[RFC][PATCH 3/7] x86: Avoid a runtime condition in native_sched_clock()

[Peter Zijlstra <peterz@infradead.org>]

[-- Attachment #1: peterz-tsc-static_key.patch --]
[-- Type: text/plain, Size: 1397 bytes --]

Use a static_key to avoid a runtime condition in native_sched_clock().

XXX: I still think tsc_disabled should die a horrid death.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
---
 arch/x86/kernel/tsc.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -11,6 +11,7 @@
 #include <linux/clocksource.h>
 #include <linux/percpu.h>
 #include <linux/timex.h>
+#include <linux/static_key.h>
 
 #include <asm/hpet.h>
 #include <asm/timer.h>
@@ -37,6 +38,8 @@ static int __read_mostly tsc_unstable;
    erroneous rdtsc usage on !cpu_has_tsc processors */
 static int __read_mostly tsc_disabled = -1;
 
+static struct static_key __use_tsc = STATIC_KEY_INIT;
+
 int tsc_clocksource_reliable;
 /*
  * Scheduler clock - returns current time in nanosec units.
@@ -53,7 +56,7 @@ u64 native_sched_clock(void)
 	 *   very important for it to be as fast as the platform
 	 *   can achieve it. )
 	 */
-	if (unlikely(tsc_disabled)) {
+	if (static_key_false(&__use_tsc)) {
 		/* No locking but a rare wrong value is not a big deal: */
 		return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
 	}
@@ -1002,7 +1005,9 @@ void __init tsc_init(void)
 		return;
 
 	/* now allow native_sched_clock() to use rdtsc */
+
 	tsc_disabled = 0;
+	static_key_slow_inc(&__use_tsc);
 
 	if (!no_sched_irq_time)
 		enable_sched_clock_irqtime();