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

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Date:   Mon, 30 Oct 2017 10:27:10 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Shile Zhang <shile.zhang@...ia.com>,
        linux-kernel@...r.kernel.org,
        Nick Desaulniers <nick.desaulniers@...il.com>,
        Douglas Anderson <dianders@...omium.org>,
        Guenter Roeck <groeck@...omium.org>
Subject: Re: [PATCH] sched/sysctl: Fix attributes of some extern declarations

El Mon, Oct 30, 2017 at 09:26:12AM +0100 Peter Zijlstra ha dit:

> On Fri, Oct 27, 2017 at 04:40:20PM -0700, Matthias Kaehlcke 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;
> 
> Since they're already declared in sched/sysctl.h this redeclaration in
> sched/sched.h seems pointless.

We could remove it, but it would require to move the definition of
const_debug to the 'globally' visible header sched/sysctl.h. Would
that be your preference? In that case we should probably use a more
unique name, like const_sched_debug.

> >   ./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
> > (or put it in a common header). 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>
> > ---
> >  include/linux/sched/sysctl.h | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/sched/sysctl.h b/include/linux/sched/sysctl.h
> > index 0f5ecd4d298e..f09afd55ecb5 100644
> > --- a/include/linux/sched/sysctl.h
> > +++ b/include/linux/sched/sysctl.h
> > @@ -1,6 +1,7 @@
> >  #ifndef _LINUX_SCHED_SYSCTL_H
> >  #define _LINUX_SCHED_SYSCTL_H
> >  
> > +#include <linux/static_key.h>
> >  #include <linux/types.h>
> >  
> >  struct ctl_table;
> 
> That extra include also seems entirely pointless.

Sorry, I wrongly associated the header with const_debug/__read_mostly
since it is included right before the definition of const_debug. I
should have double-checked.