Date: Fri, 20 Nov 2015 10:45:14 +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: <20151119200906.GS3816@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>
 <20151119172405.GA798@e105550-lin.cambridge.arm.com> <20151119200906.GS3816@twins.programming.kicks-ass.net>
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On Thu, 19 Nov 2015, Peter Zijlstra wrote:
> On Thu, Nov 19, 2015 at 05:24:07PM +0000, Morten Rasmussen wrote:
> > I would consider it an
> > emergency-only mechanism (as in emergency brake) that isn't really
> > suitable for normal thermal management. In which case: Does this sort of
> > mechanism belong in the scheduler code?
> 
> I would prefer it not to be, but Thomas is very much opposed to teaching
> the nohz code to 'work' on !idle threads.

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.

That might hurt RT tasks, but if someone really cares about real-time
and deterministic behaviour, then running the machine on its thermal
limits is simply stupid. In fact any sensible RT system will bring
itself into a safe state way before the machine runs into that
condition.

Thanks,

	tglx
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