[Xen-devel] Re: PROBLEM: 3.0-rc kernels unbootable since -rc3

["Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>]

On Tue, Jul 12, 2011 at 12:03:24PM -0400, Konrad Rzeszutek Wilk wrote:
> > > 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.

It does indeed, thank you!  Also it looks like interrupts are
disabled, and that timekeeping is similarly out of action.

> > Other possible causes include:
> 
> What is really strange is that I can only reproduce this on 32-bit builds.

Not strange at all.  If you have a 64-bit build, the function doesn't
have a loop.  ;-)

> > 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?

Disabling CONFIG_NO_HZ would be an interesting test case.

> > 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.

Thank you!

> > 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.

And could you also please try out the patch that I posted earlier?

                                                        Thaxn, Paul

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