Re: [RFC PATCH v2 3/8] cpuidle: add a new predict interface

Rafael J. Wysocki
2017-10-14T01:36:46Z

On Saturday, September 30, 2017 9:20:29 AM CEST Aubrey Li wrote:
> For the governor has predict functionality, add a new predict
> interface in cpuidle framework to call and use it.
> ---
> drivers/cpuidle/cpuidle.c | 34 ++++++++++++++++++++++++++++++++++
> drivers/cpuidle/governors/menu.c | 7 +++++++
> include/linux/cpuidle.h | 3 +++
> kernel/sched/idle.c | 1 +
> 4 files changed, 45 insertions(+)
>
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index 4066308..ef6f7dd 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -336,6 +336,40 @@ void cpuidle_entry_end(void)
> }
>
> /**
> + * cpuidle_predict - predict whether the coming idle is a fast idle or not
> + */
> +void cpuidle_predict(void)
> +{
> + struct cpuidle_device *dev = cpuidle_get_device();
> + unsigned int overhead_threshold;
> +
> + if (!dev)
> + return;
> +
> + overhead_threshold = dev->idle_stat.overhead;
> +
> + if (cpuidle_curr_governor->predict) {
> + dev->idle_stat.predicted_us = cpuidle_curr_governor->predict();
> + /*
> + * notify idle governor to avoid reduplicative
> + * prediction computation
> + */
> + dev->idle_stat.predicted = true;
> + if (dev->idle_stat.predicted_us < overhead_threshold) {
> + /*
> + * notify tick subsystem to keep ticking
> + * for the coming idle
> + */
> + dev->idle_stat.fast_idle = true;
> + } else
> + dev->idle_stat.fast_idle = false;
> + } else {
> + dev->idle_stat.predicted = false;
> + dev->idle_stat.fast_idle = false;
> + }
> +}
> +
> +/**
> * cpuidle_install_idle_handler - installs the cpuidle idle loop handler
> */
> void cpuidle_install_idle_handler(void)
> diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
> index 6bed197..90b2a10 100644
> --- a/drivers/cpuidle/governors/menu.c
> +++ b/drivers/cpuidle/governors/menu.c
> @@ -344,6 +344,12 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
> if (unlikely(latency_req == 0))
> return 0;
>
> + /*don't predict again if idle framework already did it */
> + if (!dev->idle_stat.predicted)
> + menu_predict();
> + else
> + dev->idle_stat.predicted = false;
> +
> if (CPUIDLE_DRIVER_STATE_START > 0) {
> struct cpuidle_state *s = &drv->states[CPUIDLE_DRIVER_STATE_START];
> unsigned int polling_threshold;
> @@ -518,6 +524,7 @@ static struct cpuidle_governor menu_governor = {
> .enable = menu_enable_device,
> .select = menu_select,
> .reflect = menu_reflect,
> + .predict = menu_predict,
> };
>
> /**
> diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
> index cad9b71..9ca0288 100644
> --- a/include/linux/cpuidle.h
> +++ b/include/linux/cpuidle.h
> @@ -143,6 +143,7 @@ extern int cpuidle_select(struct cpuidle_driver *drv,
> struct cpuidle_device *dev);
> extern void cpuidle_entry_start(void);
> extern void cpuidle_entry_end(void);
> +extern void cpuidle_predict(void);
> extern int cpuidle_enter(struct cpuidle_driver *drv,
> struct cpuidle_device *dev, int index);
> extern void cpuidle_reflect(struct cpuidle_device *dev, int index);
> @@ -178,6 +179,7 @@ static inline int cpuidle_select(struct cpuidle_driver *drv,
> {return -ENODEV; }
> static inline void cpuidle_entry_start(void) { }
> static inline void cpuidle_entry_end(void) { }
> +static inline void cpuidle_predict(void) { }
> static inline int cpuidle_enter(struct cpuidle_driver *drv,
> struct cpuidle_device *dev, int index)
> {return -ENODEV; }
> @@ -255,6 +257,7 @@ struct cpuidle_governor {
> int (*select) (struct cpuidle_driver *drv,
> struct cpuidle_device *dev);
> void (*reflect) (struct cpuidle_device *dev, int index);
> + unsigned int (*predict)(void);
> };
>
> #ifdef CONFIG_CPU_IDLE
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 0951dac..8704f3c 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -225,6 +225,7 @@ static void do_idle(void)
> */
> __current_set_polling();
> quiet_vmstat();
> + cpuidle_predict();

One more question here.

This changes the cpuidle code ordering such that if the ->predict callback
is present, the governor prediction will run before tick_nohz_idle_enter(),
whereas without that callback it runs in cpuidle_idle_call().

Is that actually going to work correctly for the menu governor, in particular?

> tick_nohz_idle_enter();
> cpuidle_entry_end();
>
>