Subject: Re: RCU lockup issues when CONFIG_SOFTLOCKUP_DETECTOR=n - any one else seeing this?

From: Jonathan Cameron <[email-masked]>
Date: Thu, 27 Jul 2017 08:41:56 +0100

On Wed, 26 Jul 2017 18:13:12 +0100
Jonathan Cameron <[email-masked]> wrote:

> On Wed, 26 Jul 2017 09:54:32 -0700
> David Miller <[email-masked]> wrote:
> 
> > From: "Paul E. McKenney" <[email-masked]>
> > Date: Wed, 26 Jul 2017 08:49:00 -0700
> >   
> > > On Wed, Jul 26, 2017 at 04:33:40PM +0100, Jonathan Cameron wrote:    
> > >> Didn't leave it long enough. Still bad on 4.10-rc7 just took over
> > >> an hour to occur.    
> > > 
> > > And it is quite possible that SOFTLOCKUP_DETECTOR=y and HZ_PERIODIC=y
> > > are just greatly reducing the probability of the problem rather than
> > > completely preventing it.
> > > 
> > > Still, hopefully useful information, thank you for the testing!    
> 
> Not sure it actually gives us much information, but no issues yet
> with a simple program running every cpu that wakes up every 3 seconds.
> 
> Will leave it running overnight and report back in the morning.
Perhaps unsurprisingly the above test didn't show any splats.

So it appears a userspace wakeup is enough to stop the issue happening
(or at least make it a lot less likely).

Jonathan
> 
> > 
> > I guess that invalidates my idea to test reverting recent changes to
> > the tick-sched.c code... :-/
> > 
> > In NO_HZ_IDLE mode, what is really supposed to happen on a completely
> > idle system?
> > 
> > All the cpus enter the idle loop, have no timers programmed, and they
> > all just go to sleep until an external event happens.
> > 
> > What ensures that grace periods get processed in this regime?  
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