linux-kernel - Re: [RFC PATCH 3/3] idle: store the idle state index in the struct rq

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Message-ID: <52EFA9D3.1030601@linux.intel.com>
Date:	Mon, 03 Feb 2014 06:38:11 -0800
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Morten Rasmussen <morten.rasmussen@....com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>
CC:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Len Brown <len.brown@...el.com>,
	Preeti Murthy <preeti.lkml@...il.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>
Subject: Re: [RFC PATCH 3/3] idle: store the idle state index in the struct
 rq

On 2/3/2014 4:54 AM, Morten Rasmussen wrote:

>
> I'm therefore not convinced that idle state index is the right thing to
> give the scheduler. Using a cost metric would be better in my
> opinion.

I totally agree with this, and we may need two separate cost metrics

1) A latency driven one
2) A performance impact on

first one is pretty much the exit latency related time, sort of a "expected time to first instruction"
(currently menuidle has the 99.999% worst case number, which is not useful for this, but is a first
approximation). This is obviously the dominating number for expected-short running tasks

second on is more of a "is there any cache/TLB left or is it flushed" kind of metric. It's more tricky
to compute, since what is the cost of an empty cache (or even a cache migration) after all....
.... but I suspect it's in part what the scheduler will care about more for expected-long  running tasks.

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