[PATCH 9/9] sched/clock: Print a warning recommending tsc=unstable

[Peter Zijlstra]

With our switch to stable delayed until late_initcall(), the most
likely cause of hitting mark_tsc_unstable() is the watchdog. The
watchdog typically only triggers when creative BIOS'es fiddle with the
TSC to hide SMI latency.

Since the watchdog can only detect TSC fiddling after the fact all TSC
clocks (including userspace GTOD) can already have reported funny
values.

The only way to fully avoid this, is manually marking the TSC unstable
at boot. Suggest people do this on their broken systems.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/clock.c |    1 +
 1 file changed, 1 insertion(+)

--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -175,6 +175,7 @@ static void __sched_clock_work(struct wo
 	for_each_possible_cpu(cpu)
 		per_cpu(sched_clock_data, cpu) = *scd;
 
+	printk(KERN_WARNING "TSC found unstable after boot, most likely due to broken BIOS. Use 'tsc=unstable'.\n");
 	printk(KERN_INFO "sched_clock: Marking unstable (%lld, %lld)<-(%lld, %lld)\n",
 			scd->tick_gtod, __gtod_offset,
 			scd->tick_raw,  __sched_clock_offset);