Re: [RFC 1/3] tick-sched: Remove last_tick and calculate next tick from now

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, Nov 12, 2024 at 12:43:58AM +0100, Frederic Weisbecker wrote:
> Le Fri, Nov 08, 2024 at 05:48:34PM +0000, Joel Fernandes (Google) a écrit :
> > During tick restart, we use last_tick and forward it past now.
> > 
> > Since we are forwarding past now, we can simply use now as a reference
> > instead of last_tick. This patch removes last_tick and does so.
> > 
> > This patch potentially does more mul/imul than the existing code,
> > as sometimes forwarding past now need not be done if last_tick > now.
> 
> Which is not uncommon if idle exited because of a non-timer interrupt
> (remote wake up IPI or hardware interrupt).
> 
> It's also cheaper with hrtimer_forward() if now - last_tick < TICK_NSEC
> which is not uncommon either if idle exited because of a wake-up from the tick
> (schedule_timeout for example).
> 
> > However, the patch is a cleanup which reduces LOC and reduces the size
> > of struct tick_sched.
> 
> Reducing the overhead of idle exit and consolidating its code within existing
> forward API is more important than a per-cpu field.
> 
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > ---
> >  kernel/time/tick-sched.c | 7 ++-----
> >  kernel/time/tick-sched.h | 1 -
> >  kernel/time/timer_list.c | 1 -
> >  3 files changed, 2 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 71a792cd8936..52a4eda664cf 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -837,11 +837,9 @@ EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);
> >  
> >  static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
> >  {
> > +	/* Set the time to expire on the next tick and not some far away future. */
> >  	hrtimer_cancel(&ts->sched_timer);
> > -	hrtimer_set_expires(&ts->sched_timer, ts->last_tick);
> > -
> > -	/* Forward the time to expire in the future */
> > -	hrtimer_forward(&ts->sched_timer, now, TICK_NSEC);
> > +	hrtimer_set_expires(&ts->sched_timer, DIV_ROUND_UP_ULL(now, TICK_NSEC) * TICK_NSEC);
> 
> We don't want to rewrite hrtimer_forward() but, after all, the current expiry is
> enough a relevant information.

Thanks, do you envision any way we can get past the sched_skew_tick issue
Thomas mentioned, if we still want to do something like this patch?

> How about just this? It's worth it as it now forwards after the real last programmed
> tick, which should be close enough from @now with a delta below TICK_NSEC, or even
> better @now is below the expiry. Therefore it should resume as just a no-op
> or at worst an addition within hrtimer_forward():
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 753a184c7090..ffd0c026a248 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -838,7 +838,6 @@ EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);
>  static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
>  {
>  	hrtimer_cancel(&ts->sched_timer);
> -	hrtimer_set_expires(&ts->sched_timer, ts->last_tick);
>  
>  	/* Forward the time to expire in the future */
>  	hrtimer_forward(&ts->sched_timer, now, TICK_NSEC);

For completeness, as we discussed on other thread and Thomas mentioned, we
break code if doing this.

> As for removing last_tick, I think it's a precious debugging information. But
> it's lagging behind the record of the first time only the tick got stopped within
> the last trip to idle. So it could become this instead:
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 753a184c7090..af013f7733b2 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -1042,12 +1041,11 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
>  	if (!tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
>  		calc_load_nohz_start();
>  		quiet_vmstat();
> -
> -		ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
>  		tick_sched_flag_set(ts, TS_FLAG_STOPPED);
>  		trace_tick_stop(1, TICK_DEP_MASK_NONE);
>  	}
>  
> +	ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
>  	ts->next_tick = expires;

Are you suggesting we roll this part of your diff into a new patch (to
improve debug)? I could do that with attribution to you. But I guess I don't
understand this particular part of your diff.

If the tick was already stopped, how does
hrtimer_get_expires(&ts->sched_timer) change since the last time the tick was
stopped? ->last_tick should be set only when the tick was last running and a
stop was attempted? Otherwise your diff might set ->last_tick well into the
future after the tick was already stopped, AFAICS.

thanks,

 - Joel