Re: [lkp-robot] [torture] b151f93a71: INFO:rcu_preempt_detected_stalls_on_CPUs/tasks

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Nov 29, 2017 at 02:07:03PM -0800, Paul E. McKenney wrote:
> On Wed, Nov 29, 2017 at 11:08:19AM -0800, Paul E. McKenney wrote:
> > On Tue, Nov 28, 2017 at 01:08:10PM -0800, Paul E. McKenney wrote:
> > > On Tue, Nov 28, 2017 at 12:46:19PM -0800, Paul E. McKenney wrote:
> > > > On Tue, Nov 28, 2017 at 09:35:54AM -0800, Paul E. McKenney wrote:
> > > > > On Tue, Nov 28, 2017 at 06:10:08PM +0100, Thomas Gleixner wrote:
> > > > > > On Tue, 28 Nov 2017, Paul E. McKenney wrote:
> > > > > > > On Tue, Nov 28, 2017 at 05:47:35PM +0100, Thomas Gleixner wrote:
> > > > > > > diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> > > > > > > index db774b0f217e..a3321bb565db 100644
> > > > > > > --- a/kernel/time/timer.c
> > > > > > > +++ b/kernel/time/timer.c
> > > > > > > @@ -1803,7 +1803,7 @@ signed long __sched schedule_timeout(signed long timeout)
> > > > > > >  		idx = timer_get_idx(&timer.timer);
> > > > > > >  		idx_now = calc_wheel_index(j, base->clk);
> > > > > > >  		raw_spin_unlock_irqrestore(&base->lock, flags);
> > > > > > > -		pr_info("%s: Waylayed timer idx: %u idx_now: %u\n", __func__, idx, idx_now);
> > > > > > > +		pr_info("%s: Waylayed timer base->clk: %#lx jiffies: %#lx base->next_expiry: %#lx timer->flags: %#x timer->expires %#lx idx: %u idx_now: %u\n", __func__, base->clk, j, base->next_expiry, timer.timer.flags, timer.timer.expires, idx, idx_now);
> > > > > > 
> > > > > > Please print idx and idx_now as hex values. It's simpler to decode that way.
> > > > > 
> > > > > Here you go!  Starting tests at this end, focusing on TREE01 and TREE04.
> > > > > BTW, TREE04 doesn't do any CPU hotplug, providing a counterexample to
> > > > > my long-held assumption that this only happened in the presence of CPU
> > > > > hotplug operations.
> > > > 
> > > > And here is output with changes discussed on IRC.  TREE04 managed to
> > > > have not one but two overlapping RCU CPU stall warnings, one for RCU-bh
> > > > and the second for RCU-sched.  TREE04 and TREE04.  HZ=1000.
> > > 
> > > And here is the full patch, in all its lack of aesthetic appeal.
> > 
> > And here is the list of waylaid timers from last night's testing.  The big
> > pile of them from TREE01 at the end is due to wakeups from kthread_stop(),
> > I am guessing.  The TREE04 run only had two of them, but they seem reliable
> > enough that I just might be able to bisect.  I will try that.
> 
> And it converged to 5c4991e24c69 ("sched/isolation: Split out new
> CONFIG_CPU_ISOLATION=y config from CONFIG_NO_HZ_FULL"), which is a bit
> hard to believe.  Please see below for the log.  I will be retesting
> some of the allegedly good commits, just in case.

And the bisection really did converge here.  It appears that splitting
CONFIG_CPU_ISOLATION from CONFIG_NO_HZ_FULL isn't fully cooked.
Adding CONFIG_CPU_ISOLATION=y to my tests causes them to pass, but I
get the eternal wait for a three-jiffy timeout given CONFIG_NO_HZ_FULL=y
and CONFIG_CPU_ISOLATION=n.  Both cases use CONFIG_NO_HZ_FULL_ALL=y.

Adding Frederic for his perspective.

							Thanx, Paul