[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 <[EMAIL PROTECTED]>
Cc: Ingo Molnar <[EMAIL PROTECTED]>
Cc: Thomas Gleixner <[EMAIL PROTECTED]>
Cc: john stultz <[EMAIL PROTECTED]>
Cc: Zachary Amsden <[EMAIL PROTECTED]>
Cc: James Morris <[EMAIL PROTECTED]>
Cc: Dan Hecht <[EMAIL PROTECTED]>
Cc: Paul Mackerras <[EMAIL PROTECTED]>
Cc: Martin Schwidefsky <[EMAIL PROTECTED]>
Cc: Prarit Bhargava <[EMAIL PROTECTED]>
Cc: Chris Lalancette <[EMAIL PROTECTED]>
Cc: Rick Lindsley <[EMAIL PROTECTED]>

---
 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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