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Subject[PATCH 15/15] x86: Avoid a runtime condition in native_sched_clock()
Use a static_key to avoid touching tsc_disabled and a runtime
condition in native_sched_clock() -- less cachelines touched is always
better.

MAINLINE PRE POST

sched_clock_stable: 1 1 1
(cold) sched_clock: 329841 215295 213039
(cold) local_clock: 301773 220773 216084
(warm) sched_clock: 38375 25659 25231
(warm) local_clock: 100371 27242 27601
(warm) rdtsc: 27340 24208 24203
sched_clock_stable: 0 0 0
(cold) sched_clock: 382634 237019 240055
(cold) local_clock: 396890 294819 299942
(warm) sched_clock: 38194 25609 25276
(warm) local_clock: 143452 71232 73232
(warm) rdtsc: 27345 24243 24244

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;

/*
@@ -243,7 +246,7 @@ 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);
}
@@ -1152,7 +1155,9 @@ 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();



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