From 3c829c367a1a52550378584a657768217971e587 Mon Sep 17 00:00:00 2001 From: Tim Chen Date: Sun, 30 Jul 2006 03:04:02 -0700 Subject: [PATCH] [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 Acked-by: Ingo Molnar Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/softirq.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/kernel/softirq.c b/kernel/softirq.c index aab880677c..3789ca9819 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -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); -- 2.39.5