[RFC][PATCH 7/7] x86, tsc: Employ static_branch_likely()

[Peter Zijlstra <peterz@infradead.org>]

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Because of the static_key restrictions we had to take an unconditional
jump for the most likely case, causing $I bloat.

Rewrite to use the new primitives.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/tsc.c |   22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -38,7 +38,7 @@ 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;
+static DEFINE_STATIC_KEY_FALSE(__use_tsc);
 
 int tsc_clocksource_reliable;
 
@@ -274,7 +274,12 @@ static void set_cyc2ns_scale(unsigned lo
  */
 u64 native_sched_clock(void)
 {
-	u64 tsc_now;
+	if (static_branch_likely(&__use_tsc)) {
+		u64 tsc_now = rdtsc();
+
+		/* return the value in ns */
+		return cycles_2_ns(tsc_now);
+	}
 
 	/*
 	 * Fall back to jiffies if there's no TSC available:
@@ -284,16 +289,9 @@ u64 native_sched_clock(void)
 	 *   very important for it to be as fast as the platform
 	 *   can achieve it. )
 	 */
-	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);
-	}
-
-	/* read the Time Stamp Counter: */
-	tsc_now = rdtsc();
 
-	/* return the value in ns */
-	return cycles_2_ns(tsc_now);
+	/* No locking but a rare wrong value is not a big deal: */
+	return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
 }
 
 /* We need to define a real function for sched_clock, to override the
@@ -1204,7 +1202,7 @@ void __init tsc_init(void)
 	/* now allow native_sched_clock() to use rdtsc */
 
 	tsc_disabled = 0;
-	static_key_slow_inc(&__use_tsc);
+	static_branch_enable(&__use_tsc);
 
 	if (!no_sched_irq_time)
 		enable_sched_clock_irqtime();