Re: [PATCH 3/3] tick/nohz: Don't shutdown the lowres tick from itself

From: Joel Fernandes
Date: Fri Jul 14 2023 - 14:44:56 EST


On 7/14/23 08:08, Frederic Weisbecker wrote:
In lowres dynticks mode, just like in highres dynticks mode, when there
is no tick to program in the future, the tick eventually gets
deactivated either:

* From the idle loop if in idle mode.
* From the IRQ exit if in full dynticks mode.

Therefore there is no need to deactivate it from the tick itself. This
just just brings more overhead in the idle tick path for no reason.

Fixes: 62c1256d5447 ("timers/nohz: Switch to ONESHOT_STOPPED in the low-res handler when the tick is stopped")
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
kernel/time/tick-sched.c | 24 +++++++++++++-----------
1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index dd25da8531f4..101251e103be 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1392,18 +1392,16 @@ static void tick_lowres_handler(struct clock_event_device *dev)
tick_sched_do_timer(ts, now);
tick_sched_handle(ts, regs);
- if (unlikely(ts->tick_stopped)) {
- /*
- * The clockevent device is not reprogrammed, so change the
- * clock event device to ONESHOT_STOPPED to avoid spurious
- * interrupts on devices which might not be truly one shot.
- */
- tick_program_event(KTIME_MAX, 1);
- return;
+ /*
+ * In dynticks mode, tick reprogram is deferred:
+ * - to the idle task if in dynticks-idle
+ * - to IRQ exit if in full-dynticks.
+ */
+ if (likely(!ts->tick_stopped)) {
+ hrtimer_forward(&ts->sched_timer, now, TICK_NSEC);
+ tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
}
- hrtimer_forward(&ts->sched_timer, now, TICK_NSEC);
- tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);

One slight concern here though, where in the idle loop is the removed statement "tick_program_event(KTIME_MAX, 1);" happening if the tick was already stopped before? If it is happening in tick_nohz_stop_tick(), don't we early return from there and avoid doing that "tick_program_event(KTIME_MAX, 1);" altogether, if the tick was already stopped and the next event has not changed?

/* Skip reprogram of event if its not changed */
if (ts->tick_stopped && (expires == ts->next_tick)) {
/* Sanity check: make sure clockevent is actually programmed */
if (tick == KTIME_MAX || ts->next_tick == [...]
return;
[...]
}

Also just a nit, here you can remove indent by doing:

if (unlikely(ts->tick_stopped))
return;
hrtimer_forward(&ts->sched_timer, now, TICK_NSEC);
tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);

Which is pretty much the original code except for the tick_program_event().

thanks,

- Joel


}
static inline void tick_nohz_activate(struct tick_sched *ts, int mode)
@@ -1508,7 +1506,11 @@ static enum hrtimer_restart tick_highres_handler(struct hrtimer *timer)
else
ts->next_tick = 0;
- /* No need to reprogram if we are in idle or full dynticks mode */
+ /*
+ * In dynticks mode, tick reprogram is deferred:
+ * - to the idle task if in dynticks-idle
+ * - to IRQ exit if in full-dynticks.
+ */
if (unlikely(ts->tick_stopped))
return HRTIMER_NORESTART;