to that large number. This is the diff I used:

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 433491c..e185c04 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1392,14 +1392,19 @@ static void task_waking_fair(struct task_struct *p)
 	struct sched_entity *se = &p->se;
 	struct cfs_rq *cfs_rq = cfs_rq_of(se);
 	u64 min_vruntime;
+	u64 loop_cnt = 0UL;
 
 #ifndef CONFIG_64BIT
 	u64 min_vruntime_copy;
-
+	loop_cnt = 0UL;
 	do {
 		min_vruntime_copy = cfs_rq->min_vruntime_copy;
 		smp_rmb();
 		min_vruntime = cfs_rq->min_vruntime;
+		if (loop_cnt++ > 10000000) {
+			printk(KERN_INFO "POKE!\n");
+			loop_cnt = 0UL;
+		}
 	} while (min_vruntime != min_vruntime_copy);
 #else
 	min_vruntime = cfs_rq->min_vruntime;

And the log is:
http://darnok.org/xen/loop_cnt.log

which seems to imply that we are indeed stuck in that loop
forever.

> 
> Other possible causes include:

What is really strange is that I can only reproduce this on 32-bit builds.
> 
> o	A mismatch between Xen's and RCU's ideas of how CONFIG_NO_HZ
> 	works.  If Xen thinks that the CPU is in CONFIG_NO_HZ's
> 	dyntick-idle mode, but RCU thinks otherwise, the grace period
> 	might stall.

One sure way to figure this out is to disable CONFIG_NO_HZ right?
Or will that take away task_waking_fair case as well?
> 
> o	Problems due to portions of the code attempting to use
> 	RCU read-side critical sections while in dyntick-idle mode.
> 	Frederic Weisbecker has located some of these, (though not yet
> 	in Xen) and he has some diagnositics which may be found at:
> 
> 	git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git
> 
> 	on branch eqscheck.2011.07.08a.
> 
> 	You need to enable CONFIG_PROVE_RCU for these diagnostics to
> 	be executed.

Ok, let me try those too.
> 
> o	As always, there might be bugs in RCU.  ;-)
> 
> But the loop in task_waking_fair() looks like the most prominent smoking
> gun at the moment.
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