Date: Fri, 20 Nov 2015 13:54:41 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
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
Message-ID: <20151120125441.GD17308@twins.programming.kicks-ass.net>
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 <20151119200906.GS3816@twins.programming.kicks-ass.net>
 <alpine.DEB.2.11.1511201032300.3989@nanos>
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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.


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