Re: [PATCH v12 3/4] cpuidle: Export the next timer/tick expiration for a CPU

[Rafael J. Wysocki]

On Wednesday, February 27, 2019 8:58:35 PM CET Ulf Hansson wrote:
> To be able to predict the sleep duration for a CPU that is entering idle,
> knowing when the next timer/tick is going to expire, is extremely useful.
> Both the teo and the menu cpuidle governors already makes use of this
> information, while selecting an idle state.
> 
> Moving forward, the similar prediction needs to be done, but for a group of
> idle CPUs rather than for a single idle CPU. Following changes implements a
> new genpd governor, which needs this.
> 
> Support this, by sharing a new function called
> tick_nohz_get_next_hrtimer(), which returns the next hrtimer or the next
> tick, whatever that expires first.
> 
> Additionally, when cpuidle is about to invoke the ->enter() callback, then
> call tick_nohz_get_next_hrtimer() and store its return value in the per CPU
> struct cpuidle_device, as to make it available outside cpuidle.
> 
> Do note, at the point when cpuidle calls tick_nohz_get_next_hrtimer(), the
> governor's ->select() callback has already made a decision whether to stop
> the tick or not. In this way, tick_nohz_get_next_hrtimer() actually returns
> the next timer expiration, whatever origin.
> 
> Cc: Lina Iyer <ilina@xxxxxxxxxxxxxx>
> Co-developed-by: Lina Iyer <lina.iyer@xxxxxxxxxx>
> Co-developed-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> Signed-off-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
> ---
> 
> Changes in v12:
> 	- New patch.
> 
> ---
>  drivers/cpuidle/cpuidle.c |  8 ++++++++
>  include/linux/cpuidle.h   |  1 +
>  include/linux/tick.h      |  7 ++++++-
>  kernel/time/tick-sched.c  | 12 ++++++++++++
>  4 files changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index 7f108309e871..255365b1a6ab 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -328,6 +328,14 @@ int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>  int cpuidle_enter(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>  		  int index)
>  {
> +	/*
> +	 * Store the next hrtimer, which becomes either next tick or the next
> +	 * timer event, whatever expires first. Additionally, to make this data
> +	 * useful for consumers outside cpuidle, we rely on that the governor's
> +	 * ->select() callback have decided, whether to stop the tick or not.
> +	 */
> +	dev->next_hrtimer = tick_nohz_get_next_hrtimer();

I would use WRITE_ONCE() to set next_hrtimer here and READ_ONCE() for
reading that value in the next patch, as a matter of annotation if
nothing else.

> +
>  	if (cpuidle_state_is_coupled(drv, index))
>  		return cpuidle_enter_state_coupled(dev, drv, index);
>  	return cpuidle_enter_state(dev, drv, index);

Also I would clear next_hrtimer here to avoid dragging stale values
around.

Apart from this the series LGTM.

Thanks!