Re: [PATCH v10a] timers: Move marking timer bases idle into tick_nohz_stop_tick()

Frederic Weisbecker
2024-02-20T15:25:20Z

On Tue, Feb 20, 2024 at 04:23:26PM +0100, Anna-Maria Behnsen wrote:
> Frederic Weisbecker <frederic@kernel.org> writes:
>
> > On Tue, Feb 20, 2024 at 03:00:57PM +0100, Anna-Maria Behnsen wrote:
> >> Frederic Weisbecker <frederic@kernel.org> writes:
> >>
> >> > Le Tue, Feb 20, 2024 at 01:02:18PM +0100, Anna-Maria Behnsen a écrit :
> >> >> Frederic Weisbecker <frederic@kernel.org> writes:
> >> >>
> >> >> > Le Tue, Feb 20, 2024 at 11:48:19AM +0100, Anna-Maria Behnsen a écrit :
> >> >> >> Frederic Weisbecker <frederic@kernel.org> writes:
> >> >> >> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> >> >> >> index 01fb50c1b17e..b93f0e6f273f 100644
> >> >> >> --- a/kernel/time/tick-sched.c
> >> >> >> +++ b/kernel/time/tick-sched.c
> >> >> >> @@ -895,21 +895,6 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
> >> >> >> /* Make sure we won't be trying to stop it twice in a row. */
> >> >> >> ts->timer_expires_base = 0;
> >> >> >>
> >> >> >> - /*
> >> >> >> - * If this CPU is the one which updates jiffies, then give up
> >> >> >> - * the assignment and let it be taken by the CPU which runs
> >> >> >> - * the tick timer next, which might be this CPU as well. If we
> >> >> >> - * don't drop this here, the jiffies might be stale and
> >> >> >> - * do_timer() never gets invoked. Keep track of the fact that it
> >> >> >> - * was the one which had the do_timer() duty last.
> >> >> >> - */
> >> >> >> - if (cpu == tick_do_timer_cpu) {
> >> >> >> - tick_do_timer_cpu = TICK_DO_TIMER_NONE;
> >> >> >> - ts->do_timer_last = 1;
> >> >> >> - } else if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {
> >> >> >> - ts->do_timer_last = 0;
> >> >> >> - }
> >> >> >> -
> >> >> >> /* Skip reprogram of event if it's not changed */
> >> >> >> if (ts->tick_stopped && (expires == ts->next_tick)) {
> >> >> >> /* Sanity check: make sure clockevent is actually programmed */
> >> >> >
> >> >> > That should work but then you lose the optimization that resets
> >> >> > ts->do_timer_last even if the next timer hasn't changed.
> >> >> >
> >> >>
> >> >> Beside of this optimization thing, I see onther problem. But I'm not
> >> >> sure, if I understood it correctly: When the CPU drops the
> >> >> tick_do_timer_cpu assignment and stops the tick, it is possible, that
> >> >> this CPU nevertheless executes tick_sched_do_timer() and then reassigns
> >> >> to tick_do_timer_cpu?
> >> >
> >> > Yes but in this case a timer interrupt has executed and ts->next_tick
> >> > is cleared, so the above skip reprogramm branch is not taken.
> >> >
> >>
> >> Yes... So I need to change it without dropping the
> >> optimization. Otherwise someone might complain about it.
> >>
> >> Two possible solutions:
> >>
> >> a) split out this if/else thing for dropping the tick_do_timer_cpu
> >> assignment into a separate function and call it:
> >> - before the return in the skip reprogramm branch
> >> - and after the if clause which contains stopping the tick (where it
> >> is executed in the current proposal)
> >>
> >> b) Take my current proposal and add before the return in the skip
> >> reprogramm branch the following lines:
> >>
> >> if (tick_do_timer_cpu != TICK_DO_TIMER_NONE)
> >> ts->do_timer_last = 0;
> >>
> >> as the first part of the tick_do_timer_cpu/last logic shouldn't be
> >> required (because then also ts->next_tick is already cleared).
> >>
> >> What do you prefere? Or do you prefere something else?
> >
> > Wouldn't the following work? If timer_idle is false, then the tick isn't
> > even stopped and there is nothing to do? So you can early return.
> >
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index fdd57f1af1d7..1b2984acafbd 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -924,6 +924,9 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
> > expires = ts->timer_expires;
> > }
> >
> > + if (!timer_idle)
> > + return;
> > +
> > /*
> > * If this CPU is the one which updates jiffies, then give up
> > * the assignment and let it be taken by the CPU which runs
>
> Yes... And then I can drop the if (!timer_idle) thing inside
> !ts->tick_stopped branch.
>

Right!