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

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Date:   Mon, 23 Apr 2018 11:45:30 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc:     Philipp Klocke <Phil_K97@....de>, 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 11:29:33PM +0200, Lukas Bulwahn wrote:
> 
> On Fri, 20 Apr 2018, Peter Zijlstra wrote:
> 
> > 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.
> > 
> 
> There are some advantages of having a second compiler that can compile
> the kernel (https://lwn.net/Articles/734071/). Some people in the kernel 
> community and LLVM community are trying to get that to work.

Sure, not arguing against that. Just saying clang isn't there yet and it
has much bigger problems than a stray warning.

> We also want a zero-warning policy for clang, similar to gcc. 
> Hence, this motivates to have a look at those few clang warnings and come 
> up with patches for them.
> 
> This does not imply to make changes at any cost, and we need to determine 
> a proper patch to either change the source code, disable the warning in 
> the build script or annotate the file with some clang-specific pragmas.
> 
> To us, a minor change in the source sounded most reasonable after looking
> at all three possible patches. Philipp might need another iteration, but
> it generally looks sound to me if we get the details flattened out. 

Given the history of compiler warnings; I would really like to have some
text that explains why the warning is useful and should be worked
around.

To me the warning under discussion seems very dodgy and I would propose
to disable it entirely. Using a value other than 0/1 for boolean
expressions is fairly common, it being a compile time constant doesn't
seem to make much difference to me.