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SubjectRe: [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:
> Make sched_feat(x) return 1 or 0 instead of 2**x or 0 when
> sysctl_sched_features is constant, by changing the left shift of the
> mask-bit to a right shift of the bitmap. The behaviour of the code
> remains unchanged.
> We prove this by showing that the compiler can evaluate this shift
> during compile time, which results in generating the same
> machine code as before.
>
> 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))
> ~^~~~~~~~~~~~~~~~~~~~~~~~~~
>
> This resolves the warning, leaves the code base largely as is.
>
> Tested with gcc 7.3.1 and clang 6.0.0 on x86_64, comparing resulting
> binaries with diff.
> Applicable to all modern compilers and architectures
>
> Signed-off-by: Philipp Klocke <Phil_K97@gmx.de>
> Suggested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Nicholas Mc Guire <der.herr@hofr.at>

> ---
> kernel/sched/sched.h | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index fb5fc458547f..d2aedee6ab0f 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -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
>

The changed sched_feat(y) line is fine but I do not get/like the
of the ifdef - keeping the change minimal is ok if there is a
relevant impact but here there is no effective difference (you write
the object code is the same for the !CONFIG_SCHED_DEBUG case)
So I think the ifdef should be kicked here and the proposed change
seems fine to me.

> #endif /* SCHED_DEBUG && HAVE_JUMP_LABEL */
>
> --
> 2.17.0
>

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