Subject: Re: [v7 PATCH 3/7]: x86: refactor x86 idle power management code and remove all instances of pm_idle.

From: Arun R Bharadwaj <[email-masked]>
Date: Wed, 7 Oct 2009 22:15:36 +0530

* Peter Zijlstra <[email-masked]> [2009-10-07 16:45:50]:

> On Tue, 2009-10-06 at 21:01 +0530, Arun R Bharadwaj wrote:
> > +++ linux.trees.git/arch/x86/kernel/process.c
> > @@ -9,6 +9,7 @@
> >  #include <linux/pm.h>
> >  #include <linux/clockchips.h>
> >  #include <linux/random.h>
> > +#include <linux/cpuidle.h>
> >  #include <trace/events/power.h>
> >  #include <asm/system.h>
> >  #include <asm/apic.h>
> > @@ -244,12 +245,6 @@ int sys_vfork(struct pt_regs *regs)
> >  unsigned long boot_option_idle_override = 0;
> >  EXPORT_SYMBOL(boot_option_idle_override);
> >  
> > -/*
> > - * Powermanagement idle function, if any..
> > - */
> > -void (*pm_idle)(void);
> > -EXPORT_SYMBOL(pm_idle);
> > -
> >  #ifdef CONFIG_X86_32
> >  /*
> >   * This halt magic was a workaround for ancient floppy DMA
> > @@ -329,17 +324,15 @@ static void do_nothing(void *unused)
> >  }
> >  
> >  /*
> > - * cpu_idle_wait - Used to ensure that all the CPUs discard old value of
> > - * pm_idle and update to new pm_idle value. Required while changing pm_idle
> > - * handler on SMP systems.
> > + * cpu_idle_wait - Required while changing idle routine handler on SMP systems.
> >   *
> > - * Caller must have changed pm_idle to the new value before the call. Old
> > - * pm_idle value will not be used by any CPU after the return of this function.
> > + * Caller must have changed idle routine to the new value before the call. Old
> > + * value will not be used by any CPU after the return of this function.
> >   */
> >  void cpu_idle_wait(void)
> >  {
> >         smp_mb();
> > -       /* kick all the CPUs so that they exit out of pm_idle */
> > +       /* kick all the CPUs so that they exit out of idle loop */
> >         smp_call_function(do_nothing, NULL, 1);
> >  }
> >  EXPORT_SYMBOL_GPL(cpu_idle_wait);
> > @@ -518,15 +511,59 @@ static void c1e_idle(void)
> >                 default_idle();
> >  }
> >  
> > +static void (*local_idle)(void);
> > +DEFINE_PER_CPU(struct cpuidle_device, idle_devices);
> > +
> > +struct cpuidle_driver cpuidle_default_driver = {
> > +       .name =         "cpuidle_default",
> > +};
> > +
> > +static int local_idle_loop(struct cpuidle_device *dev, struct cpuidle_state *st)
> > +{
> > +       ktime_t t1, t2;
> > +       s64 diff;
> > +       int ret;
> > +
> > +       t1 = ktime_get();
> > +       local_idle();
> > +       t2 = ktime_get();
> > +
> > +       diff = ktime_to_us(ktime_sub(t2, t1));
> > +       if (diff > INT_MAX)
> > +               diff = INT_MAX;
> > +       ret = (int) diff;
> > +
> > +       return ret;
> > +}
> > +
> > +static int setup_cpuidle_simple(void)
> > +{
> > +       struct cpuidle_device *dev;
> > +       int cpu;
> > +
> > +       if (!cpuidle_curr_driver)
> > +               cpuidle_register_driver(&cpuidle_default_driver);
> > +
> > +       for_each_online_cpu(cpu) {
> > +               dev = &per_cpu(idle_devices, cpu);
> > +               dev->cpu = cpu;
> > +               dev->states[0].enter = local_idle_loop;
> > +               dev->state_count = 1;
> > +               cpuidle_register_device(dev);
> > +       }
> > +       return 0;
> > +}
> > +device_initcall(setup_cpuidle_simple);
> > +
> >  void __cpuinit select_idle_routine(const struct cpuinfo_x86 *c)
> >  {
> >  #ifdef CONFIG_SMP
> > -       if (pm_idle == poll_idle && smp_num_siblings > 1) {
> > +       if (local_idle == poll_idle && smp_num_siblings > 1) {
> >                 printk(KERN_WARNING "WARNING: polling idle and HT enabled,"
> >                         " performance may degrade.\n");
> >         }
> >  #endif
> > -       if (pm_idle)
> > +       if (local_idle)
> >                 return;
> >  
> >         if (cpu_has(c, X86_FEATURE_MWAIT) && mwait_usable(c)) {
> > @@ -534,18 +571,20 @@ void __cpuinit select_idle_routine(const
> >                  * One CPU supports mwait => All CPUs supports mwait
> >                  */
> >                 printk(KERN_INFO "using mwait in idle threads.\n");
> > -               pm_idle = mwait_idle;
> > +               local_idle = mwait_idle;
> >         } else if (check_c1e_idle(c)) {
> >                 printk(KERN_INFO "using C1E aware idle routine\n");
> > -               pm_idle = c1e_idle;
> > +               local_idle = c1e_idle;
> >         } else
> > -               pm_idle = default_idle;
> > +               local_idle = default_idle;
> > +
> > +       return;
> >  }
> >  
> >  void __init init_c1e_mask(void)
> >  {
> >         /* If we're using c1e_idle, we need to allocate c1e_mask. */
> > -       if (pm_idle == c1e_idle)
> > +       if (local_idle == c1e_idle)
> >                 zalloc_cpumask_var(&c1e_mask, GFP_KERNEL);
> >  }
> >  
> > @@ -556,7 +595,7 @@ static int __init idle_setup(char *str)
> >  
> >         if (!strcmp(str, "poll")) {
> >                 printk("using polling idle threads.\n");
> > -               pm_idle = poll_idle;
> > +               local_idle = poll_idle;
> >         } else if (!strcmp(str, "mwait"))
> >                 force_mwait = 1;
> >         else if (!strcmp(str, "halt")) {
> > @@ -567,7 +606,7 @@ static int __init idle_setup(char *str)
> >                  * To continue to load the CPU idle driver, don't touch
> >                  * the boot_option_idle_override.
> >                  */
> > -               pm_idle = default_idle;
> > +               local_idle = default_idle;
> >                 idle_halt = 1;
> >                 return 0;
> >         } else if (!strcmp(str, "nomwait")) {
> 
> 
> What guarantees that the cpuidle bits actually select this
> cpuidle_default driver when you do idle=poll?
> 

When we do a idle=poll, it sets boot_option_idle_override = 1, which
is checked during cpuidle_register_device in acpi/processor_idle.c

So cpuidle devices are not even registered if this option is set.

But, in acpi/processor_core.c where cpuidle_register_driver happens,
this check is not made currently. So, I guess this check must be added
before we register acpi_idle driver.

> Also, cpuidle already has a poll loop in it, why duplicate that?
>

Suppose the arch doesnt have a poll loop of its own, it can use the
one provided by cpuidle. I have just retained this from the earlier
implementation.

--arun