Re: [RFC/RFT][PATCH v2 1/6] time: tick-sched: Reorganize idle tick management code

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Mar 8, 2018 at 4:14 PM, Frederic Weisbecker <frederic@kernel.org> wrote:
> On Thu, Mar 08, 2018 at 10:22:26AM +0100, Rafael J. Wysocki wrote:
>> On Thursday, March 8, 2018 12:18:29 AM CET Frederic Weisbecker wrote:
>> > On Tue, Mar 06, 2018 at 10:02:01AM +0100, Rafael J. Wysocki wrote:
>> > > Index: linux-pm/kernel/time/tick-sched.c
>> > > ===================================================================
>> > > --- linux-pm.orig/kernel/time/tick-sched.c
>> > > +++ linux-pm/kernel/time/tick-sched.c
>> > > +
>> > > +/**
>> > > + * tick_nohz_idle_prepare - prepare for entering idle on the current CPU.
>> > > + *
>> > > + * Called when we start the idle loop.
>> > > + */
>> > > +void tick_nohz_idle_prepare(void)
>> > > +{
>> > > + struct tick_sched *ts;
>> > > +
>> > > + __tick_nohz_idle_prepare();
>> > > +
>> > > + local_irq_disable();
>> > > +
>> > > + ts = this_cpu_ptr(&tick_cpu_sched);
>> > > + ts->inidle = 1;
>> > > +
>> > > + local_irq_enable();
>> > > +}
>> >
>> > Why not calling tick_nohz_start_idle() from there? This is going to
>> > simplify the rest, you won't need to call tick_nohz_idle_go_idle()
>> > from places that don't want to stop the tick and you can then remove
>> > the stop_tick argument.
>>
>> So I guess I would then use ts->idle_entrytime as "now" in the
>> tick_nohz_get_sleep_length() computation, right?
>
> Ah right, I missed the need for ktime_get().
>
> You can't use ts->idle_entrytime in tick_nohz_get_sleep_length() because
> full dynticks doesn't rely on it.
>
> But I think you can just do the following, with a comment explaining that
> idle_entrytime is expected to be fresh enough at this point:
>
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 57b3de4..8e61796 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -923,7 +923,7 @@ static void __tick_nohz_idle_enter(struct tick_sched *ts, bool stop_tick)
>
>                 ts->idle_calls++;
>
> -               expires = tick_nohz_stop_sched_tick(ts, now, cpu);
> +               expires = tick_nohz_stop_sched_tick(ts, ts->idle_entrytime, cpu);
>                 if (expires > 0LL) {
>                         ts->idle_sleeps++;
>                         ts->idle_expires = expires;
>

That's what I was thinking about, but ktime_get() seems to be working too.