On Wed, 26 Jul 2017 18:13:12 +0100
Jonathan Cameron [off-list ref] wrote:

On Wed, 26 Jul 2017 09:54:32 -0700
David Miller [off-list ref] wrote:

quoted

From: "Paul E. McKenney" <redacted>
Date: Wed, 26 Jul 2017 08:49:00 -0700
  

quoted

On Wed, Jul 26, 2017 at 04:33:40PM +0100, Jonathan Cameron wrote:    

quoted

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

quoted

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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