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

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Date:	Mon, 6 Oct 2008 13:57:15 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	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, 06 Oct 2008 22:46:09 +0200
Andi Kleen <andi@...stfloor.org> wrote:
> >  
> > +	trace_power_mark(&it, POWER_PSTATE, next_perf_state);
> 
> Wouldn't that be better higher up in the cpufreq system?  It would
> seem bad to duplicate that in all low level cpufreq modules.

the problem is that higher up the actual P state isn't known.

> 
> 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.

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

> 
> > +{
> > +	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
the tracer is in use.

> 

> 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.


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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