Re: [PATCH v3 05/17] timer: Rework idle logic

Frederic Weisbecker
2022-10-26T13:27:33Z

On Tue, Oct 25, 2022 at 03:58:38PM +0200, Anna-Maria Behnsen wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
>
> To improve readability of the code, split base->idle calculation and
> expires calculation into separate parts.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
> ---
> kernel/time/timer.c | 29 ++++++++++++++---------------
> 1 file changed, 14 insertions(+), 15 deletions(-)
>
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index f34bc75ff848..cb4194ecca60 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -1727,21 +1727,20 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
> base->clk = nextevt;
> }
>
> - if (time_before_eq(nextevt, basej)) {
> - expires = basem;
> - base->is_idle = false;
> - } else {
> - if (base->timers_pending)
> - expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
> - /*
> - * If we expect to sleep more than a tick, mark the base idle.
> - * Also the tick is stopped so any added timer must forward
> - * the base clk itself to keep granularity small. This idle
> - * logic is only maintained for the BASE_STD base, deferrable
> - * timers may still see large granularity skew (by design).
> - */
> - if ((expires - basem) > TICK_NSEC)
> - base->is_idle = true;
> + /*
> + * Base is idle if the next event is more than a tick away. Also
> + * the tick is stopped so any added timer must forward the base clk
> + * itself to keep granularity small. This idle logic is only
> + * maintained for the BASE_STD base, deferrable timers may still
> + * see large granularity skew (by design).
> + */
> + base->is_idle = time_after(nextevt, basej + 1);

A subtle change, yet welcome, is introduced here. If the next event is just
one jiffy ahead, base->is_idle used to be left untouched. So the behaviour
was:

1) If the tick was already stopped (so we must be after an idle IRQ),
base->is_idle remains true
2) If the tick was not yet stopped, base->is_idle remains false

After this patch, 1) becomes:

1) If the tick was already stopped, turn base->is_idle to false

As a result, we may spare an IPI if we remotely enqueue a timer to
an idle CPU that is going to tick on the next jiffy.

Whether it's worth mentioning in the changelog, I leave it to you.

> +
> + if (base->timers_pending) {
> + /* If we missed a tick already, force 0 delta */
> + if (time_before_eq(nextevt, basej))
> + nextevt = basej;

Can be time_before() then, right?

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

> + expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
> }
> raw_spin_unlock(&base->lock);
>
> --
> 2.30.2
>