linux-kernel - Re: [PATCH v1] sched/topology: Make compiler happy about unused constant definitions

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Message-ID: <20200825112405.GU1891694@smile.fi.intel.com>
Date:   Tue, 25 Aug 2020 14:24:05 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v1] sched/topology: Make compiler happy about unused
 constant definitions

On Tue, Aug 25, 2020 at 11:12:21AM +0100, Valentin Schneider wrote:
> On 25/08/20 10:03, Valentin Schneider wrote:
> > On 25/08/20 09:26, Andy Shevchenko wrote:
> >> On Mon, Aug 24, 2020 at 06:09:41PM +0100, Valentin Schneider wrote:
> >>> On 24/08/20 16:39, Andy Shevchenko wrote:
> >>> > Compilation of almost each file ends up with
> >>> >
> >>> >  In file included from .../include/linux/energy_model.h:10,
> >>> >                 from .../include/linux/device.h:16,
> >>> >                 from .../drivers/spi/spi.c:8:
> >>> >  .../include/linux/sched/topology.h:30:27: warning: ‘SD_DEGENERATE_GROUPS_MASK’ defined but not used [-Wunused-const-variable=]
> >>> >     30 | static const unsigned int SD_DEGENERATE_GROUPS_MASK =
> >>> >        |                           ^~~~~~~~~~~~~~~~~~~~~~~~~
> >>> >  ...
> >>> >
> >>> > Make compiler happy by annotating the static constants with __maybwe_unused.
> 
> > Sorry, that's what I get for trying to be too succinct; what I tried to say
> > was that SD_DEGENERATE_GROUPS_MASK should very much be used for SMP. If the
> > build is !SMP, it shouldn't even be defined, IOW I'm perplexed as to where
> > this is coming from.
> 
> So I see how having this as a constvar rather than a constexpr is somewhat
> daft (we get an instance per compilation unit), but none of my compilers
> seem to complain (even with W=1). AFAIA the kernelbot didn't catch any of
> it either.
> 
> Out of curiosity, what's your compiler & compiler version?

% gcc --version
gcc (Debian 10.2.0-5) 10.2.0

% make O=... W=1 C=1 CF=-D__CHECK_ENDIAN__ -j64

> Alternatively we can make this a "proper" constant expression with the
> below.

It doesn't fix another static const in the same file, though that one requires
DEBUG_SCHED to be enabled.

Do you want me to split this fix and fix only the other case?

> ---
> 
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index 2d59ca77103e..b1331c4c48e9 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -27,9 +27,11 @@ enum {
> 
>  /* Generate a mask of SD flags with the SDF_NEEDS_GROUPS metaflag */
>  #define SD_FLAG(name, mflags) (name * !!((mflags) & SDF_NEEDS_GROUPS)) |
> -static const unsigned int SD_DEGENERATE_GROUPS_MASK =
> +enum {
> +	SD_DEGENERATE_GROUPS_MASK =
>  #include <linux/sched/sd_flags.h>
> -0;
> +0
> +};
>  #undef SD_FLAG
> 
>  #ifdef CONFIG_SCHED_DEBUG

-- 
With Best Regards,
Andy Shevchenko