linux-kernel - Re: PATCH] ftrace: Add a C/P state tracer to help power optimization

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Message-ID: <20081006213418.GM3180@one.firstfloor.org>
Date:	Mon, 6 Oct 2008 23:34:18 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>, mingo@...e.hu,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, lenb@...nel.org
Subject: Re: PATCH] ftrace: Add a C/P state tracer to help power optimization

On Mon, Oct 06, 2008 at 02:21:31PM -0700, Arjan van de Ven wrote:
> > > the link between P states and frequency is... rather lose.
> > > Especially with Turbo Mode it no longer is really relevant to list
> > > frequencies.
> > 
> > It would probably be less confusing for everyone if the higher level
> > cpufreq layers reported the correct frequency for turbo mode too.
> > I haven't checked how complicated this would be.
> 
> it's impossible until after the fact; you don't know which frequencies
> you got until you check back later.

Well it could do that couldn't it? Ok not sure how big the cost
would be.

I can just imagine Turbo mode becoming a FAQ on this list and having
better reporting upfront might mitigate this a bit.

> 
> 
> > Ok.
> > 
> > That means that when a CPU is idle forever there won't be any output?
> 
> correct; it'll wait until it stops being idle before telling  you how
> long it was idle.
> 
> if it really bothers you we could do a dummy broadcast ipi on shutting
> down the tracer.. in practice it's not a problem.

Or just mark entry/exit, but you just need a wakeup threshold to avoid
the loop. A wakeup threshold seems like a good idea anyways though, just
to get better efficiency on larger systems.

> (we wake up all cpus all the time)

I assume that will change in the future and might even not true
anymore on some special stripped down configurations.

-Andi

-- 
ak@...ux.intel.com
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