powerpc: Make decrementer interrupt robust against offlined CPUs

With some implementations, it is possible that a timer interrupt
occurs every few seconds on an offline CPU. In this case, just
re-arm the decrementer and return immediately

Signed-off-by: Benjamin Herrenschmidt <[e-mail]>

arch/powerpc/kernel/time.c

···
 	struct clock_event_device *evt = &decrementer->event;
 	u64 now;
 
+	/* Ensure a positive value is written to the decrementer, or else
+	 * some CPUs will continue to take decrementer exceptions.
+	 */
+	set_dec(DECREMENTER_MAX);
+
+	/* Some implementations of hotplug will get timer interrupts while
+	 * offline, just ignore these
+	 */
+	if (!cpu_online(smp_processor_id()))
+		return;
+
 	trace_timer_interrupt_entry(regs);
 
 	__get_cpu_var(irq_stat).timer_irqs++;
-
-	/* Ensure a positive value is written to the decrementer, or else
-	 * some CPUs will continuue to take decrementer exceptions */
-	set_dec(DECREMENTER_MAX);
 
 #if defined(CONFIG_PPC32) && defined(CONFIG_PMAC)
 	if (atomic_read(&ppc_n_lost_interrupts) != 0)