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

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Message-ID: <alpine.DEB.2.11.1511201150480.3989@nanos>
Date:	Fri, 20 Nov 2015 11:58:12 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Morten Rasmussen <morten.rasmussen@....com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Len Brown <len.brown@...el.com>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>,
	Eduardo Valentin <edubezval@...il.com>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH 3/4] sched: introduce synchronized idle injection

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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