On Wed, Aug 22, 2012 at 12:01:09PM -0700, Paul E. McKenney wrote:
> > 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.
> >
> >
> > >
> > > Thanx, Paul
> > >
> > > > 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();
}
}
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.
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