linux-kernel - Re: [PATCH] sched/fair: Change sched_feat(x) in !CONFIG_SCHED

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Date:   Fri, 20 Apr 2018 09:57:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Philipp Klocke <Phil_K97@....de>
Cc:     lukas.bulwahn@...il.com, kernelnewbies@...nelnewbies.org,
        llvmlinux@...ts.linuxfoundation.org, sil2review@...ts.osadl.org,
        der.herr@...r.at, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Change sched_feat(x) in !CONFIG_SCHED_DEBUG
 case

On Mon, Apr 16, 2018 at 10:54:26AM +0200, Philipp Klocke wrote:

> This patch is motivated by the clang warning Wconstant-logical-operand,
> issued when logically comparing a variable to a constant integer that is
> neither 1 nor 0.  It happens for sched_feat(x) when sysctl_sched_features
> is constant, i.e., CONFIG_SCHED_DEBUG is not set.
> 
> kernel/sched/fair.c:3927:14: warning: use of logical '&&' with constant operand [-Wconstant-logical-operand]
>         if (initial && sched_feat(START_DEBIT))
>                     ^  ~~~~~~~~~~~~~~~~~~~~~~~
> kernel/sched/fair.c:3927:14: note: use '&' for a bitwise operation
>         if (initial && sched_feat(START_DEBIT))
>                     ^~
>                     &
> kernel/sched/fair.c:3927:14: note: remove constant to silence this warning
>         if (initial && sched_feat(START_DEBIT))
>                    ~^~~~~~~~~~~~~~~~~~~~~~~~~~


> @@ -1305,7 +1305,11 @@ static const_debug __maybe_unused unsigned int sysctl_sched_features =
>  	0;
>  #undef SCHED_FEAT
>  
> +#ifdef CONFIG_SCHED_DEBUG
>  #define sched_feat(x) (sysctl_sched_features & (1UL << __SCHED_FEAT_##x))
> +#else
> +#define sched_feat(x) ((sysctl_sched_features >> __SCHED_FEAT_##x) & 1UL)
> +#endif

So this is extra ugly, for no gain?

WTH does clang complain about a constant? Can't you just disable that
stupid warning?

Also, if sysctl_sched_features is a constant, the both expressions
_should_ really result in a constant and clang should still warn about
it.

I'm really not seeing why we'd want to do this. Just fix clang to not be
stupid.