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SubjectRe: [PATCH v12 3/4] cpuidle: Export the next timer/tick expiration for a CPU
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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@codeaurora.org>
> Co-developed-by: Lina Iyer <lina.iyer@linaro.org>
> Co-developed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
> Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
> ---
>
> 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!

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