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

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Date:	Wed, 18 Nov 2015 12:27:04 +0000
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	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 Wed, Nov 18, 2015 at 11:35:41AM +0100, Peter Zijlstra wrote:
> On Wed, Nov 18, 2015 at 09:36:22AM +0100, Ingo Molnar wrote:
> > What will such throttling do to latencies, as observed by user-space tasks? What's 
> > the typical expected frequency of the throttling frequency that you are targeting?
> 
> The default has 5ms (iirc) of forced idle, so depending on what you do,
> noticeable to outright painful.

IIUC, it is 5 ticks, not ms.

Which raises the question, doesn't that mean that we get disturbed four
times on each cpu during the forced idle period? So idle injection only
makes sense if the platform has package states with a target residency
less than a jiffy. Or, do we enter NOHZ idle? I haven't looked closely
enough to figure out yet.
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