linux-kernel - Re: [RFC PATCH v2 4/8] tick/nohz: keep tick on for a fast idle

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Date:   Sat, 14 Oct 2017 02:51:10 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Aubrey Li <aubrey.li@...el.com>
Cc:     tglx@...utronix.de, peterz@...radead.org, len.brown@...el.com,
        ak@...ux.intel.com, tim.c.chen@...ux.intel.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, Aubrey Li <aubrey.li@...ux.intel.com>
Subject: Re: [RFC PATCH v2 4/8] tick/nohz: keep tick on for a fast idle

On Saturday, September 30, 2017 9:20:30 AM CEST Aubrey Li wrote:
> If the next idle is expected to be a fast idle, we should keep tick
> on before going into idle
> 
> Signed-off-by: Aubrey Li <aubrey.li@...ux.intel.com>

This also can be merged with the previous patch (and the [2/8]) IMO.

> ---
>  drivers/cpuidle/cpuidle.c | 14 ++++++++++++++
>  include/linux/cpuidle.h   |  2 ++
>  kernel/time/tick-sched.c  |  4 ++++
>  3 files changed, 20 insertions(+)
> 
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index ef6f7dd..6cb7e17 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -370,6 +370,20 @@ void cpuidle_predict(void)
>  }
>  
>  /**
> + * cpuidle_fast_idle - predict whether or not the coming idle is a fast idle
> + * This function can be called in irq exit path, make it as soon as possible
> + */
> +bool cpuidle_fast_idle(void)
> +{
> +	struct cpuidle_device *dev = cpuidle_get_device();
> +
> +	if (!dev)
> +		return false;
> +
> +	return dev->idle_stat.fast_idle;

return dev && dev->idle_stat.fast_idle;

> +}
> +
> +/**
>   * cpuidle_install_idle_handler - installs the cpuidle idle loop handler
>   */
>  void cpuidle_install_idle_handler(void)
> diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
> index 9ca0288..791db15 100644
> --- a/include/linux/cpuidle.h
> +++ b/include/linux/cpuidle.h
> @@ -144,6 +144,7 @@ extern int cpuidle_select(struct cpuidle_driver *drv,
>  extern void cpuidle_entry_start(void);
>  extern void cpuidle_entry_end(void);
>  extern void cpuidle_predict(void);
> +extern bool cpuidle_fast_idle(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);
> @@ -180,6 +181,7 @@ static inline int cpuidle_select(struct cpuidle_driver *drv,
>  static inline void cpuidle_entry_start(void) { }
>  static inline void cpuidle_entry_end(void) { }
>  static inline void cpuidle_predict(void) { }
> +static inline void cpuidle_fast_idle(void) {return false; }
>  static inline int cpuidle_enter(struct cpuidle_driver *drv,
>  				struct cpuidle_device *dev, int index)
>  {return -ENODEV; }
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index c7a899c..d663fab 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -27,6 +27,7 @@
>  #include <linux/irq_work.h>
>  #include <linux/posix-timers.h>
>  #include <linux/context_tracking.h>
> +#include <linux/cpuidle.h>
>  
>  #include <asm/irq_regs.h>
>  
> @@ -916,6 +917,9 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
>  			return false;
>  	}
>  
> +	if (cpuidle_fast_idle())
> +		return false;
> +
>  	return true;

return !cpuidle_fast_idle();

>  }
>  
> 

And IMO there is quite a bit too much marketing in the "fast_idle" name,
as it seems all about avoiding to stop the tick if the predicted idle
duration is short enough.