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

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Message-ID: <20140204091405.GB19156@gmail.com>
Date:	Tue, 4 Feb 2014 10:14:05 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	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

* Peter Zijlstra <peterz@...radead.org> wrote:

> [...]
>
> The reason Ingo took it out was that these measured numbers would 
> slightly vary from boot to boot making it hard to compare 
> performance numbers across boots.
> 
> There's something to be said for either case I suppose.

Yeah, so we could put the parameters back by measuring it in 
user-space via a nice utility in tools/, and by matching it to 
relevant hardware signatures (CPU type and cache sizes), plus doing 
some defaults for when we don't have any signature... possibly based 
on a fuzzy search to find the 'closest' system in the table of 
constants.

That would stabilize the boot-to-boot figures while still keeping most 
of the system specific-ness, in a maintainable fashion.

The downside is that we'd have to continuously maintain a table of all 
this info, with new entries added when new CPUs are introduced on the 
market. That's an upside too, btw.

Thanks,

	Ingo
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