add touch_all_softlockup_watchdogs()

Add touch_all_softlockup_watchdogs() to allow the softlockup watchdog
timers on all cpus to be updated.  This is used to prevent sysrq-t from
generating a spurious watchdog message when generating lots of output.

Softlockup watchdogs use sched_clock() as its timebase, which is inherently
per-cpu (at least, when it is measuring unstolen time).  Because of this,
it isn't possible for one CPU to directly update the other CPU's timers,
but it is possible to tell the other CPUs to do update themselves
appropriately.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Acked-by: Chris Lalancette <clalance@redhat.com>
Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: Rick Lindsley <ricklind@us.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 15 insertions, 2 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 0227f1625a75..5530ed211f72 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4750,6 +4750,8 @@ void show_state_filter(unsigned long state_filter)
                         show_task(p);
         } while_each_thread(g, p);
 
+        touch_all_softlockup_watchdogs();
+
         read_unlock(&tasklist_lock);
         /*
          * Only show locks if all tasks are dumped:
diff --git a/kernel/softlockup.c b/kernel/softlockup.c
index 5ea631742dbc..8fa7040247ad 100644
--- a/kernel/softlockup.c
+++ b/kernel/softlockup.c
@@ -50,6 +50,16 @@ void touch_softlockup_watchdog(void)
 }
 EXPORT_SYMBOL(touch_softlockup_watchdog);
 
+void touch_all_softlockup_watchdogs(void)
+{
+        int cpu;
+
+        /* Cause each CPU to re-update its timestamp rather than complain */
+        for_each_online_cpu(cpu)
+                per_cpu(touch_timestamp, cpu) = 0;
+}
+EXPORT_SYMBOL(touch_all_softlockup_watchdogs);
+
 /*
  * This callback runs from the timer interrupt, and checks
  * whether the watchdog thread has hung or not:
@@ -61,9 +71,10 @@ void softlockup_tick(void)
         unsigned long print_timestamp;
         unsigned long now;
 
-        /* watchdog task hasn't updated timestamp yet */
-        if (touch_timestamp == 0)
+        if (touch_timestamp == 0) {
+                touch_softlockup_watchdog();
                 return;
+        }
 
         print_timestamp = per_cpu(print_timestamp, this_cpu);