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

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Message-ID: <46a6019c-b029-4764-8c66-ad61f4191716@intel.com>
Date: Fri, 3 May 2024 13:46:22 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: John Hubbard <jhubbard@...dia.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

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:
>>> ...
>>>>> diff --git a/tools/testing/selftests/resctrl/mbm_test.c b/tools/testing/selftests/resctrl/mbm_test.c
>>>>> index d67ffa3ec63a..c873793d016d 100644
>>>>> --- a/tools/testing/selftests/resctrl/mbm_test.c
>>>>> +++ b/tools/testing/selftests/resctrl/mbm_test.c
>>>>> @@ -33,7 +33,7 @@ show_bw_info(unsigned long *bw_imc, unsigned long *bw_resc, size_t span)
>>>>>          avg_bw_imc = sum_bw_imc / 4;
>>>>>        avg_bw_resc = sum_bw_resc / 4;
>>>>> -    avg_diff = (float)labs(avg_bw_resc - avg_bw_imc) / avg_bw_imc;
>>>>> +    avg_diff = (float)(avg_bw_resc - avg_bw_imc) / avg_bw_imc;
>>>>>        avg_diff_per = (int)(avg_diff * 100);
>>>>>          ret = avg_diff_per > MAX_DIFF_PERCENT;
>>>>
>>>> But how are these two cases same after your change when you ended up
>>>> removing taking the absolute value entirely?
>>>
>>> All of the arguments are unsigned integers, so all arithmetic results
>>> are interpreted as unsigned, so taking the absolute value of that is
>>> always a no-op.
>>
>> It does not seem as though clang can see when values have been casted.
>> I tried to do so explicitly with a:
>>       avg_diff = labs((long)avg_bw_resc - avg_bw_imc) / (float)avg_bw_imc;
> 
> The subtraction result will get promoted to an unsigned long, before being
> passed into labs(3).
> 
>>
>> But that still triggers:
>> warning: taking the absolute value of unsigned type 'unsigned long' has no effect [-Wabsolute-value]
> 
> As expected, yes.
> 
>>
>> Looks like we may need to be more explicit types and not rely on casting so much
>> to make the compiler happy.
>>
> 
> 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

> 
> ...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?

Reinette