Need help understanding the logic of __cpuidle_set_driver

[AYAN KUMAR HALDER]

On Fri, Aug 15, 2014 at 3:50 PM, Mohammad Merajul Islam Molla
<meraj.enigma at gmail.com> wrote:
> Hello,
>
> I was looking into the code of drivers/cpuidle/driver.c. I have some
> doubts regarding the implementation of __cpuidle_set_driver function
> when CONFIG_CPU_IDLE_MULTIPLE_DRIVERS is defined.
>
> If CONFIG_CPU_IDLE_MULTIPLE_DRIVERS is defined, the code for
> __cpuidle_set_driver/__cpuidle_unset_driver looks as -
>
>  39  * __cpuidle_unset_driver - unset per CPU driver variables.
>  40  * @drv: a valid pointer to a struct cpuidle_driver
>  41  *
>  42  * For each CPU in the driver's CPU mask, unset the registered
> driver per CPU
>  43  * variable. If @drv is different from the registered driver, the
> corresponding
>  44  * variable is not cleared.
>  45  */
>  46 static inline void __cpuidle_unset_driver(struct cpuidle_driver *drv)
>  47 {
>  48         int cpu;
>  49
>  50         for_each_cpu(cpu, drv->cpumask) {
>  51
>  52                 if (drv != __cpuidle_get_cpu_driver(cpu))
>  53                         continue;
>  54
>  55                 per_cpu(cpuidle_drivers, cpu) = NULL;
>  56         }
>  57 }
>  58
>  59 /**
>  60  * __cpuidle_set_driver - set per CPU driver variables for the given driver.
>  61  * @drv: a valid pointer to a struct cpuidle_driver
>  62  *
>  63  * For each CPU in the driver's cpumask, unset the registered driver per CPU
>  64  * to @drv.
>  65  *
>  66  * Returns 0 on success, -EBUSY if the CPUs have driver(s) already.
>  67  */
>  68 static inline int __cpuidle_set_driver(struct cpuidle_driver *drv)
>  69 {
>  70         int cpu;
>  71
>  72         for_each_cpu(cpu, drv->cpumask) {
>  73
>  74                 if (__cpuidle_get_cpu_driver(cpu)) {
>  75                         __cpuidle_unset_driver(drv);
>  76                         return -EBUSY;
>  77                 }
>  78
>  79                 per_cpu(cpuidle_drivers, cpu) = drv;
>  80         }
>  81
>  82         return 0;
>  83 }
>
> Apparently, the comment should be - "set/register the driver per CPU
> to @drv" instead of "unset the registered driver per CPU to @drv" in
> case of __cpuidle_set_driver.

I would like to slightly differ here. As per my understanding, the
comments given
for the function is correct. The function __cpuidle_set_driver(drv)
would do the following:-

1. If the cpu has any registered idle driver which is same as @drv,
the registered
driver would be unset. ie "unset the registered driver per CPU to @drv"
2. If the cpu has any registered idle driver which is different from
@drv, do nothing
3. If the cpu has no registered idle driver, set the idle driver to @drv

> However, regarding the logic, I have a few doubts -
>
> 1. for each cpu in drv->cpumask, if there is already a driver
> registered, its calling __cpuidle_unset_driver which loops over for
> each cpu in drv->cpumask again. Isn't it unnecessary to do this nested
> calls?

__cpuidle_unset_driver :- This function gets called from
"__cpuidle_unregister_driver()" too.
So it needs to loop over each cpu to see if its registered driver is
same as @drv. What
you might be trying to convey here is that instead of calling
__cpuidle_unset_driver, we could
have done the following:-
static inline int __cpuidle_set_driver(struct cpuidle_driver *drv)
{
        int cpu;
        struct cpuidle_driver *tmp = NULL;
        for_each_cpu(cpu, drv->cpumask) {

                tmp = __cpuidle_get_cpu_driver(cpu); /* This would
prevent nesting of loops */
                if ( tmp != NULL ) {
                        if ( tmp == drv )
                              per_cpu(cpuidle_drivers, cpu) = NULL;
                        return -EBUSY;
                }

                per_cpu(cpuidle_drivers, cpu) = drv;
        }

        return 0;
}



> 2. after calling __cpuidle_unset_driver, if drv equals already
> registered driver, it sets per_cpu driver to null? Isn't it wrong when
> we are trying to set to a new driver? Why do we need to unset and make
> the driver null when we are returning EBUSY from __cpuidle_set_driver?

My understanding is that if there is a previously registered cpuidle
driver, returning
EBUSY is fine. But I do share the same doubt as you have that if the previous
registered cpuidle driver is same as the new one, then why should it
be unset and NULLed.

> Would it be correct and cleaner if the code is written as below -
>
>  static inline int __cpuidle_set_driver(struct cpuidle_driver *drv)
>  {
>          int ret = -EBUSY;
>          int cpu;
>
>         for_each_cpu(cpu, drv->cpumask) {
>                 if (drv == __cpuidle_get_cpu_driver(cpu))    [if drv
> is already the registered driver, do nothing]
>                          continue;
>
>                  per_cpu(cpuidle_drivers, cpu) = drv;  [if only drv !=
> already registered driver, set per_cpu driver to drv and set ret 0]
>                  ret = 0;
>          }
>
>          return ret;     [only if all cpus already had drv as
> registered driver, return -EBUSY. Otherwise return 0]
>  }
>
The difference that might cause some trouble is that the timer
broadcast notification is
not sent while changing the cpuide drivers.

Regards,
Ayan Kumar Halder