linux-kernel - Re: [REDO PATCH v7] perf/x86/amd/power: Add AMD accumulated power reporting mechanism

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Date:	Thu, 16 Jun 2016 13:38:14 +0800
From:	Huang Rui <ray.huang@....com>
To:	Vince Weaver <vincent.weaver@...ne.edu>
CC:	Borislav Petkov <bp@...e.de>, Thomas Gleixner <tglx@...utronix.de>,
	"Peter Zijlstra" <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Andy Lutomirski" <luto@...capital.net>,
	Robert Richter <rric@...nel.org>,
	"Jacob Shin" <jacob.w.shin@...il.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Kan Liang <kan.liang@...el.com>,
	<linux-kernel@...r.kernel.org>, <x86@...nel.org>,
	Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
	Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>,
	Borislav Petkov <bp@...en8.de>,
	Guenter Roeck <linux@...ck-us.net>,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [REDO PATCH v7] perf/x86/amd/power: Add AMD accumulated power
 reporting mechanism

Hi Vince,

Thanks for asking.

On Wed, Jun 15, 2016 at 09:13:59PM -0400, Vince Weaver wrote:
> 
> three questions about this functionality:
> 
> 1.  In theory this should also work on an amd fam16h model 30h
>     processor too, correct?  The current code limits things to fam15h
>     even though the fam16mod30h has all the proper cpuid flags.
> 

I was told this feature would be supported on fam15h 60h, 70h and
later processors before. Just checked the fam16h model 30h BKDG, yes,
it should be also supported. But I didn't test that platform, if you
confirm it works in your side. We can enable it.

>     I've tested the functionality a bit and it seems to work but for
>     some reason the ptsc seems to occasionally count backwards
>     on my machine.  Any reason that would be?  (It doesn't seem to be
>     an overflow, just reading the ptsc 5ms apart and the values are 
>     slightly lower after than before).
> 

PTSC's frequency is about 100Mhz, it shouldn't be overflow.

> 2.  Unless I'm misunderstanding things, the code seems to be accumulating 
> 	Power. (see chunk below) Power is an instantaneous measurement, it 
> 	makes no sense to add values.  If you use 5W for 1ms and 10W for
> 	1ms, the average power across the 2ms interval is not 15W.
> 
> 	You can add energy, but not power.
> 
> > +	delta *= cpu_pwr_sample_ratio * 1000;
> > +	tdelta = new_ptsc - prev_ptsc;
> > +
> > +	do_div(delta, tdelta);
> > +	local64_add(delta, &event->count);
> 

You're right. Nice catch! The average power is per compute unit. We
cannot add the power simplely for each processor/package.

So here, the average power per package should be (delta1 + delta2 + ... + deltaN)/(tdelta_avg).
I will work out a fix. Thanks to point out.

> 3.  The actual results gathered seem rediculously low.  341 seconds of
>     calculation and only using 183 mWatts of power?
> 

mWatts are for processor power not system power. Below data is
calculated on fam15h model 60h which is low power platform. Even
though the method has a minor mistake, the processor power should be
in mWatts field.

> >    Performance counter stats for 'system wide':
> > 
> >               183.44 mWatts power/power-pkg/
> > 
> >        341.837270111 seconds time elapsed
> > 
> >   root@...zp:/home/ray/tip# ./tools/perf/perf stat -a -e 'power/power-pkg/' sleep 10
> 

Thanks,
Rui