[PATCH] Reducing local_bh_enable/disable overhead in irqtrace
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>
diff --git a/kernel/softirq.c b/kernel/softirq.c
index aab8806..3789ca9 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -65,6 +65,7 @@
* This one is for softirq.c-internal use,
* where hardirqs are disabled legitimately:
*/
+#ifdef CONFIG_TRACE_IRQFLAGS
static void __local_bh_disable(unsigned long ip)
{
unsigned long flags;
@@ -80,6 +81,13 @@
trace_softirqs_off(ip);
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)
{
@@ -121,12 +129,16 @@
void local_bh_enable(void)
{
+#ifdef CONFIG_TRACE_IRQFLAGS
unsigned long flags;
WARN_ON_ONCE(in_irq());
+#endif
WARN_ON_ONCE(irqs_disabled());
+#ifdef CONFIG_TRACE_IRQFLAGS
local_irq_save(flags);
+#endif
/*
* Are softirqs going to be turned on now:
*/
@@ -142,18 +154,22 @@
do_softirq();
dec_preempt_count();
+#ifdef CONFIG_TRACE_IRQFLAGS
local_irq_restore(flags);
+#endif
preempt_check_resched();
}
EXPORT_SYMBOL(local_bh_enable);
void local_bh_enable_ip(unsigned long ip)
{
+#ifdef CONFIG_TRACE_IRQFLAGS
unsigned long flags;
WARN_ON_ONCE(in_irq());
local_irq_save(flags);
+#endif
/*
* Are softirqs going to be turned on now:
*/
@@ -169,7 +185,9 @@
do_softirq();
dec_preempt_count();
+#ifdef CONFIG_TRACE_IRQFLAGS
local_irq_restore(flags);
+#endif
preempt_check_resched();
}
EXPORT_SYMBOL(local_bh_enable_ip);
