Re: PREEMPT_RT with Full Tickless

[Mike Galbraith <umgwanakikbuti@gmail.com>]

On Sat, 2015-12-12 at 14:11 -0600, Gary Robertson wrote:
> On Fri, Dec 11, 2015 at 11:50 AM, Sebastian Andrzej Siewior
> <bigeasy@linutronix.de> wrote:
> > * Mike Galbraith | 2015-12-11 15:36:17 [+0100]:
> > 
> > > On Fri, 2015-12-11 at 19:29 +0800, GeHao Kang wrote:
> > > > When I enable CONFIG_PREEMPT_RT_FULL and CONFIG_NO_HZ_FULL
> > > > in Linux Kernel v4.1.12-rt12, I find the tick on the nohz_full
> > > > cpu
> > > > never stops.
> > > 
> 
> 
> Also - since in RT the tick timer interrupt results in a schedulable
> thread running to handle the software interrupt work stimulated by
> the
> tick - upon exit from the interrupt handler there are always at least
> 2 runnable threads in the CPU running the tick if there was a
> runnable
> task interrupted by the tick event.

Yup, handling that was one of the hacks that was dropped.  If you look
at what all was dropped in one 'let's con NOHZ_FULL into working'
patch, you'd think twice about submitting it for review.

	-Mike