Re: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression

Tim Chen
2006-10-04T00:08:41Z

Sorry my mailer screwed up the line-wrap. Here's a resent of the patch.

Tim

The patch "Let WARN_ON/WARN_ON_ONCE return the condition"
http://kernel.org/git/?
p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=684f978347deb42d180373ac4c427f82ef963171

introduced 40% more 2nd level cache miss to tbench workload
being run in a loop back mode on a Core 2 machine. I think the
introduction of the local variables to WARN_ON and WARN_ON_ONCE

typeof(x) __ret_warn_on = (x);
typeof(condition) __ret_warn_once = (condition);

results in the extra cache misses. In our test workload profile, we see
heavily used functions like do_softirq and local_bh_enable
takes a lot longer to execute.

The modification below helps fix the problem. I made a slight
modification to sched.c to get around a gcc bug.

Signed-off-by: Tim Chen <tim.c.chen@intel.com>
diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index a525089..05ed388 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -17,13 +17,12 @@ #endif

#ifndef HAVE_ARCH_WARN_ON
#define WARN_ON(condition) ({ \
- typeof(condition) __ret_warn_on = (condition); \
- if (unlikely(__ret_warn_on)) { \
+ if (unlikely(condition)) { \
printk("BUG: warning at %s:%d/%s()\n", __FILE__, \
__LINE__, __FUNCTION__); \
dump_stack(); \
} \
- unlikely(__ret_warn_on); \
+ unlikely(condition); \
})
#endif

@@ -43,12 +42,16 @@ #endif

#define WARN_ON_ONCE(condition) ({ \
static int __warn_once = 1; \
- typeof(condition) __ret_warn_once = (condition);\
\
- if (likely(__warn_once)) \
- if (WARN_ON(__ret_warn_once)) \
- __warn_once = 0; \
- unlikely(__ret_warn_once); \
+ if (unlikely(condition)){ \
+ if (likely(__warn_once)){ \
+ __warn_once=0; \
+ printk("BUG: warning at %s:%d/%s()\n", __FILE__, \
+ __LINE__, __FUNCTION__);\
+ dump_stack(); \
+ } \
+ } \
+ unlikely(condition); \
})

#ifdef CONFIG_SMP
diff --git a/kernel/sched.c b/kernel/sched.c
index 5c848fd..8ae972c 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5629,7 +5629,8 @@ static unsigned long domain_distance(int
struct sched_domain *sd;

for_each_domain(cpu1, sd) {
- WARN_ON(!cpu_isset(cpu1, sd->span));
+ if (unlikely(!cpu_isset(cpu1, sd->span)))
+ WARN_ON(1);
if (cpu_isset(cpu2, sd->span))
return distance;
distance++;