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

[Thomas Gleixner]

On Fri, 20 Nov 2015, Peter Zijlstra wrote:

> On Fri, Nov 20, 2015 at 10:45:14AM +0100, Thomas Gleixner wrote:
> 
> > The whole concept of faking idle is simply crap.
> 
> :-)
> 
> > If you want to avoid that stuff in the scheduler, then create a
> > mechanism which just defers the next timer interrupt for X
> > milliseconds and does not any fiddling with NOHZ state and such.
> 
> Not sure that will actually result in the machine going idle. Device
> interrupts will still wake tasks and get them ran.

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.

Thanks,

	tglx
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