sched/clock: Delay switching sched_clock to stable

Currently we switch to the stable sched_clock if we guess the TSC is
usable, and then switch back to the unstable path if it turns out TSC
isn't stable during SMP bringup after all.

Delay switching to the stable path until after SMP bringup is
complete. This way we'll avoid switching during the time we detect the
worst of the TSC offences.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 26 insertions, 28 deletions

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index 5d6dd38b449c..b3466d4e0cc2 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -77,6 +77,11 @@ EXPORT_SYMBOL_GPL(sched_clock);
 
 __read_mostly int sched_clock_running;
 
+void sched_clock_init(void)
+{
+	sched_clock_running = 1;
+}
+
 #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
 static DEFINE_STATIC_KEY_FALSE(__sched_clock_stable);
 static int __sched_clock_stable_early;
@@ -96,12 +101,18 @@ void set_sched_clock_stable(void)
 {
 	__sched_clock_stable_early = 1;
 
-	smp_mb(); /* matches sched_clock_init() */
-
-	if (!sched_clock_running)
-		return;
+	smp_mb(); /* matches sched_clock_init_late() */
 
-	__set_sched_clock_stable();
+	/*
+	 * This really should only be called early (before
+	 * sched_clock_init_late()) when guestimating our sched_clock() is
+	 * solid.
+	 *
+	 * After that we test stability and we can negate our guess using
+	 * clear_sched_clock_stable, possibly from a watchdog.
+	 */
+	if (WARN_ON_ONCE(sched_clock_running == 2))
+		__set_sched_clock_stable();
 }
 
 static void __clear_sched_clock_stable(struct work_struct *work)
@@ -117,12 +128,10 @@ void clear_sched_clock_stable(void)
 {
 	__sched_clock_stable_early = 0;
 
-	smp_mb(); /* matches sched_clock_init() */
-
-	if (!sched_clock_running)
-		return;
+	smp_mb(); /* matches sched_clock_init_late() */
 
-	schedule_work(&sched_clock_work);
+	if (sched_clock_running == 2)
+		schedule_work(&sched_clock_work);
 }
 
 struct sched_clock_data {
@@ -143,20 +152,9 @@ static inline struct sched_clock_data *cpu_sdc(int cpu)
 	return &per_cpu(sched_clock_data, cpu);
 }
 
-void sched_clock_init(void)
+void sched_clock_init_late(void)
 {
-	u64 ktime_now = ktime_to_ns(ktime_get());
-	int cpu;
-
-	for_each_possible_cpu(cpu) {
-		struct sched_clock_data *scd = cpu_sdc(cpu);
-
-		scd->tick_raw = 0;
-		scd->tick_gtod = ktime_now;
-		scd->clock = ktime_now;
-	}
-
-	sched_clock_running = 1;
+	sched_clock_running = 2;
 
 	/*
 	 * Ensure that it is impossible to not do a static_key update.
@@ -362,11 +360,6 @@ EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
 
 #else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
 
-void sched_clock_init(void)
-{
-	sched_clock_running = 1;
-}
-
 u64 sched_clock_cpu(int cpu)
 {
 	if (unlikely(!sched_clock_running))
@@ -374,6 +367,7 @@ u64 sched_clock_cpu(int cpu)
 
 	return sched_clock();
 }
+
 #endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
 
 /*
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a129b34b8206..96a4267e6020 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7498,6 +7498,7 @@ void __init sched_init_smp(void)
 	init_sched_dl_class();
 
 	sched_init_smt();
+	sched_clock_init_late();
 
 	sched_smp_initialized = true;
 }
@@ -7513,6 +7514,7 @@ early_initcall(migration_init);
 void __init sched_init_smp(void)
 {
 	sched_init_granularity();
+	sched_clock_init_late();
 }
 #endif /* CONFIG_SMP */
 
@@ -7556,6 +7558,8 @@ void __init sched_init(void)
 	int i, j;
 	unsigned long alloc_size = 0, ptr;
 
+	sched_clock_init();
+
 	for (i = 0; i < WAIT_TABLE_SIZE; i++)
 		init_waitqueue_head(bit_wait_table + i);