Re: [PATCH 3/4] sched: introduce synchronized idle injection

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 20, 2015 at 11:58:12AM +0100, Thomas Gleixner wrote:
> That's not what I meant. If you don't want to control all that from
> the scheduler than you are back to that thread which "runs" at RT
> priority and does
> 
> 	 if (machine_on_fire) {
> 	        defer_timer_interrupt(5ms);
> 		end = now + 5ms:
> 	    	while (now < end)
> 		      mwait();
>          }
> 
> That's what the existing code does, but the above does not longer
> claim it's idle and confuses the hell out of nohz and whatever.  It's
> just a "runaway" RT task which "hogs" the CPU for 5ms and makes the
> next timer interrupt firing late.

Right; so the naive way of implementing that is by simply programing the
timer hardware 5ms in the future and leaving it at that.

The problem with that would be a device interrupt happening and mucking
with timers, this would result in the timer hardware being reprogrammed
to a 'sane' value. I see two solutions for that:

 - add another check in tick_program_event(); or,

 - muck about with the evtdev pointer, such that we (temporarily) neuter
   its clock_event_device::set_next_*() methods.

The later is fugly, but avoids any runtime overhead.

This all makes the idle-injection muck hard depend on hres_active, but I
think that's a sane constraint anyway.