linux-kernel - Re: [PATCH 2/2] x86 cpufreq: Make trace_power

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Message-Id: <201004301200.39076.trenn@suse.de>
Date:	Fri, 30 Apr 2010 12:00:38 +0200
From:	Thomas Renninger <trenn@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	x86@...nel.org, linux-trace-users@...r.kernel.org,
	linux-perf-users@...r.kernel.org, robert.schoene@...dresden.de,
	cpufreq@...r.kernel.org, linux-kernel@...r.kernel.org,
	davej@...hat.com, arjan@...radead.org,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"Fr??d??ric Weisbecker" <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 2/2] x86 cpufreq: Make trace_power_frequency cpufreq driver independent

On Friday 30 April 2010 11:08:14 Ingo Molnar wrote:
> 
> * Thomas Renninger <trenn@...e.de> wrote:
> 
> > +		trace_power_start(POWER_CSTATE, 1, smp_processor_id());
> > +	trace_power_start(POWER_CSTATE, (ax>>4)+1, smp_processor_id());
> > +		trace_power_start(POWER_CSTATE, 1, smp_processor_id());
> > +	trace_power_start(POWER_CSTATE, 0, smp_processor_id());
> > +                trace_power_frequency(POWER_PSTATE, freqs->new, freqs->cpu);
> > +	trace_power_end(smp_processor_id());
> 
> Extending power tracing to all cpufreq modules is obviously a good thing.
> 
> But why is trace_power_start() adding a CPU ID argument? CPU ids are already 
> available and can be sampled via PERF_SAMPLE_CPU if needed. AFAICS only 
> power_frequency needs a new 'target_cpu_id' field.
Currently the C-states get triggered on the machine you are running on.
But you can have C-state dependencies (ACPI spec at least defines this 
the same way as done for P-states, not sure whether this exists in
reality already, Linux does not evaluate them yet).
The same way we run into trouble with P-states (SW_ALL, SW_ANY, HW).
Not sure how this will/could show up in reality in HW or implementation, but
as these dependencies are already defined in spec, it sounds like a good idea
to pass the CPU through the POWER_CSTATE events as well.

Compare with chapter 8.4.2.2 _CSD (C-State Dependency)
of an ACPI spec 3.0 or newer.

There are also other architectures starting to use processor sleep states.

So this is not actually used (smp_processor_id() is the same as already
tracked by trace event internally), but to make the interface more robust
for the future. Changes in the trace_power layout later may hurt.
 
> > diff --git a/tools/perf/builtin-timechart.c b/tools/perf/builtin-timechart.c
> > index 0d4d8ff..7809bef 100644
> > --- a/tools/perf/builtin-timechart.c
> > +++ b/tools/perf/builtin-timechart.c
> 
> Timechart is maintained by Arjan so we need an ack from him as well. I've seen 
> some back and forth in the discussions - what's the technical resolution of 
> that?
> 
> (Also, there's some whitespace noise in the patch.)
Oops. Tell me if you like to push it and I send a checkpatch cleaned up version.
Sorry about that.

     Thomas
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