linux-kernel - Re: [peterz-queue:sched/core 13/19] kernel/sched/fair.c:892:34: warning: variable 'stats' set but not used

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Date:   Sat, 11 Sep 2021 16:30:41 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     kernel test robot <lkp@...el.com>,
        Yafang Shao <laoar.shao@...il.com>, kbuild-all@...ts.01.org,
        linux-kernel@...r.kernel.org, linux-toolchains@...r.kernel.org
Subject: Re: [peterz-queue:sched/core 13/19] kernel/sched/fair.c:892:34: warning: variable 'stats' set but not used

Hi!

On Sat, Sep 11, 2021 at 02:20:49PM +0200, Peter Zijlstra wrote:
> On Sat, Sep 11, 2021 at 02:21:26AM +0800, kernel test robot wrote:
> > compiler: m68k-linux-gcc (GCC) 11.2.0
> 
> >    kernel/sched/fair.c: In function 'update_curr':
> >    kernel/sched/fair.c:860:42: warning: unused variable 'stats' [-Wunused-variable]
> >      860 |                 struct sched_statistics *stats = __schedstats_from_se(curr);
> >          |                                          ^~~~~
> 
> OK, compiler guys, this code reads like:
> 
> #define schedstats_enabled()	(0)
> #define __schedstat_set(x, y)	do { } while (0)
> 
> 
> 	if (schedstats_enabled()) {
> 		struct sched_statistics *stats = __schedstats_from_se(curr);
> 
> 		__schedstat_set(stats->exec_max,
> 				max(delta_exec, stats->exec_max));
> 	}
> 
> So yes, we initialize a variable that then isn't used, but the whole
> bloody thing is inside if (0) which will not ever get ran *anyway*.
> 
> This is a crap warning if ever I saw one...

Yes, we really should warn "do not use a preprocessor macro if what you
want is a function"?  The variable really *is* unused, with this macro.

If we would remove dead code before warning about unused variables
there would be many *more* false positives, fwiw.


Segher