On Fri, Aug 15, 2014 at 3:50 PM, Mohammad Merajul Islam Molla
<meraj.enigma@xxxxxxxxx> 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
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