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

[Paul E. McKenney]

On Sun, Mar 27, 2016 at 08:40:18AM -0700, Paul E. McKenney wrote:
> On Sun, Mar 27, 2016 at 01:48:55PM +0000, Mathieu Desnoyers wrote:
> > ----- On Mar 26, 2016, at 9:34 PM, Paul E. McKenney paulmck@xxxxxxxxxxxxxxxxxx wrote:
> > > On Sat, Mar 26, 2016 at 10:22:57PM +0000, Mathieu Desnoyers wrote:
> > >> ----- On Mar 26, 2016, at 2:49 PM, Paul E. McKenney paulmck@xxxxxxxxxxxxxxxxxx
> > >> wrote:
> > >> > On Sat, Mar 26, 2016 at 08:28:16AM -0700, Paul E. McKenney wrote:
> > >> >> On Sat, Mar 26, 2016 at 12:29:31PM +0000, Mathieu Desnoyers wrote:

[ . . . ]

> > >> >> > Perhaps we could try with those commits reverted ?
> > >> >> > 
> > >> >> > commit e3baac47f0e82c4be632f4f97215bb93bf16b342
> > >> >> > Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > >> >> > Date:   Wed Jun 4 10:31:18 2014 -0700
> > >> >> > 
> > >> >> >     sched/idle: Optimize try-to-wake-up IPI
> > >> >> > 
> > >> >> > commit fd99f91aa007ba255aac44fe6cf21c1db398243a
> > >> >> > Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > >> >> > Date:   Wed Apr 9 15:35:08 2014 +0200
> > >> >> > 
> > >> >> >     sched/idle: Avoid spurious wakeup IPIs
> > >> >> > 
> > >> >> > They appeared in 3.16.
> > >> >> 
> > >> >> At this point, I am up for trying pretty much anything.  ;-)
> > >> >> 
> > >> >> Will give it a go.
> > >> > 
> > >> > And those certainly don't revert cleanly!  Would patching the kernel
> > >> > to remove the definition of TIF_POLLING_NRFLAG be useful?  Or, more
> > >> > to the point, is there some other course of action that would be more
> > >> > useful?  At this point, the test times are measured in weeks...
> > >> 
> > >> Indeed, patching the kernel to remove the TIF_POLLING_NRFLAG
> > >> definition would have an effect similar to reverting those two
> > >> commits.
> > >> 
> > >> Since testing takes a while, we could take a more aggressive
> > >> approach towards reproducing a possible race condition: we
> > >> could re-implement the _TIF_POLLING_NRFLAG vs _TIF_NEED_RESCHED
> > >> dance, along with the ttwu pending lock-list queue, within
> > >> a dummy test module, with custom data structures, and
> > >> stress-test the invariants. We could also create a Promela
> > >> model of these ipi-skip optimisations trying to validate
> > >> progress: whenever a wakeup is requested, there should
> > >> always be a scheduling performed, even if no further wakeup
> > >> is encountered.
> > >> 
> > >> Each of the two approaches proposed above might be a significant
> > >> endeavor, and would only validate my specific hunch. So it might
> > >> be a good idea to just let a test run for a few weeks with
> > >> TIF_POLLING_NRFLAG disabled meanwhile.
> > > 
> > > This makes a lot of sense.  I did some short runs, and nothing broke
> > > too badly.  However, I left some diagnostic stuff in that obscured
> > > the outcome.  I disabled the diagnostic stuff and am running overnight.
> > > I might need to go further and revert some of my diagnostic patches,
> > > but let's see where it is in the morning.
> > 
> > Here is another idea that might help us reproduce this issue faster.
> > If you can afford it, you might want to just throw more similar hardware
> > at the problem. Assuming the problem shows up randomly, but its odds
> > of showing up make it happen only once per week, if we have 100 machines
> > idling in the same way in parallel, we should be able to reproduce it
> > within about 1-2 hours.
> > 
> > Of course, if the problem really need each machine to "degrade" for
> > a week (e.g. memory fragmentation), that would not help. It's only for
> > races that appear to be showing up randomly.
> 
> Certain rcutorture tests sometimes hit it within an hour (TREE03).
> Last night's TREE03 ran six hours without incident, which is unusual
> given that I didn't enable any tracepoints, but does not any significant
> level of statitstical confidence.  The set will finish in a few hours,
> at which point I will start parallel batches of TREE03 to see what
> comes up.
> 
> Feel free to take a look at kernel/rcu/waketorture.c for my (feeble
> thus far) attempt to speed things up.  I am thinking that I need to
> push sleeping tasks onto idle CPUs to make it happen more often.
> My current approach to this is to run with CPU utilizations of about
> 40% and using hrtimer with a prime number of microseconds to avoid
> synchronization.  That should in theory get me a 40% chance of hitting
> an idle CPU with a wakeup, and a reasonable chance of racing with a
> CPU-hotplug operation.  But maybe the wakeup needs to be remote or
> some such, in which case waketorture also needs to move stuff around.
> 
> Oh, and the patch I am running with is below.  I am running x86, and so
> some other architectures would of course need the corresponding patch
> on that architecture.

And it passed a full set of six-hour runs.  Unusual of late, but not
unheard of.  Next step is to focus on TREE03 overnight.

							Thanx, Paul