Hi,
On Mon, May 16, 2011 at 3:03 PM, <Valdis.Kletnieks@xxxxxx> wrote:
> On Fri, 13 May 2011 10:09:09 +0200, Jean-Christophe PLAGNIOL-VILLARD said:
>> On 10:52 Mon 09 May , Michal Marek wrote:
>> > Do you have proof of concept patches that make use of the
>> > config_is_xxx macros? Acked by the respective subsystem maintainers?
>> > It would be a good idea to send them along to show that this feature
>> > is going to be actually used.
>> I've seen thousands of place in the kernel we can use
>> so I'll just take one example on x86
>>
>> the patch attached is just an example
>
> Out of curiosity, will this Do The Right Thing for cases where things simply won't
> build for some configs? For example, consider this code snippet from kernel/timer.c,
> in __mod_timer() (near line 682):
>
> debug_activate(timer, expires);
>
> cpu = smp_processor_id();
>
> #if defined(CONFIG_NO_HZ) && defined(CONFIG_SMP)
> if (!pinned && get_sysctl_timer_migration() && idle_cpu(cpu))
> cpu = get_nohz_timer_target();
> #endif
> new_base = per_cpu(tvec_bases, cpu);
>
> If you convert this to an if statement, will it still compile? Which will
> happen first, dead code elimination, or the warning that get_nohz_timer_target()
> is an implicit declaration because the definition in the .h file is also
> guarded by #ifdef CONFIG_NO_HZ?
>
I already exposed this case, but let's prove it:
% grep CONFIG_SMP .config
# CONFIG_SMP is not set
% git diff
diff --git a/kernel/timer.c b/kernel/timer.c
index fd61986..ea4a5ba 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -681,10 +681,8 @@ __mod_timer(struct timer_list *timer, unsigned
long expires,
cpu = smp_processor_id();
-#if defined(CONFIG_NO_HZ) && defined(CONFIG_SMP)
- if (!pinned && get_sysctl_timer_migration() && idle_cpu(cpu))
+ if (0 && 0 && !pinned && get_sysctl_timer_migration() && idle_cpu(cpu))
cpu = get_nohz_timer_target();
-#endif
new_base = per_cpu(tvec_bases, cpu);
% gmake kernel/timer.o
CHK include/linux/version.h
CHK include/generated/utsrelease.h
CALL scripts/checksyscalls.sh
CC kernel/timer.o
kernel/timer.c: In function '__mod_timer':
kernel/timer.c:685:3: error: implicit declaration of function
'get_nohz_timer_target'
gmake[1]: *** [kernel/timer.o] Error 1
gmake: *** [kernel/timer.o] Error 2
- Arnaud
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