linux-kernel - Re: [PATCH v2] sched/sysctl: Fix attributes of some extern declarations

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Date:   Tue, 31 Oct 2017 10:57:58 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Shile Zhang <shile.zhang@...ia.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Nick Desaulniers <nick.desaulniers@...il.com>,
        Douglas Anderson <dianders@...omium.org>,
        Guenter Roeck <groeck@...omium.org>
Subject: Re: [PATCH v2] sched/sysctl: Fix attributes of some extern
 declarations


* Matthias Kaehlcke <mka@...omium.org> wrote:

> The definition of sysctl_sched_migration_cost, sysctl_sched_nr_migrate
> and sysctl_sched_time_avg includes the attribute const_debug. This
> attribute is not part of the extern declaration of these variables in
> include/linux/sched/sysctl.h, as a result clang generates warnings like
> this:
> 
>   kernel/sched/sched.h:1618:33: warning: section attribute is specified on
>     redeclared variable [-Wsection]
>   extern const_debug unsigned int sysctl_sched_time_avg;
>                                 ^
>   ./include/linux/sched/sysctl.h:42:21: note: previous declaration is here
>   extern unsigned int sysctl_sched_time_avg;
> 
> The header only declares the variables when CONFIG_SCHED_DEBUG is defined,
> therefore it is not necessary to duplicate the definition of const_debug.
> Instead we can use the attribute __read_mostly, which is the expansion of
> const_debug when CONFIG_SCHED_DEBUG is set.
> 
> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> Reviewed-by: Nick Desaulniers <nick.desaulniers@...il.com>
> ---
> Changes in v2:
> - removed pointless include of linux/static_key.h
> - added Reviewed-by tag
> 
>  include/linux/sched/sysctl.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/sched/sysctl.h b/include/linux/sched/sysctl.h
> index 0f5ecd4d298e..d34c823f3d36 100644
> --- a/include/linux/sched/sysctl.h
> +++ b/include/linux/sched/sysctl.h
> @@ -37,9 +37,9 @@ extern unsigned int sysctl_numa_balancing_scan_period_max;
>  extern unsigned int sysctl_numa_balancing_scan_size;
>  
>  #ifdef CONFIG_SCHED_DEBUG
> -extern unsigned int sysctl_sched_migration_cost;
> -extern unsigned int sysctl_sched_nr_migrate;
> -extern unsigned int sysctl_sched_time_avg;
> +extern __read_mostly unsigned int sysctl_sched_migration_cost;
> +extern __read_mostly unsigned int sysctl_sched_nr_migrate;
> +extern __read_mostly unsigned int sysctl_sched_time_avg;

So I hate this change, because it pointlessly duplicates an attribute that should 
only matter at the definition site. The Clang warning:

>   kernel/sched/sched.h:1618:33: warning: section attribute is specified on
>     redeclared variable [-Wsection]

suggests that the -Wsection warning can be turned off. The Clang build should 
probably do that.

Thanks,

	Ingo