El Tue, Oct 30, 2017 at 10:57:58AM +0100 Ingo Molnar ha dit: > * Matthias Kaehlcke <m...@chromium.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 <m...@chromium.org> > > Reviewed-by: Nick Desaulniers <nick.desaulni...@gmail.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.
It's certainly not ideal, and then again essentially the same is done in kernel/sched/sched.h, just that here the specific attribute is hidden behind const_debug. > 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. That can definitely be done. I don't have a really strong opinion on this, just wonder if the warning could be useful in other circumstances to spot conflicting declarations. This instance looks like a somewhat special case with the declarations in two header files and it might be worth to pay the price of the pointlessly duplicate attribute to get a useful warning in others. Adding kbuild maintainer Masahiro Yamada to the thread, who tends to be reluctant about disabling warnings globally.
