linux-kernel - Re: [PATCH v2] selftests/resctrl: fix clang build warnings related to abs(), labs() calls

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Date: Wed, 8 May 2024 11:00:15 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Reinette Chatre <reinette.chatre@...el.com>
cc: John Hubbard <jhubbard@...dia.com>, 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 v2] selftests/resctrl: fix clang build warnings related
 to abs(), labs() calls

On Tue, 7 May 2024, Reinette Chatre wrote:

> Hi John,
> 
> On 5/7/2024 6:25 PM, John Hubbard wrote:
> > On 5/7/24 6:21 PM, Reinette Chatre wrote:
> >> Hi John,
> > ...
> >>
> >> The following (what was in v1) looks good to me. What am I missing?
> >>
> >> diff --git a/tools/testing/selftests/resctrl/cmt_test.c b/tools/testing/selftests/resctrl/cmt_test.c
> >> index a81f91222a89..05a241519ae8 100644
> >> --- a/tools/testing/selftests/resctrl/cmt_test.c
> >> +++ b/tools/testing/selftests/resctrl/cmt_test.c
> >> @@ -40,11 +40,11 @@ static int show_results_info(unsigned long sum_llc_val, int no_of_bits,
> >>       int ret;
> >>         avg_llc_val = sum_llc_val / num_of_runs;
> >> -    avg_diff = (long)abs(cache_span - avg_llc_val);
> >> +    avg_diff = (long)(cache_span - avg_llc_val);
> > 
> > This deletes the abs() call, because I incorrectly let clang's warning
> > lead me to believe that the abs() call was a no-op. But both you and Ilpo
> > pointed out that the math breaks if you do that.
> 
> To me the extra abs() was unnecessary anyway ...
> 
> > 
> >>       diff_percent = ((float)cache_span - avg_llc_val) / cache_span * 100;
> >>         ret = platform && abs((int)diff_percent) > max_diff_percent &&
> >> -          abs(avg_diff) > max_diff;
> >> +          labs(avg_diff) > max_diff;
> > 
> 
> .. because it is repeated here.

Yes, there are two *abs() calls in this function.

In this case is okay to remove the first one since it didn't remove 
absolute value completely, whereas in the MBA/MBM cases v1 removed *abs() 
call entirely which was wrong thing to do.

I explicitly noted in my v1 comment that this CMT change is okay but the 
other two were not.

-- 
 i.