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

Anna-Maria Behnsen
2024-01-22T11:48:23Z

Frederic Weisbecker <frederic@kernel.org> writes:

> 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.
>

I would like to take 'either'. I thought the changelog already mentioned
it. But maybe I have to make it more explicit. I'll go and rework the
comments once more.

Thanks,

Anna-Maria