Ignore stolen time in the softlockup watchdog

The softlockup watchdog is currently a nuisance in a virtual machine, since
the whole system could have the CPU stolen from it for a long period of
time. While it would be unlikely for a guest domain to be denied timer
interrupts for over 10s, it could happen and any softlockup message would
be completely spurious.

Earlier I proposed that sched_clock() return time in unstolen nanoseconds,
which is how Xen and VMI currently implement it. If the softlockup
watchdog uses sched_clock() to measure time, it would automatically ignore
stolen time, and therefore only report when the guest itself locked up.
When running native, sched_clock() returns real-time nanoseconds, so the
behaviour would be unchanged.

Note that sched_clock() used this way is inherently per-cpu, so this patch
makes sure that the per-processor watchdog thread initialized its own
timestamp.

Signed-off-by: Jeremy Fitzhardinge <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: john stultz <[e-mail]>
Cc: Zachary Amsden <[e-mail]>
Cc: James Morris <[e-mail]>
Cc: Dan Hecht <[e-mail]>
Cc: Paul Mackerras <[e-mail]>
Cc: Martin Schwidefsky <[e-mail]>
Cc: Prarit Bhargava <[e-mail]>
Cc: Chris Lalancette <[e-mail]>
Cc: Rick Lindsley <[e-mail]>
Cc: Eric Dumazet <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/softlockup.c

···
 	.notifier_call = softlock_panic,
 };
 
+/*
+ * Returns seconds, approximately.  We don't need nanosecond
+ * resolution, and we don't need to waste time with a big divide when
+ * 2^30ns == 1.074s.
+ */
+static unsigned long get_timestamp(void)
+{
+	return sched_clock() >> 30;  /* 2^30 ~= 10^9 */
+}
+
 void touch_softlockup_watchdog(void)
 {
-	__raw_get_cpu_var(touch_timestamp) = jiffies;
+	__raw_get_cpu_var(touch_timestamp) = get_timestamp();
 }
 EXPORT_SYMBOL(touch_softlockup_watchdog);
 
···
 {
 	int this_cpu = smp_processor_id();
 	unsigned long touch_timestamp = per_cpu(touch_timestamp, this_cpu);
+	unsigned long print_timestamp;
+	unsigned long now;
 
-	/* prevent double reports: */
-	if (per_cpu(print_timestamp, this_cpu) == touch_timestamp ||
+	/* watchdog task hasn't updated timestamp yet */
+	if (touch_timestamp == 0)
+		return;
+
+	print_timestamp = per_cpu(print_timestamp, this_cpu);
+
+	/* report at most once a second */
+	if (print_timestamp < (touch_timestamp + 1) ||
 		did_panic ||
 			!per_cpu(watchdog_task, this_cpu))
 		return;
···
 		return;
 	}
 
+	now = get_timestamp();
+
 	/* Wake up the high-prio watchdog task every second: */
-	if (time_after(jiffies, touch_timestamp + HZ))
+	if (now > (touch_timestamp + 1))
 		wake_up_process(per_cpu(watchdog_task, this_cpu));
 
 	/* Warn about unreasonable 10+ seconds delays: */
-	if (time_after(jiffies, touch_timestamp + 10*HZ)) {
+	if (now > (touch_timestamp + 10)) {
 		per_cpu(print_timestamp, this_cpu) = touch_timestamp;
 
 		spin_lock(&print_lock);
···
 
 	sched_setscheduler(current, SCHED_FIFO, &param);
 	current->flags |= PF_NOFREEZE;
+
+	/* initialize timestamp */
+	touch_softlockup_watchdog();
 
 	/*
 	 * Run briefly once per second to reset the softlockup timestamp.
···
 			printk("watchdog for %i failed\n", hotcpu);
 			return NOTIFY_BAD;
 		}
-  		per_cpu(touch_timestamp, hotcpu) = jiffies;
+  		per_cpu(touch_timestamp, hotcpu) = 0;
   		per_cpu(watchdog_task, hotcpu) = p;
 		kthread_bind(p, hotcpu);
  		break;