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

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Message-ID: <20180420165139.GP4064@hirez.programming.kicks-ass.net>
Date:   Fri, 20 Apr 2018 18:51:39 +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 Fri, Apr 20, 2018 at 06:29:07PM +0200, Philipp Klocke wrote:
> The gain is stopping a warning that clutters the output log of clang.

Well, you should not be using clang anyway. It is known to miscompile
the kernel.

> To improve readability, one can drop the ifdef-structure and just keep
> the right shift version, like Nicholas suggested. This will have a (very
> small)
> impact on performance in CONFIG_SCHED_DEBUG case, but when
> debugging, performance is no problem anyways.

See that is two bad choices.

> > Also, if sysctl_sched_features is a constant, the both expressions
> > _should_ really result in a constant and clang should still warn about
> > it.
> No, because clang only warns if the constant is neither 1 nor 0.
> (These being the 'best' integer representations of booleans)

Then won't something like so work?

#define sched_feat(x) !!(sysctl_sched_features & (1UL << __SCHED_FEAT_##x))

That forces it into _Bool space (0/1) and per the above rule will no
longer warn.