From 7708d5f04de4dd5d2110df3244372b1e3f61bc7c Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri, 21 Apr 2017 12:52:52 +0200
Subject: [PATCH] sched/clock: Print a warning recommending 'tsc=unstable'
Git-commit: 7708d5f04de4dd5d2110df3244372b1e3f61bc7c
Patch-mainline: v4.13-rc1
References: bsc#1052904

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@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>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>

---
 kernel/sched/clock.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index a2f847c6ada8..1a0d389d2f2b 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -175,6 +175,7 @@ static void __sched_clock_work(struct work_struct *work)
 	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);
-- 
2.12.0