x86, trace: Introduce entering/exiting_irq()

When implementing tracepoints in interrupt handers, if the tracepoints are
simply added in the performance sensitive path of interrupt handers,
it may cause potential performance problem due to the time penalty.

To solve the problem, an idea is to prepare non-trace/trace irq handers and
switch their IDTs at the enabling/disabling time.

So, let's introduce entering_irq()/exiting_irq() for pre/post-
processing of each irq handler.

A way to use them is as follows.

Non-trace irq handler:
smp_irq_handler()
{
	entering_irq();		/* pre-processing of this handler */
	__smp_irq_handler();	/*
				 * common logic between non-trace and trace handlers
				 * in a vector.
				 */
	exiting_irq();		/* post-processing of this handler */

}

Trace irq_handler:
smp_trace_irq_handler()
{
	entering_irq();		/* pre-processing of this handler */
	trace_irq_entry();	/* tracepoint for irq entry */
	__smp_irq_handler();	/*
				 * common logic between non-trace and trace handlers
				 * in a vector.
				 */
	trace_irq_exit();	/* tracepoint for irq exit */
	exiting_irq();		/* post-processing of this handler */

}

If tracepoints can place outside entering_irq()/exiting_irq() as follows,
it looks cleaner.

smp_trace_irq_handler()
{
	trace_irq_entry();
	smp_irq_handler();
	trace_irq_exit();
}

But it doesn't work.
The problem is with irq_enter/exit() being called. They must be called before
trace_irq_enter/exit(),  because of the rcu_irq_enter() must be called before
any tracepoints are used, as tracepoints use  rcu to synchronize.

As a possible alternative, we may be able to call irq_enter() first as follows
if irq_enter() can nest.

smp_trace_irq_hander()
{
	irq_entry();
	trace_irq_entry();
	smp_irq_handler();
	trace_irq_exit();
	irq_exit();
}

But it doesn't work, either.
If irq_enter() is nested, it may have a time penalty because it has to check if it
was already called or not. The time penalty is not desired in performance sensitive
paths even if it is tiny.

Signed-off-by: Seiji Aguchi <seiji.aguchi@hds.com>
Link: http://lkml.kernel.org/r/51C3238D.9040706@hds.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 27 insertions, 8 deletions

diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
index 48d2b7ded42..d85837574a7 100644
--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -249,32 +249,51 @@ finish:
 /*
  * Reschedule call back.
  */
-void smp_reschedule_interrupt(struct pt_regs *regs)
+static inline void __smp_reschedule_interrupt(void)
 {
-	ack_APIC_irq();
 	inc_irq_stat(irq_resched_count);
 	scheduler_ipi();
+}
+
+void smp_reschedule_interrupt(struct pt_regs *regs)
+{
+	ack_APIC_irq();
+	__smp_reschedule_interrupt();
 	/*
 	 * KVM uses this interrupt to force a cpu out of guest mode
 	 */
 }
 
-void smp_call_function_interrupt(struct pt_regs *regs)
+static inline void call_function_entering_irq(void)
 {
 	ack_APIC_irq();
 	irq_enter();
+}
+
+static inline void __smp_call_function_interrupt(void)
+{
 	generic_smp_call_function_interrupt();
 	inc_irq_stat(irq_call_count);
-	irq_exit();
 }
 
-void smp_call_function_single_interrupt(struct pt_regs *regs)
+void smp_call_function_interrupt(struct pt_regs *regs)
+{
+	call_function_entering_irq();
+	__smp_call_function_interrupt();
+	exiting_irq();
+}
+
+static inline void __smp_call_function_single_interrupt(void)
 {
-	ack_APIC_irq();
-	irq_enter();
 	generic_smp_call_function_single_interrupt();
 	inc_irq_stat(irq_call_count);
-	irq_exit();
+}
+
+void smp_call_function_single_interrupt(struct pt_regs *regs)
+{
+	call_function_entering_irq();
+	__smp_call_function_single_interrupt();
+	exiting_irq();
 }
 
 static int __init nonmi_ipi_setup(char *str)