Re: rcu_preempt self-detected stall on CPU from 4.5-rc3, since 3.17

[Paul E. McKenney]

On Tue, Feb 23, 2016 at 08:34:11PM +0000, Mathieu Desnoyers wrote:
> ----- On Feb 21, 2016, at 1:15 PM, Ross Green rgkernel@xxxxxxxxx wrote:
> 
> > On Sun, Feb 21, 2016 at 4:04 PM, Ross Green <rgkernel@xxxxxxxxx> wrote:
> >> On Sat, Feb 20, 2016 at 5:32 PM, Paul E. McKenney
> >> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >>> On Sat, Feb 20, 2016 at 03:34:30PM +1100, Ross Green wrote:
> >>>> On Sat, Feb 20, 2016 at 4:33 AM, Paul E. McKenney
> >>>> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >>>> > On Thu, Feb 18, 2016 at 08:13:18PM -0800, John Stultz wrote:
> >>>> >> On Thu, Feb 18, 2016 at 7:56 PM, Ross Green <rgkernel@xxxxxxxxx> wrote:
> >>>> >> > Well a bonus extra!
> >>>> >> > Kept everything running and there was another stall.
> >>>> >> > So i have included the demsg output for perusal.
> >>>> >> >
> >>>> >> > Just to clear things up there is no hotplug involved in this system.
> >>>> >> > It is a standard Pandaboard ES Ti4460 two processor system.
> >>>> >> > I use this for testing as a generic armv7 processor, plus can keep it
> >>>> >> > just running along for testing for a long time. the system has a total
> >>>> >> > of 23-25 process running on average. Mainly standard daemons. There is
> >>>> >> > certainly no heavy processing going on. I run a series of benchmarks
> >>>> >> > that are cpu intensive for the first 20 miinutes after boot and then
> >>>> >> > just leave it idle away. checking every so often to see how it has
> >>>> >> > gone.
> >>>> >> > As mentioned I have observed these stalls going back to 3.17 kernel.
> >>>> >> > It will often take up to a week to record such a stall. I will
> >>>> >> > typically test every new release kernel, so the -rc? series will get
> >>>> >> > around a weeks testing.
> >>>> >>
> >>>> >> Sorry. Kind of hopping in a bit late here. Is this always happening
> >>>> >> with just the pandaboard? Or are you seeing this on different
> >>>> >> machines?
> >>>> >>
> >>>> >> Have you tried enabling CONFIG_DEBUG_TIMEKEEPING just in case
> >>>> >> something is going awry there?
> >>>> >
> >>>> > Excellent point -- timekeeping issues have caused this sort of issue
> >>>> > in the past.
> >>>> >
> >>>> > Ross, on your next test, could you please enable CONFIG_DEBUG_TIMEKEEPING
> >>>> > as John suggests?
> >>>> >
> >>>> >                                                         Thanx, Paul
> >>>> >
> >>>> As John has suggested have already enabled CONFIG_DEBUG_TIMEKEEPING.
> >>>>
> >>>> So far just on 1 day running.
> >>>>
> >>>> Sigh...!! Nothing to report as yet, only one day on the clock.
> >>>> Its like watching grass grow!
> >>>
> >>> I hear you!  Though I was thinking in terms of watching paint dry...
> >>>
> >>>                                                         Thanx, Paul
> >>>
> >> Yes,
> >>
> >> but with paint drying there is an end point!
> >> Grass just keeps on growing ...
> >>
> >> More like the children in the back of the car ...
> >> Are we there yet? ...
> >>
> >> Well still nothing .. to report. I have just built a 4.5-rc5, but will
> >> wait till I get some outcome from the previous test. That can't be too
> >> much longer!
> >>
> >> In hope,
> >>
> >> Ross Green
> > Patience little ones ...
> > 
> > Well after 2 days plus running pulled another stall.
> > This is with 4.5-rc4 and CONFIG_DEBUG_TIMEKEEPING set.
> > 
> > Can't see anything related to the TIMEKEEPING.
> > 
> > Anyway here is the dmesg output for people to look at.
> > 
> > Paul, I was going to move onto 4.5-rc5 kernel, is there something else
> > that you want me to test with that, Anyone else have any suggestions
> > or ideas?
> 
> Starting from kernel 3.17, if we request a e.g. 1000 jiffies schedule_timeout
> on a HZ=1000 kernel, is there an upper bound to the number of jiffies it
> can actually take before the timeout happens, especially on idle systems ?
> I remember a talk from Thomas Gleixner on the new timer wheel which could
> add some imprecision to those timeouts. Not sure in which kernel version it
> got in though.
> 
> My thinking is that it might be a good idea to try using hrtimers rather than
> a jiffies-based timeout to awaken the RCU thread if it's really important to
> run in a bounded amount of jiffies. Or else the jiffies-based sanity check
> that triggers the warning is perhaps too strict.

Some imprecision I could well understand.  21 seconds of imprecision
on a three-millisecond timeout I am having somewhat more difficulty
understanding.  ;-)

But I would rather avoid hrtimers for the RCU grace-period kthread
because a few milliseconds of adjustment for energy-efficiency
reasons is a very good thing.

Still working on getting decent traces...

							Thanx, Paul