[RFC][PATCH 1/4] clocksource: Add optional cs::mark_unstable() method

[Peter Zijlstra <peterz@infradead.org>]

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From: Thomas Gleixner <tglx@linutronix.de>

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: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/tsc.c       |   11 +++++++++++
 include/linux/clocksource.h |    3 +++
 kernel/time/clocksource.c   |    4 ++++
 3 files changed, 18 insertions(+)

--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1101,6 +1101,16 @@ static cycle_t read_tsc(struct clocksour
 	return (cycle_t)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()
  */
@@ -1112,6 +1122,7 @@ static struct clocksource clocksource_ts
 	.flags                  = CLOCK_SOURCE_IS_CONTINUOUS |
 				  CLOCK_SOURCE_MUST_VERIFY,
 	.archdata               = { .vclock_mode = VCLOCK_TSC },
+	.mark_unstable		= tsc_cs_mark_unstable,
 };
 
 void mark_tsc_unstable(char *reason)
--- a/include/linux/clocksource.h
+++ b/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
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -140,6 +140,10 @@ static void __clocksource_unstable(struc
 {
 	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);
 }