RE: [PATCH] cpuidle: coupled: fix dead loop corner case

[Neil Zhang]

> -----Original Message-----
> From: Colin Cross [mailto:ccross@xxxxxxxxxx]
> Sent: 2013年8月23日 5:08
> To: Neil Zhang
> Cc: Rafael J. Wysocki; Daniel Lezcano; Linux PM list; lkml
> Subject: Re: [PATCH] cpuidle: coupled: fix dead loop corner case
> 
> On Mon, Aug 19, 2013 at 10:17 PM, Neil Zhang <zhangwm@xxxxxxxxxxx>
> wrote:
> > There is a corener case when no peripheral irqs route to secondary
> > cores.
> > Let's take dual core system for example, the sequence is as following:
> >
> >                 Core 0                          Core1
> > 1.                                 set waiting bit and enter waiting
> loop
> > 2. set waiting bit and poke core1
> > 3.                                 clear poke in irq and enter safe
> state
> > 4. set ready bit and enter ready loop
> >
> > Since there is no peripheral irq route to core 1, so it will stay in
> > safe state forever, and core 0 will dead loop in the following code.
> >         while (!cpuidle_coupled_cpus_ready(coupled)) {
> >                 /* Check if any other cpus bailed out of idle. */
> >                 if (!cpuidle_coupled_cpus_waiting(coupled))
> >         }
> >
> > The solution is don't let secondary core enter safe state when it has
> > already handled the poke interrupt.
> >
> > Signed-off-by: Neil Zhang <zhangwm@xxxxxxxxxxx>
> > Reviewed-by: Fangsuo Wu <fswu@xxxxxxxxxxx>
> > ---
> >  drivers/cpuidle/coupled.c |    7 +++++++
> >  1 files changed, 7 insertions(+), 0 deletions(-)
> >
> > diff --git a/drivers/cpuidle/coupled.c b/drivers/cpuidle/coupled.c
> > index 2a297f8..a37c718 100644
> > --- a/drivers/cpuidle/coupled.c
> > +++ b/drivers/cpuidle/coupled.c
> > @@ -119,6 +119,7 @@ struct cpuidle_coupled {
> >  #define CPUIDLE_COUPLED_NOT_IDLE       (-1)
> >
> >  static DEFINE_MUTEX(cpuidle_coupled_lock);
> > +static DEFINE_PER_CPU(bool, poke_sync);
> >  static DEFINE_PER_CPU(struct call_single_data,
> > cpuidle_coupled_poke_cb);
> >
> >  /*
> > @@ -295,6 +296,7 @@ static void cpuidle_coupled_poked(void *info)  {
> >         int cpu = (unsigned long)info;
> >         cpumask_clear_cpu(cpu, &cpuidle_coupled_poked_mask);
> > +       __this_cpu_write(poke_sync, true);
> >  }
> >
> >  /**
> > @@ -473,6 +475,7 @@ retry:
> >          * allowed for a single cpu.
> >          */
> >         while (!cpuidle_coupled_cpus_waiting(coupled)) {
> > +               __this_cpu_write(poke_sync, false);
> >                 if (cpuidle_coupled_clear_pokes(dev->cpu)) {
> >                         cpuidle_coupled_set_not_waiting(dev->cpu,
> coupled);
> >                         goto out;
> > @@ -483,6 +486,10 @@ retry:
> >                         goto out;
> >                 }
> >
> > +               if (cpuidle_coupled_cpus_waiting(coupled)
> > +                       && __this_cpu_read(poke_sync))
> > +                       break;
> > +
> >                 entered_state = cpuidle_enter_state(dev, drv,
> >                         dev->safe_state_index);
> >         }
> > --
> > 1.7.4.1
> >
> > --
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> 
> I have a similar patch that avoids adding another check for
> cpuidle_coupled_cpus_waiting, and uses the return value from
> cpuidle_coupled_clear_pokes instead of adding a percpu bool.  I will post it
> shortly.
> 
> Do you have a test case that can reproduce this easily?

It's not easy to reproduce.
We only catch one time till now.

Best Regards,
Neil Zhang
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