Re: [PATCH 01/10] alpha: Add missing RCU idle APIs on idle loop

On Thu, Aug 23, 2012 at 12:42:11PM +0200, Frederic Weisbecker wrote:

On Wed, Aug 22, 2012 at 12:01:09PM -0700, Paul E. McKenney wrote:

quoted

quoted

The current code is preemptable, at least it appears so because it calls
schedule() directly. And if I call rcu_idle_enter() in a preemptable section,
I'm in trouble because I'll schedule while in extended QS.

Thus I need to disable preemption here at least until I call rcu_idle_exit().

Now this is an endless loop so there is no need to re-enable
preemption after the loop. And schedule_preempt_disabled()
takes care of enabling preemption before schedule() and redisabling
it afterward.

quoted

							Thanx, Paul

quoted

 	while (1) {
 		/* FIXME -- EV6 and LCA45 know how to power down
 		   the CPU.  */

+		rcu_idle_enter();
 		while (!need_resched())
 			cpu_relax();
-		schedule();
+		rcu_idle_exit();
+		schedule_preempt_disabled();
 	}

Understood, but what I don't understand is why you don't need a
preempt_enable() right here.

Look, let's inline the content of schedule_preempt_disabled(), the code
then looks like:

void cpu_idle(void)
{
	set_thread_flag(TIF_POLLING_NRFLAG);

	preempt_disable();
	while (1) {
		/* FIXME -- EV6 and LCA45 know how to power down
		   the CPU.  */

		rcu_idle_enter();
		while (!need_resched())
			cpu_relax();
		rcu_idle_exit();

		sched_preempt_enable_no_resched();
	        schedule();
	        preempt_disable();
	}

	preempt_enable();  /* Why is this not needed? */

}

So there is a preempt_enable() before we schedule, then we re-disable
preemption after schedule.

Now I realize cpu_idle() is supposed to be called with preemption disabled
already so I shouldn't add an explicit preempt_disable() or it's going to be worse.
But that means there is an existing bug here in alpha, it should call schedule_preempt_disabled()
instead of schedule(). cpu_idle() is called with preemption disabled on the boot CPU.
And it should as well from the secondary CPUs entry but alpha doesn't seem to do that.

So I need to fix that first. I'll respin.

OK, look forward to seeing the respin.

							Thanx, Paul