Date: Fri, 20 Nov 2015 11:58:12 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
cc: Morten Rasmussen <morten.rasmussen@arm.com>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>, John Stultz <john.stultz@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Len Brown <len.brown@intel.com>,
        Rafael Wysocki <rafael.j.wysocki@intel.com>,
        Eduardo Valentin <edubezval@gmail.com>, Paul Turner <pjt@google.com>
Subject: Re: [PATCH 3/4] sched: introduce synchronized idle injection
In-Reply-To: <20151120102014.GC17308@twins.programming.kicks-ass.net>
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References: <1447444387-23525-1-git-send-email-jacob.jun.pan@linux.intel.com> <1447444387-23525-4-git-send-email-jacob.jun.pan@linux.intel.com> <20151118154457.GD30184@e105550-lin.cambridge.arm.com> <564C9E93.8030901@linux.intel.com>
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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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