Re: [PATCH v9 16/32] timers: Optimization for timer_base_try_to_set_idle()

Anna-Maria Behnsen
2023-12-05T12:12:47Z

Sebastian Siewior <bigeasy@linutronix.de> writes:

> On 2023-12-01 10:26:38 [+0100], Anna-Maria Behnsen wrote:
>> 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
>> nohz_get_next_event() call which was executed before tick_nohz_stop_tick().
>
> as of #15 tick_stopped is set later in tick_nohz_stop_tick() and both
> (tick_sched::tick_stopped and timer_base::is_idle) are cleared in
> tick_nohz_restart_sched_tick().
>
> Then we have tick_nohz_idle_retain_tick() with only one caller and is
> only clearing timer_base::is_idle. Now, wouldn't it make sense to
> preload timer_idle based on timer_base::is_idle?

When revisting the code, this timer_clear_idle() is no longer required
in tick_nohz_idle_retain_tick(). This is only called when the tick is
not stopped - so timer base is not idle as well and this call is
superfluous.

As we keep both states in sync (tick_sched::tick_stopped and
timer_base::is_idle) it doesn't matter which one is used. In
tick_nohz_stop_tick() I don't have access to timer base. I could add it
to timer_base_try_to_set_idle() but it will not make a difference.

> I don't know if it there is a different outcome if timer_base::is_idle
> gets cleared in the idle path vs tick_sched::tick_stopped.
> I can't find nohz_get_next_event().

s/nohz_get_next_event/tick_nohz_next_event/ ...

>> Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
>
> Sebastian

Thanks,

Anna-Maria