Re: [PATCH] rcu: Only pin GP kthread when full dynticks is actually used

[Frederic Weisbecker <fweisbec@gmail.com>]

On Fri, Jun 13, 2014 at 01:49:03PM -0700, Paul E. McKenney wrote:
> On Fri, Jun 13, 2014 at 06:21:32PM +0200, Frederic Weisbecker wrote:
> > On Fri, Jun 13, 2014 at 09:16:30AM -0700, Paul E. McKenney wrote:
> > > > Is it because we have dynticks CPUs staying too long in the kernel without
> > > > taking any quiescent states? Are we perhaps missing some rcu_user_enter() or
> > > > things?
> > > 
> > > Sort of the former, but combined with the fact that in-kernel CPUs still
> > > need scheduling-clock interrupts for RCU to make progress.  I could
> > > move this to RCU's context-switch hook, but that could be very bad for
> > > workloads that do lots of context switching.
> > 
> > Or I can restart the tick if the CPU stays in the kernel for too long without
> > a tick. I think that's what we were doing before but we removed that because
> > we never implemented it correctly (we sent scheduler IPI that did nothing...)
> 
> That would work for me!
> 
> Just out of curiosity, what would you use to determine that the CPU
> had been in the kernel too long?

I'd rather deduce that when grace periods completion go past some delay.
I think that's the requirement for calling rcu_kick_nohz_cpu()?