[for-next][PATCH 7/7] ftrace/x86: Do no run CPU sync when there is only one CPU online

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>

Moving enabling of function tracing to early boot, even before scheduling is
enabled, means that it is not safe to enable interrupts. When function
tracing was enabled at boot up, it use to happen after scheduling and the
other CPUs were brought up. That required running a sync across all CPUs
when modifying the function hook locations in the code. To do the
synchronization, interrupts had to be enabled. Now function tracing can be
started before the other CPUs are brought up, and enabling interrupts in
that case is dangerous. As only tho boot CPU is active, there is no reason
to run the synchronization. If the online CPU count is one, do not bother
doing the synchronization. This removes the need to enable interrupts.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 arch/x86/kernel/ftrace.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 8f3d9cf26ff9..70945fbd1258 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -533,7 +533,13 @@ static void do_sync_core(void *data)
 
 static void run_sync(void)
 {
-	int enable_irqs = irqs_disabled();
+	int enable_irqs;
+
+	/* No need to sync if there's only one CPU */
+	if (num_online_cpus() == 1)
+		return;
+
+	enable_irqs = irqs_disabled();
 
 	/* We may be called with interrupts disabled (on bootup). */
 	if (enable_irqs)
-- 
2.10.2