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

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Date:	Mon, 6 Oct 2008 23:19:39 +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 01:57:15PM -0700, Arjan van de Ven wrote:
> 
> the problem is that higher up the actual P state isn't known.

My main worry is that every architecture is going to implement
their own version of this plugin if there isn't some abstraction.
 
> > Also I suspect some higher level format would be good here too.
> > Just put the frequency in? 
> 
> 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.

> 
> > > +			ret = trace_seq_printf(s, "[%5ld.%09ld]
> > > CSTATE: Going to C%i on cpu %i for %ld.%09ld\n",
> > > +					  stamp.tv_sec,
> > > +					  stamp.tv_nsec,
> > > +					  it->state, iter->cpu,
> > > +					  duration.tv_sec,
> > > +					  duration.tv_nsec);
> > > +		if (it->type == POWER_PSTATE)
> > > +			ret = trace_seq_printf(s, "[%5ld.%09ld]
> > > PSTATE: Going to P%i on cpu %i\n",
> > > +					  stamp.tv_sec,
> > > +					  stamp.tv_nsec,
> > > +					  it->state, iter->cpu);
> > 
> > I suspect a less verbose output format would be better.
> why?

to shuffle less data around

> It's fine as it is, and it's actually human readable as well.

I suspect humans could well do with something more compact too

e.g.

CPU %d -> P%d

> 
> > 
> > > +{
> > > +	if (!trace_power_enabled)
> > > +		return;
> > > +
> > > +	memset(it, 0, sizeof(struct power_trace));
> > 
> > The memset seems redundant.
> 
> it's free and it initializes the datastructure cleanly; and only when

AFAIK all data fields are init'ed anyways.

How is it free? I'm not sure gcc will optimize it away completely.

I thought the point of ftrace tracers was to be reasonably efficient
when they are enabled?  If efficiency doesn't matter you could
as well use printk.

> > Hmm, that does a unconditional wake_up() in idle. Doesn't this cause
> > a loop on UP?
> > 
> > idle -> wakeup -> idle -> wakeup -> ... etc.
> > 
> > Am I missing something?
> 
> yes you're missing something ;-)
> this code is called when going out of idle, not when going into idle.

Ok.

That means that when a CPU is idle forever there won't be any output?

-Andi

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