linux-kernel - Re: [RESEND PATCH v1 2/2] cpuidle: menu: Dismiss tick impaction on correction factors

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Message-ID: <CAJZ5v0ggZXDCvQ7EbO1gtZhMewK6ToRW9JSiDzwpdZqZLDY31g@mail.gmail.com>
Date:   Thu, 9 Aug 2018 23:06:07 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [RESEND PATCH v1 2/2] cpuidle: menu: Dismiss tick impaction on
 correction factors

On Thu, Aug 9, 2018 at 7:20 PM, Leo Yan <leo.yan@...aro.org> wrote:
> If the idle duration predictor detects the tick is triggered, and with
> meeting the condition 'data->next_timer_us > TICK_USEC', it will give a
> big compensation for the 'measured' interval; this is purposed to avoid
> artificially small correction factor values.  Unfortunately, this still
> cannot cover all cases of the tick impaction on correction factors,
> e.g. if the predicted next event is less than ITCK_USEC, then all
> wakening up by the ticks will be taken as usual case and reducing exit
> latency, as results the tick events heavily impacts the correction
> factors.
>
> Moreover, the coming tick sometimes is very soon, especially
> at the first time when the CPU becomes idle the tick expire time might
> be vary, so ticks can introduce big deviation on correction factors.
>
> If idle governor deliberately doesn't stop the tick timer, the tick
> event is coming as expected with fixed interval, so the tick event is
> predictable; if the tick event is coming early than other normal timer
> event and other possible wakeup events, we need to dismiss the tick
> impaction on correction factors, this can let the correction factor
> array is purely used for other wakeup events correctness rather than
> sched tick.
>
> This patch is to check if it's a tick wakeup, it takes the CPU can
> stay in the idle state for enough time so it gives high compensation
> for the measured' interval, this can avoid tick impaction on the
> correction factor array.

Well, again, this is questionable.

Yes, you can remove the tick influence on correction factors this way,
but will the resulting idle duration predictions be actually better
because of that?

Do you have any data to demonstrate the difference?

>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Signed-off-by: Leo Yan <leo.yan@...aro.org>
> ---
>  drivers/cpuidle/governors/menu.c | 14 ++++++--------
>  1 file changed, 6 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
> index 2ce4068..43cbde3 100644
> --- a/drivers/cpuidle/governors/menu.c
> +++ b/drivers/cpuidle/governors/menu.c
> @@ -525,15 +525,13 @@ static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
>          * assume the state was never reached and the exit latency is 0.
>          */
>
> -       if (data->tick_wakeup && data->next_timer_us > TICK_USEC) {
> +       if (data->tick_wakeup) {
>                 /*
> -                * The nohz code said that there wouldn't be any events within
> -                * the tick boundary (if the tick was stopped), but the idle
> -                * duration predictor had a differing opinion.  Since the CPU
> -                * was woken up by a tick (that wasn't stopped after all), the
> -                * predictor was not quite right, so assume that the CPU could
> -                * have been idle long (but not forever) to help the idle
> -                * duration predictor do a better job next time.
> +                * Since the CPU was woken up by a tick (that wasn't stopped
> +                * after all), the predictor was not quite right, so assume

This part of the comment is not valid any more IMO.

The fact that the CPU was woken up by the tick alone doesn't tell you
much about the prediction.  The tick may have not been stopped,
because the nohz code saw timer events within the tick boundary, in
which case the CPU could not be idle very long.  The next_timer_us
check is there to see what the nohz code told us.

> +                * that the CPU could have been idle long (but not forever)
> +                * to help the idle duration predictor do a better job next
> +                * time.
>                  */
>                 measured_us = 9 * MAX_INTERESTING / 10;
>         } else {
> --