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

Nicolas Pitre
2014-01-29T20:14:45Z

On Wed, 29 Jan 2014, Olof Johansson wrote:

> Hi,
>
> On Wed, Jan 29, 2014 at 9:45 AM, Nicolas Pitre <nicolas.pitre@linaro.org> wrote:
> > The core idle loop now takes care of it.
> >
> > Signed-off-by: Nicolas Pitre <nico@linaro.org>
> > Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
> > ---
> > 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 {