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:
> > > 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();
> }
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
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