Re: [PATCH v2 5/6] X86: remove redundant cpuidle_idle_call()

[Nicolas Pitre]

On Wed, 29 Jan 2014, Olof Johansson wrote:

> Hi,
> 
> On Wed, Jan 29, 2014 at 9:45 AM, Nicolas Pitre <nicolas.pitre@xxxxxxxxxx> wrote:
> > The core idle loop now takes care of it.
> >
> > Signed-off-by: Nicolas Pitre <nico@xxxxxxxxxx>
> > Acked-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> > ---
> >  arch/x86/kernel/process.c | 5 +----
> >  1 file changed, 1 insertion(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> > index 3fb8d95ab8..4505e2a950 100644
> > --- a/arch/x86/kernel/process.c
> > +++ b/arch/x86/kernel/process.c
> > @@ -298,10 +298,7 @@ void arch_cpu_idle_dead(void)
> >   */
> >  void arch_cpu_idle(void)
> >  {
> > -       if (cpuidle_idle_call())
> > -               x86_idle();
> > -       else
> > -               local_irq_enable();
> > +       x86_idle();
> 
> You're taking out the local_irq_enable() here but I don't see the
> equivalent of adding it back in the 1/6 patch that moves the
> cpuidle_idle_call() up to common code. It seems that one of the call
> paths through cpuidle_idle_call() don't re-enable it on its own.

When cpuidle_idle_call() returns non-zero, IRQs are left disabled.  When 
it returns zero then IRQs should be disabled.  Same goes for cpuidle 
drivers.  That's the theory at least.

Looking into some cpuidle drivers for x86 I found at least one that 
doesn't respect this convention.  Damn.

> Even if this is the right thing to do, why it's OK to do so should
> probably be documented in the patch description.

Better yet, I'm going to amend patch 1/6 with the below to make things 
more reliable while still identifying misbehaving drivers.

diff --git a/kernel/cpu/idle.c b/kernel/cpu/idle.c
index ffcd3ee9af..14ca43430a 100644
--- a/kernel/cpu/idle.c
+++ b/kernel/cpu/idle.c
@@ -98,7 +98,8 @@ static void cpu_idle_loop(void)
 					rcu_idle_enter();
 					if (cpuidle_idle_call())
 						arch_cpu_idle();
-					WARN_ON_ONCE(irqs_disabled());
+					if (WARN_ON_ONCE(irqs_disabled()))
+						local_irq_enable();
 					rcu_idle_exit();
 					start_critical_timings();
 				} else {
--
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