linux-kernel - Re: [PATCH 3/7] cpufreq: intel_pstate: Add debugfs file stats

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Date:	Tue, 10 Jun 2014 20:45:34 +0300
From:	Stratos Karafotis <stratosk@...aphore.gr>
To:	Dirk Brandewie <dirk.brandewie@...il.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Dirk Brandewie <dirk.j.brandewie@...el.com>
CC:	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/7] cpufreq: intel_pstate: Add debugfs file stats

On 10/06/2014 08:05 μμ, Dirk Brandewie wrote:
> On 06/10/2014 09:21 AM, Stratos Karafotis wrote:
>> On 10/06/2014 06:47 μμ, Dirk Brandewie wrote:
>>> On 06/09/2014 02:00 PM, Stratos Karafotis wrote:
>>>> Add stats file in debugfs under driver's parent directory
>>>> (pstate_snb) which counts the time in nsecs per requested
>>>> P state and the number of times the specific state
>>>> was requested.
>>>>
>>>> The file presents the statistics per logical CPU in the
>>>> following format. The time is displayed in msecs:
>>>>
>>>
>>> NAK
>>>
>>> This adds significantly to the memory footprint to gather information
>>> that is available by post processing the perf tracepoint information.
>>> The increase isn't horrible on single socket desktop processor machines
>>> but gets big with server class machines.  One vendor I have talked to considers
>>> a machine with 1024 cpus to be a SMALL machine.
>>>
>>
>> If I am not wrong the sizeof pstate_stat is 20B. On my CPU with 20 P states, we
>> need 400B per logical CPU (3200B total in my desktop) plus 64B for stats pointers.
>>
>> In your example this would need about 400KB - 500KB?
>> Is it too much for 1024 a CPUs system?
> 
> For something that will likely not be used IMO yes.
> 
>>
>> I think it's a useful piece of info that we can have it directly without
>> post processing tracepoint.
>> Is it acceptable to conditionally compile it with a new CONFIG option?
> 
> 
> I can see where the information could be useful but the set of people
> that would find it useful is very small.  Having information about residency 
> since boot is interesting but just barely.  This file will encourage people
> to build tools/scripts that rely on this file and they will complain bitterly
> if/when it changes or goes away so you would be creating a defacto ABI in
> debugfs.
> 
> 
> This functionality will *not* be supportable in up coming processors where HWP
> is being used.  See section 14.4 of the current SDM vol. 3 
> http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-system-programming-manual-325384.pdf
> 

I will drop this patch in v2.
Thanks a lot for your comments and your time!

Stratos

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