Re: [PATCH v10 04/20] timers: Optimization for timer_base_try_to_set_idle()

Frederic Weisbecker
2024-01-17T16:45:35Z

Le Mon, Jan 15, 2024 at 03:37:27PM +0100, Anna-Maria Behnsen a écrit :
> When tick is stopped also the timer base is_idle flag is set. When
> reentering the timer_base_try_to_set_idle() with the tick stopped, there is
> no need to check whether the timer base needs to be set idle again. When a
> timer was enqueued in the meantime, this is already handled by the
> tick_nohz_next_event() call which was executed before
> tick_nohz_stop_tick().
>
> Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
> ---
> kernel/time/tick-sched.c | 2 +-
> kernel/time/timer.c | 11 ++++++++---
> 2 files changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index c6223afc801f..27f1a2ae7f39 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -886,7 +886,7 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
> struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
> unsigned long basejiff = ts->last_jiffies;
> u64 basemono = ts->timer_expires_base;
> - bool timer_idle;
> + bool timer_idle = ts->tick_stopped;
> u64 expires;
>
> /* Make sure we won't be trying to stop it twice in a row. */
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index 3a668060692e..2f69a485a070 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -1999,13 +1999,18 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
> * timer_base_try_to_set_idle() - Try to set the idle state of the timer bases
> * @basej: base time jiffies
> * @basem: base time clock monotonic
> - * @idle: pointer to store the value of timer_base->is_idle
> + * @idle: pointer to store the value of timer_base->is_idle on return;
> + * *idle contains the information whether tick was already stopped
> *
> - * Returns the tick aligned clock monotonic time of the next pending
> - * timer or KTIME_MAX if no timer is pending.
> + * Returns the tick aligned clock monotonic time of the next pending timer or
> + * KTIME_MAX if no timer is pending. When tick was already stopped KTIME_MAX is
> + * returned as well.
> */
> u64 timer_base_try_to_set_idle(unsigned long basej, u64 basem, bool *idle)
> {
> + if (*idle)
> + return KTIME_MAX;

Ok now I see the reason behind the behavioural change.

So either:

* We remove the old behaviour consisting in clearing base->is_idle if the new
next timer is within a jiffy while the tick is stopped. But then the changelog
from the previous patch should state that and comments must be clarified.

or:

* We restore the old behaviour, making things a bit more complicated I guess.

Thanks.