clockevents: prevent stale tick update on offline cpu

Taking a cpu offline removes the cpu from the online mask before the
CPU_DEAD notification is done. The clock events layer does the cleanup
of the dead CPU from the CPU_DEAD notifier chain. tick_do_timer_cpu is
used to avoid xtime lock contention by assigning the task of jiffies
xtime updates to one CPU. If a CPU is taken offline, then this
assignment becomes stale. This went unnoticed because most of the time
the offline CPU went dead before the online CPU reached __cpu_die(),
where the CPU_DEAD state is checked. In the case that the offline CPU did
not reach the DEAD state before we reach __cpu_die(), the code in there
goes to sleep for 100ms. Due to the stale time update assignment, the
system is stuck forever.

Take the assignment away when a cpu is not longer in the cpu_online_mask.
We do this in the last call to tick_nohz_stop_sched_tick() when the offline
CPU is on the way to the final play_dead() idle entry.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index b416995..8c3fef1 100644 (file)
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -160,6 +160,18 @@ void tick_nohz_stop_sched_tick(void)
        cpu = smp_processor_id();
        ts = &per_cpu(tick_cpu_sched, cpu);
 
+       /*
+        * If this cpu is offline and it is the one which updates
+        * jiffies, then give up the assignment and let it be taken by
+        * the cpu which runs the tick timer next. If we don't drop
+        * this here the jiffies might be stale and do_timer() never
+        * invoked.
+        */
+       if (unlikely(!cpu_online(cpu))) {
+               if (cpu == tick_do_timer_cpu)
+                       tick_do_timer_cpu = -1;
+       }
+
        if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE))
                goto end;