[patch 1/2] Ignore stolen time in the softlockup watchdog

[Jeremy Fitzhardinge]

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 <jeremy@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: john stultz <johnstul@xxxxxxxxxx>
Cc: Zachary Amsden <zach@xxxxxxxxxx>
Cc: James Morris <jmorris@xxxxxxxxx>
Cc: Dan Hecht <dhecht@xxxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Cc: Prarit Bhargava <prarit@xxxxxxxxxx>
Cc: Chris Lalancette <clalance@xxxxxxxxxx>
Cc: Rick Lindsley <ricklind@xxxxxxxxxx>

---
 kernel/softlockup.c |   28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

===================================================================
--- a/kernel/softlockup.c
+++ b/kernel/softlockup.c
@@ -17,8 +17,8 @@
 
 static DEFINE_SPINLOCK(print_lock);
 
-static DEFINE_PER_CPU(unsigned long, touch_timestamp);
-static DEFINE_PER_CPU(unsigned long, print_timestamp);
+static DEFINE_PER_CPU(unsigned long long, touch_timestamp);
+static DEFINE_PER_CPU(unsigned long long, print_timestamp);
 static DEFINE_PER_CPU(struct task_struct *, watchdog_task);
 
 static int did_panic = 0;
@@ -37,7 +37,7 @@ static struct notifier_block panic_block
 
 void touch_softlockup_watchdog(void)
 {
-	__raw_get_cpu_var(touch_timestamp) = jiffies;
+	__raw_get_cpu_var(touch_timestamp) = sched_clock();
 }
 EXPORT_SYMBOL(touch_softlockup_watchdog);
 
@@ -48,10 +48,15 @@ void softlockup_tick(void)
 void softlockup_tick(void)
 {
 	int this_cpu = smp_processor_id();
-	unsigned long touch_timestamp = per_cpu(touch_timestamp, this_cpu);
+	unsigned long long touch_timestamp = per_cpu(touch_timestamp, this_cpu);
+	unsigned long 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;
+
+	/* report at most once a second */
+	if (per_cpu(print_timestamp, this_cpu) < (touch_timestamp + NSEC_PER_SEC) ||
 		did_panic ||
 			!per_cpu(watchdog_task, this_cpu))
 		return;
@@ -62,12 +67,14 @@ void softlockup_tick(void)
 		return;
 	}
 
+	now = sched_clock();
+
 	/* Wake up the high-prio watchdog task every second: */
-	if (time_after(jiffies, touch_timestamp + HZ))
+	if (now > (touch_timestamp + NSEC_PER_SEC))
 		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 + 10ull*NSEC_PER_SEC)) {
 		per_cpu(print_timestamp, this_cpu) = touch_timestamp;
 
 		spin_lock(&print_lock);
@@ -87,6 +94,9 @@ static int watchdog(void * __bind_cpu)
 
 	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.
@@ -120,7 +130,7 @@ cpu_callback(struct notifier_block *nfb,
 			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;

-- 

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