> > It would be better if you hung rcu_irq_enter in the irq_enter() if 
> > statement that checks if the task was idle or not. This way it would
> > be zero overhead for interruptions of non busy CPUs, keeping 

Sorry that should have been "non idle CPUs" of course.

> > it out of many fast paths.
> > 
> > Haven't read everything, sorry.
> 
> So that I lose the #else above, and so that irq_enter() and irq_exit()
> look something like the following (with additional adjustments to suit)?

Yes looks good.

BTW I wonder if the compiler CSEs the idle_cpu() check properly.

> void irq_enter(void)
> {
> #ifdef CONFIG_NO_HZ
> 	int cpu = smp_processor_id();
> 	if (idle_cpu(cpu) && !in_interrupt())
> 		tick_nohz_stop_idle(cpu);
> #endif
> 	__irq_enter();
> #ifdef CONFIG_NO_HZ
> 	if (idle_cpu(cpu)) {
> 		rcu_irq_enter();
> 		tick_nohz_update_jiffies();
> 	}
> #endif


-Andi