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[PATCH] Reducing local_bh_enable/disable overhead in irqtrace

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The recent changes from irqtrace feature has added overheads to
local_bh_disable and local_bh_enable that reduces UDP performance across
x86_64 and IA64, even though IA64 does not support the irqtrace feature.
Patch in question is

[PATCH]lockdep: irqtrace subsystem, core
http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=c
ommit;h=de30a2b355ea85350ca2f58f3b9bf4e5bc007986

Prior to this patch, local_bh_disable was a short macro.  Now it is a
function which calls __local_bh_disable with added irq flags save and
restore.  The irq flags save and restore were also added to
local_bh_enable, probably for injecting the trace irqs code.

This overhead is on the generic code path across all architectures.  On a
IA_64 test machine (Itanium-2 1.6 GHz) running a benchmark like netperf's
UDP streaming test, the added overhead results in a drop of 3% in
throughput, as udp_sendmsg calls the local_bh_enable/disable several times.

Other workloads that have heavy usages of local_bh_enable/disable could
also be affected.  The patch ideally should not have affected IA-64
performance as it does not have IRQ tracing support.  A significant portion
of the overhead is in the added irq flags save and restore, which I think
is not needed if IRQ tracing is unused.  A suggested patch is attached
below that recovers the lost performance.  However, the "ifdef"s in the
patch are a bit ugly.

Signed-off-by: Tim Chen <tim.c.chen@intel.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Tim Chen 2006-07-30 03:04:02 -07:00 committed by Linus Torvalds
parent 953a7f2066
commit 3c829c367a
1 changed files with 18 additions and 0 deletions
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18
kernel/softirq.c
View file

@ -65,6 +65,7 @@ static inline void wakeup_softirqd(void)
* This one is for softirq.c-internal use, * This one is for softirq.c-internal use,
* where hardirqs are disabled legitimately: * where hardirqs are disabled legitimately:
*/ */
#ifdef CONFIG_TRACE_IRQFLAGS
static void __local_bh_disable(unsigned long ip) static void __local_bh_disable(unsigned long ip)
{ {
unsigned long flags; unsigned long flags;
@ -80,6 +81,13 @@ static void __local_bh_disable(unsigned long ip)
trace_softirqs_off(ip); trace_softirqs_off(ip);
raw_local_irq_restore(flags); raw_local_irq_restore(flags);
} }
#else /* !CONFIG_TRACE_IRQFLAGS */
static inline void __local_bh_disable(unsigned long ip)
{
add_preempt_count(SOFTIRQ_OFFSET);
barrier();
}
#endif /* CONFIG_TRACE_IRQFLAGS */
void local_bh_disable(void) void local_bh_disable(void)
{ {
@ -121,12 +129,16 @@ EXPORT_SYMBOL(_local_bh_enable);
void local_bh_enable(void) void local_bh_enable(void)
{ {
#ifdef CONFIG_TRACE_IRQFLAGS
unsigned long flags; unsigned long flags;
WARN_ON_ONCE(in_irq()); WARN_ON_ONCE(in_irq());
#endif
WARN_ON_ONCE(irqs_disabled()); WARN_ON_ONCE(irqs_disabled());
#ifdef CONFIG_TRACE_IRQFLAGS
local_irq_save(flags); local_irq_save(flags);
#endif
/* /*
* Are softirqs going to be turned on now: * Are softirqs going to be turned on now:
*/ */
@ -142,18 +154,22 @@ void local_bh_enable(void)
do_softirq(); do_softirq();
dec_preempt_count(); dec_preempt_count();
#ifdef CONFIG_TRACE_IRQFLAGS
local_irq_restore(flags); local_irq_restore(flags);
#endif
preempt_check_resched(); preempt_check_resched();
} }
EXPORT_SYMBOL(local_bh_enable); EXPORT_SYMBOL(local_bh_enable);
void local_bh_enable_ip(unsigned long ip) void local_bh_enable_ip(unsigned long ip)
{ {
#ifdef CONFIG_TRACE_IRQFLAGS
unsigned long flags; unsigned long flags;
WARN_ON_ONCE(in_irq()); WARN_ON_ONCE(in_irq());
local_irq_save(flags); local_irq_save(flags);
#endif
/* /*
* Are softirqs going to be turned on now: * Are softirqs going to be turned on now:
*/ */
@ -169,7 +185,9 @@ void local_bh_enable_ip(unsigned long ip)
do_softirq(); do_softirq();
dec_preempt_count(); dec_preempt_count();
#ifdef CONFIG_TRACE_IRQFLAGS
local_irq_restore(flags); local_irq_restore(flags);
#endif
preempt_check_resched(); preempt_check_resched();
} }
EXPORT_SYMBOL(local_bh_enable_ip); EXPORT_SYMBOL(local_bh_enable_ip);