Re: [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu_stop() enable tick on all online CPUs

[Peter Zijlstra]

On Sun, Aug 04, 2019 at 09:19:01PM -0700, Paul E. McKenney wrote:
> On Sun, Aug 04, 2019 at 01:24:46PM -0700, Paul E. McKenney wrote:

> > For whatever it is worth, the things on my list include using 25 rounds
> > of resched_cpu() on each CPU with ten-jiffy wait between each (instead of
> > merely 10 rounds), using waitqueues or some such to actually force a
> > meaningful context switch on the other CPUs, etc.

That really should not be needed. What are those other CPUs doing?

> Which appears to have reduced the bug rate by about a factor of two.
> (But statistics and all that.)

Which is just weird..

> I am now trying the same test, but with CONFIG_PREEMPT=y and without
> quite so much hammering on the scheduler.  This is keying off Peter's
> earlier mention of preemption.  If this turns out to be solid, perhaps
> we outlaw CONFIG_PREEMPT=n && CONFIG_NO_HZ_FULL=y?

CONFIG_PREEMPT=n should work just fine, _something_ is off.