linux-kernel - Re: [PATCH] cpufreq, Fix overflow in busy

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Message-ID: <5579BE65.5020008@redhat.com>
Date:	Thu, 11 Jun 2015 12:59:17 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	Doug Smythies <dsmythies@...us.net>
CC:	"'Rafael J. Wysocki'" <rjw@...ysocki.net>,
	linux-kernel@...r.kernel.org,
	"'Kristen Carlson Accardi'" <kristen@...ux.intel.com>,
	"'Viresh Kumar'" <viresh.kumar@...aro.org>,
	linux-pm@...r.kernel.org
Subject: Re: [PATCH] cpufreq, Fix overflow in busy_scaled due to long delay



On 06/11/2015 12:17 PM, Doug Smythies wrote:
> 
> On 2015.06.11 08:01 Prarit Bhargava wrote:
>> On 06/11/2015 10:51 AM, Doug Smythies wrote:
>>>
>>> On 2015.06.10 16:46 Rafael J. Wysocki wrote:
>>>> On Wednesday, June 10, 2015 09:18:45 AM Prarit Bhargava wrote:
>>>>> I looked into switching to div64_s64() instead of the 32-bit version in
>>>>> div_fp(), however, this would result in sample_ratio and core_busy returning
>>>>> 0 which is something we don't want.
>>>
>>> ???
>>> Due to a great many overflow related issues, div_fp() was changed to div64_s64()
>>> a long time ago.
> 
>> Doug,
>>
>> Nope -- in a linux.git tree (up-to-date as of 7:00AM ET this AM)
>>
>> static inline int32_t div_fp(int32_t x, int32_t y)
>> {
>>        return div_s64((int64_t)x << FRAC_BITS, y);
>> }
> 
>> If we do want this to be div64_s64, I can make that change, however, I feel that
>> a long delay like this should be ignored in the performance calculations in the
>> driver and that's why I chose to go the direction I did.
> 
> Prarit,
> 
> Apologies to you and the list for the distraction. I mis-read "div_s64" as "div64_s64".
> Your suggestion is a good one.

Thanks and no problems.  It's always good to have someone make me go back and
double check ;)

> 
> I do maintain that my point about the duration method being flawed is valid.
> I proposed a fix for that some time ago.

Okay, I'll go back and look at switching to div64_s64() and do some additional
testing.

P.

> 
> 
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