Commit 3c829c36 authored Jul 30, 2006 by Tim Chen Committed by Linus Torvalds Jul 31, 2006

[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=de30a2b3

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>

parent 953a7f20

kernel/softirq.c

+18 −0

Original line number	Diff line number	Diff line
		@@ -65,6 +65,7 @@ static inline void wakeup_softirqd(void)
		* 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 @@ static void __local_bh_disable(unsigned long ip)
		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 @@ EXPORT_SYMBOL(_local_bh_enable);

		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 @@ void local_bh_enable(void)
		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 @@ void local_bh_enable_ip(unsigned long ip)
		do_softirq();

		dec_preempt_count();
		#ifdef CONFIG_TRACE_IRQFLAGS
		local_irq_restore(flags);
		#endif
		preempt_check_resched();
		}
		EXPORT_SYMBOL(local_bh_enable_ip);

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