sched/clock, clocksource: Add optional cs::mark_unstable() method

PeterZ reported that we'd fail to mark the TSC unstable when the
clocksource watchdog finds it unsuitable.

Allow a clocksource to run a custom action when its being marked
unstable and hook up the TSC unstable code.
Reported-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/tsc.c

@@ -1106,6 +1106,16 @@ static u64 read_tsc(struct clocksource *cs)
 	return (u64)rdtsc_ordered();
 }
 
+static void tsc_cs_mark_unstable(struct clocksource *cs)
+{
+	if (tsc_unstable)
+		return;
+	tsc_unstable = 1;
+	clear_sched_clock_stable();
+	disable_sched_clock_irqtime();
+	pr_info("Marking TSC unstable due to clocksource watchdog\n");
+}
+
 /*
  * .mask MUST be CLOCKSOURCE_MASK(64). See comment above read_tsc()
  */
@@ -1118,6 +1128,7 @@ static struct clocksource clocksource_tsc = {
 				  CLOCK_SOURCE_MUST_VERIFY,
 	.archdata               = { .vclock_mode = VCLOCK_TSC },
 	.resume			= tsc_resume,
+	.mark_unstable		= tsc_cs_mark_unstable,
 };
 
 void mark_tsc_unstable(char *reason)

include/linux/clocksource.h

@@ -62,6 +62,8 @@ struct module;
  * @archdata:		arch-specific data
  * @suspend:		suspend function for the clocksource, if necessary
  * @resume:		resume function for the clocksource, if necessary
+ * @mark_unstable:	Optional function to inform the clocksource driver that
+ *			the watchdog marked the clocksource unstable
  * @owner:		module reference, must be set by clocksource in modules
  *
  * Note: This struct is not used in hotpathes of the timekeeping code
@@ -93,6 +95,7 @@ struct clocksource {
 	unsigned long flags;
 	void (*suspend)(struct clocksource *cs);
 	void (*resume)(struct clocksource *cs);
+	void (*mark_unstable)(struct clocksource *cs);
 
 	/* private: */
 #ifdef CONFIG_CLOCKSOURCE_WATCHDOG

kernel/time/clocksource.c

@@ -141,6 +141,10 @@ static void __clocksource_unstable(struct clocksource *cs)
 {
 	cs->flags &= ~(CLOCK_SOURCE_VALID_FOR_HRES | CLOCK_SOURCE_WATCHDOG);
 	cs->flags |= CLOCK_SOURCE_UNSTABLE;
+
+	if (cs->mark_unstable)
+		cs->mark_unstable(cs);
+
 	if (finished_booting)
 		schedule_work(&watchdog_work);
 }