linux-kernel - Re: [PATCH] selftests/resctrl: fix clang build warnings related to abs(), labs() calls

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Date: Fri, 3 May 2024 15:00:20 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Reinette Chatre <reinette.chatre@...el.com>,
	Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
CC: Shuah Khan <shuah@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
	"Nick Desaulniers" <ndesaulniers@...gle.com>, Bill Wendling
	<morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, Fenghua Yu
	<fenghua.yu@...el.com>, Valentin Obst <kernel@...entinobst.de>,
	<linux-kselftest@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
	<llvm@...ts.linux.dev>
Subject: Re: [PATCH] selftests/resctrl: fix clang build warnings related to
 abs(), labs() calls

On 5/3/24 1:46 PM, Reinette Chatre wrote:
> Hi John,
> On 5/3/2024 12:12 PM, John Hubbard wrote:
>> On 5/3/24 11:37 AM, Reinette Chatre wrote:
>>> On 5/3/2024 9:52 AM, John Hubbard wrote:
>>>> On 5/3/24 1:00 AM, Ilpo Järvinen wrote:
>>>>> On Thu, 2 May 2024, John Hubbard wrote:
>>>> ...
..
>> I assumed that this code did not expect to handle negative numbers,
>> because it is using unsigned math throughout.
>>
>> If you do expect it to handle cases where, for example, this happens:
>>
>>     avg_bw_imc > avg_bw_resc
> 
> The existing code seems to handle this ok. A sample program with this
> scenario comparing existing computation with your first proposal is
> below:
> 
> #include <stdio.h>
> #include <stdlib.h>
> 
> void main(void) {
> 	unsigned long avg_bw_resc = 20000;
> 	unsigned long avg_bw_imc = 40000;
> 	float avg_diff;
> 
> 	/* Existing code */
> 	avg_diff = (float)labs(avg_bw_resc - avg_bw_imc) / avg_bw_imc;
> 	printf("Existing code: avg_diff = %f\n", avg_diff);
> 
> 	/* Original proposed fix */
> 	avg_diff = (float)(avg_bw_resc - avg_bw_imc) / avg_bw_imc;
> 	printf("Original proposed fix: avg_diff = %f\n", avg_diff);
> }
> 
> output:
> Existing code: avg_diff = 0.500000
> Original proposed fix: avg_diff = 461168590192640.000000

That seems "a little bit" wrong. haha :)

> 
>>
>> ...then a proper solution is easy, and looks like this:
>>
>> diff --git a/tools/testing/selftests/resctrl/mbm_test.c b/tools/testing/selftests/resctrl/mbm_test.c
>> index c873793d016d..b87f91a41494 100644
>> --- a/tools/testing/selftests/resctrl/mbm_test.c
>> +++ b/tools/testing/selftests/resctrl/mbm_test.c
>> @@ -17,8 +17,8 @@
>>   static int
>>   show_bw_info(unsigned long *bw_imc, unsigned long *bw_resc, size_t span)
>>   {
>> -       unsigned long avg_bw_imc = 0, avg_bw_resc = 0;
>> -       unsigned long sum_bw_imc = 0, sum_bw_resc = 0;
>> +       long avg_bw_imc = 0, avg_bw_resc = 0;
>> +       long sum_bw_imc = 0, sum_bw_resc = 0;
>>          int runs, ret, avg_diff_per;
>>          float avg_diff = 0;
>>
>> Should I resend the patch with that approach?
> 
> ok. That indeed makes the computations easier to understand. I assume
> you intend to fix the snippet in mba_test.c also?
> 

Yes, will do that. Thanks for spotting the bug in the original "fix"!

thanks,
-- 
John Hubbard
NVIDIA