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

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Date:	Tue, 10 Jun 2014 10:05:02 -0700
From:	Dirk Brandewie <dirk.brandewie@...il.com>
To:	Stratos Karafotis <stratosk@...aphore.gr>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Dirk Brandewie <dirk.j.brandewie@...el.com>
CC:	dirk.brandewie@...il.com,
	"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 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


> 
> 
> Thanks,
> Stratos
> 

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