Re: [PATCH v9 11/32] timers: Rework idle logic

Frederic Weisbecker
2023-12-20T14:00:55Z

Le Fri, Dec 01, 2023 at 10:26:33AM +0100, Anna-Maria Behnsen a écrit :
> From: Thomas Gleixner <tglx@linutronix.de>
>
> To improve readability of the code, split base->idle calculation and
> expires calculation into separate parts. While at it, update the comment
> about timer base idle marking.
>
> Thereby the following subtle change happens if the next event is just one
> jiffy ahead and the tick was already stopped: Originally base->is_idle
> remains true in this situation. Now base->is_idle turns to false. This may
> spare an IPI if a timer is enqueued remotely to an idle CPU that is going
> to tick on the next jiffy.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
> ---
> v9: Re-ordering to not hurt the eyes and update comment
> v4: Change condition to force 0 delta and update commit message (Frederic)
> ---
> kernel/time/timer.c | 31 ++++++++++++++++---------------
> 1 file changed, 16 insertions(+), 15 deletions(-)
>
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index fee42dda8237..0826018d9873 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -1943,22 +1943,23 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
> */
> __forward_timer_base(base, basej);
>
> - 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;
> + if (base->timers_pending) {
> + /* If we missed a tick already, force 0 delta */
> + if (time_before(nextevt, basej))
> + nextevt = basej;
> + expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
> }
> +
> + /*
> + * Base is idle if the next event is more than a tick away.
> + *
> + * If the base is marked idle then any timer add operation 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);
> +

Much better, thanks! :-)