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

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Message-ID: <20151118124946.GF3816@twins.programming.kicks-ass.net>
Date:	Wed, 18 Nov 2015 13:49:46 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
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 12:27:04PM +0000, Morten Rasmussen wrote:
> 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.

The code uses hrtimers (badly), this means there _should_ not be a tick
dependency.

> 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.

The idea is to hit NOHZ.
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