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

Paul E. McKenney
2011-07-12T19:00:28Z

On Tue, Jul 12, 2011 at 02:01:51PM -0400, Konrad Rzeszutek Wilk wrote:
> > > 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.
>
> .. With the latest patch the time looks to be advancing.

Sounds like an improvement. ;-)

> > Disabling CONFIG_NO_HZ would be an interesting test case.
>
> Hadn't done that yet. Compiling a kernel with "# CONFIG_NO_HZ is not set"
> right now.
> >
> > > > 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!
>
> Will shortly do this.
> >
> > > > 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?
>
> With the previous patch and the .. this is getting confusing. With this patch:
> http://darnok.org/xen/loop_cnt-extra.patch

That is indeed the patch I intended.

> I get this output: http://darnok.org/xen/log.loop_cnt-extra-patch (one guest
> with 4 VCPUS) and http://darnok.org/xen/loop_cnt-extra-patch.log (the guest with 16 VCPUs)

OK, so the infinite loop in task_waking_fair() happens even if RCU callbacks
are deferred until after the scheduler is fully initialized. Sounds like
one for the scheduler guys. ;-)

Thanx, Paul