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

[Frederic Weisbecker <frederic@kernel.org>]

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.

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);


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;
 
 	/*

Thanks!