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Date: Wed, 22 Oct 2014 16:30:03 -0400 (EDT)
From: Nicolas Pitre <nicolas.pitre@...aro.org>
To: Daniel Lezcano <daniel.lezcano@...aro.org>
cc: rjw@...ysocki.net, peterz@...radead.org, linux-pm@...r.kernel.org,
linux-kernel@...r.kernel.org, linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH 1/5] sched: idle: cpuidle: Check the latency req before
idle
On Mon, 20 Oct 2014, Daniel Lezcano wrote:
> When the pmqos latency requirement is set to zero that means "poll in all the
> cases".
>
> That is correctly implemented on x86 but not on the other archs.
>
> As how is written the code, if the latency request is zero, the governor will
> return zero, so corresponding, for x86, to the poll function, but for the
> others arch the default idle function. For example, on ARM this is wait-for-
> interrupt with a latency of '1', so violating the constraint.
>
> In order to fix that, do the latency requirement check *before* calling the
> cpuidle framework in order to jump to the poll function without entering
> cpuidle. That has several benefits:
>
> 1. It clarifies and unifies the code
> 2. It fixes x86 vs other archs behavior
> 3. Factors out the call to the same function
> 4. Prevent to enter the cpuidle framework with its expensive cost in
> calculation
>
> As the latency_req is needed in all the cases, change the select API to take
> the latency_req as parameter in case it is not equal to zero.
>
> As a positive side effect, it introduces the latency constraint specified
> externally, so one more step to the cpuidle/scheduler integration.
>
> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
Acked-by: Nicolas Pitre <nico@...aro.org>
> ---
> drivers/cpuidle/cpuidle.c | 5 +++--
> drivers/cpuidle/governors/ladder.c | 9 +--------
> drivers/cpuidle/governors/menu.c | 8 ++------
> include/linux/cpuidle.h | 7 ++++---
> kernel/sched/idle.c | 18 ++++++++++++++----
> 5 files changed, 24 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index ee9df5e..372c36f 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -158,7 +158,8 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
> *
> * Returns the index of the idle state.
> */
> -int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
> +int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
> + int latency_req)
> {
> if (off || !initialized)
> return -ENODEV;
> @@ -169,7 +170,7 @@ int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
> if (unlikely(use_deepest_state))
> return cpuidle_find_deepest_state(drv, dev);
>
> - return cpuidle_curr_governor->select(drv, dev);
> + return cpuidle_curr_governor->select(drv, dev, latency_req);
> }
>
> /**
> diff --git a/drivers/cpuidle/governors/ladder.c b/drivers/cpuidle/governors/ladder.c
> index 044ee0d..18f0da9 100644
> --- a/drivers/cpuidle/governors/ladder.c
> +++ b/drivers/cpuidle/governors/ladder.c
> @@ -64,18 +64,11 @@ static inline void ladder_do_selection(struct ladder_device *ldev,
> * @dev: the CPU
> */
> static int ladder_select_state(struct cpuidle_driver *drv,
> - struct cpuidle_device *dev)
> + struct cpuidle_device *dev, int latency_req)
> {
> struct ladder_device *ldev = &__get_cpu_var(ladder_devices);
> struct ladder_device_state *last_state;
> int last_residency, last_idx = ldev->last_state_idx;
> - int latency_req = pm_qos_request(PM_QOS_CPU_DMA_LATENCY);
> -
> - /* Special case when user has set very strict latency requirement */
> - if (unlikely(latency_req == 0)) {
> - ladder_do_selection(ldev, last_idx, 0);
> - return 0;
> - }
>
> last_state = &ldev->states[last_idx];
>
> diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
> index 34db2fb..96f8fb0 100644
> --- a/drivers/cpuidle/governors/menu.c
> +++ b/drivers/cpuidle/governors/menu.c
> @@ -287,10 +287,10 @@ again:
> * @drv: cpuidle driver containing state data
> * @dev: the CPU
> */
> -static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
> +static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
> + int latency_req)
> {
> struct menu_device *data = &__get_cpu_var(menu_devices);
> - int latency_req = pm_qos_request(PM_QOS_CPU_DMA_LATENCY);
> int i;
> unsigned int interactivity_req;
> unsigned long nr_iowaiters, cpu_load;
> @@ -302,10 +302,6 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
>
> data->last_state_idx = CPUIDLE_DRIVER_STATE_START - 1;
>
> - /* Special case when user has set very strict latency requirement */
> - if (unlikely(latency_req == 0))
> - return 0;
> -
> /* determine the expected residency time, round up */
> data->next_timer_us = ktime_to_us(tick_nohz_get_sleep_length());
>
> diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
> index 25e0df6..fb465c1 100644
> --- a/include/linux/cpuidle.h
> +++ b/include/linux/cpuidle.h
> @@ -122,7 +122,7 @@ struct cpuidle_driver {
> extern void disable_cpuidle(void);
>
> extern int cpuidle_select(struct cpuidle_driver *drv,
> - struct cpuidle_device *dev);
> + struct cpuidle_device *dev, int latency_req);
> extern int cpuidle_enter(struct cpuidle_driver *drv,
> struct cpuidle_device *dev, int index);
> extern void cpuidle_reflect(struct cpuidle_device *dev, int index);
> @@ -150,7 +150,7 @@ extern struct cpuidle_driver *cpuidle_get_cpu_driver(struct cpuidle_device *dev)
> #else
> static inline void disable_cpuidle(void) { }
> static inline int cpuidle_select(struct cpuidle_driver *drv,
> - struct cpuidle_device *dev)
> + struct cpuidle_device *dev, int latency_req)
> {return -ENODEV; }
> static inline int cpuidle_enter(struct cpuidle_driver *drv,
> struct cpuidle_device *dev, int index)
> @@ -205,7 +205,8 @@ struct cpuidle_governor {
> struct cpuidle_device *dev);
>
> int (*select) (struct cpuidle_driver *drv,
> - struct cpuidle_device *dev);
> + struct cpuidle_device *dev,
> + int latency_req);
> void (*reflect) (struct cpuidle_device *dev, int index);
>
> struct module *owner;
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 11e7bc4..25ba94d 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -5,6 +5,7 @@
> #include <linux/cpu.h>
> #include <linux/cpuidle.h>
> #include <linux/tick.h>
> +#include <linux/pm_qos.h>
> #include <linux/mm.h>
> #include <linux/stackprotector.h>
>
> @@ -74,7 +75,7 @@ void __weak arch_cpu_idle(void)
> * set, and it returns with polling set. If it ever stops polling, it
> * must clear the polling bit.
> */
> -static void cpuidle_idle_call(void)
> +static void cpuidle_idle_call(unsigned int latency_req)
> {
> struct cpuidle_device *dev = __this_cpu_read(cpuidle_devices);
> struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
> @@ -107,7 +108,7 @@ static void cpuidle_idle_call(void)
> * Ask the cpuidle framework to choose a convenient idle state.
> * Fall back to the default arch idle method on errors.
> */
> - next_state = cpuidle_select(drv, dev);
> + next_state = cpuidle_select(drv, dev, latency_req);
> if (next_state < 0) {
> use_default:
> /*
> @@ -182,6 +183,8 @@ exit_idle:
> */
> static void cpu_idle_loop(void)
> {
> + unsigned int latency_req;
> +
> while (1) {
> /*
> * If the arch has a polling bit, we maintain an invariant:
> @@ -205,19 +208,26 @@ static void cpu_idle_loop(void)
> local_irq_disable();
> arch_cpu_idle_enter();
>
> + latency_req = pm_qos_request(PM_QOS_CPU_DMA_LATENCY);
> +
> /*
> * In poll mode we reenable interrupts and spin.
> *
> + * If the latency req is zero, we don't want to
> + * enter any idle state and we jump to the poll
> + * function directly
> + *
> * Also if we detected in the wakeup from idle
> * path that the tick broadcast device expired
> * for us, we don't want to go deep idle as we
> * know that the IPI is going to arrive right
> * away
> */
> - if (cpu_idle_force_poll || tick_check_broadcast_expired())
> + if (!latency_req || cpu_idle_force_poll ||
> + tick_check_broadcast_expired())
> cpu_idle_poll();
> else
> - cpuidle_idle_call();
> + cpuidle_idle_call(latency_req);
>
> arch_cpu_idle_exit();
> }
> --
> 1.9.1
>
>
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