Re: WARN_ON_ONCE() in process_one_work()?

[Paul E. McKenney]

On Fri, Jun 23, 2017 at 09:41:42AM -0700, Paul E. McKenney wrote:
> On Wed, Jun 21, 2017 at 08:30:35AM -0700, Paul E. McKenney wrote:
> > On Tue, Jun 20, 2017 at 09:45:23AM -0700, Paul E. McKenney wrote:
> > > On Sun, Jun 18, 2017 at 06:40:00AM -0400, Tejun Heo wrote:
> > > > Hello,
> > > > 
> > > > On Sat, Jun 17, 2017 at 10:31:05AM -0700, Paul E. McKenney wrote:
> > > > > On Sat, Jun 17, 2017 at 07:53:14AM -0400, Tejun Heo wrote:
> > > > > > Hello,
> > > > > > 
> > > > > > On Fri, Jun 16, 2017 at 10:36:58AM -0700, Paul E. McKenney wrote:
> > > > > > > And no test failures from yesterday evening.  So it looks like we get
> > > > > > > somewhere on the order of one failure per 138 hours of TREE07 rcutorture
> > > > > > > runtime with your printk() in the mix.
> > > > > > >
> > > > > > > Was the above output from your printk() output of any help?
> > > > > > 
> > > > > > Yeah, if my suspicion is correct, it'd require new kworker creation
> > > > > > racing against CPU offline, which would explain why it's so difficult
> > > > > > to repro.  Can you please see whether the following patch resolves the
> > > > > > issue?
> > > > > 
> > > > > That could explain why only Steve Rostedt and I saw the issue.  As far
> > > > > as I know, we are the only ones who regularly run CPU-hotplug stress
> > > > > tests.  ;-)
> > > > 
> > > > I was a bit confused.  It has to be racing against either new kworker
> > > > being created on the wrong CPU or rescuer trying to migrate to the
> > > > CPU, and it looks like we're mostly seeing the rescuer condition, but,
> > > > yeah, this would only get triggered rarely.  Another contributing
> > > > factor could be the vmstat work putting on a workqueue w/ rescuer
> > > > recently.  It runs quite often, so probably has increased the chance
> > > > of hitting the right condition.
> > > 
> > > Sounds like too much fun!  ;-)
> > > 
> > > But more constructively...  If I understand correctly, it is now possible
> > > to take a CPU partially offline and put it back online again.  This should
> > > allow much more intense testing of this sort of interaction.
> > > 
> > > And no, I haven't yet tried this with RCU because I would probably need
> > > to do some mix of just-RCU online/offline and full-up online-offline.
> > > Plus RCU requires pretty much a full online/offline cycle to fully
> > > exercise it.  :-/
> > > 
> > > > > I have a weekend-long run going, but will give this a shot overnight on
> > > > > Monday, Pacific Time.  Thank you for putting it together, looking forward
> > > > > to seeing what it does!
> > > > 
> > > > Thanks a lot for the testing and patience.  Sorry that it took so
> > > > long.  I'm not completely sure the patch is correct.  It might have to
> > > > be more specifc about which type of migration or require further
> > > > synchronization around migration, but hopefully it'll at least be able
> > > > to show that this was the cause of the problem.
> > > 
> > > And last night's tests had no failures.  Which might actually mean
> > > something, will get more info when I run without your patch this
> > > evening.  ;-)
> > 
> > And it didn't fail without the patch, either.  45 hours of test vs.
> > 60 hours with the patch.  This one is not going to be easy to prove
> > either way.  I will try again this evening without the patch and see
> > what that gets us.
> 
> And another 36 hours (total of 81 hours) without the patch, still no
> failure.  Sigh.
> 
> In the sense that the patch doesn't cause any new problem:
> 
> Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> But I clearly have nothing of statistical significance, so any confidence
> in the fix is coming from your reproducer.

And for whatever it is worth, I did finally get a reproduction without
the patch.  The probability of occurrence is quite low with my test setup,
so please queue this patch.  I will accumulate test time on it over the
months to come.  :-/

							Thanx, Paul