[Question] timers: trigger_dyntick_cpu() vs TIMER_DEFERRABLE

[Valentin Schneider <vschneid@redhat.com>]

Hi,

I've been incidentally staring at some NOHZ bits related to the timer
wheels, and trigger_dyntick_cpu() confuses me:

  static void
  trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
  {
          [...]
          /*
           * TODO: This wants some optimizing similar to the code below, but we
           * will do that when we switch from push to pull for deferrable timers.
           */
          if ((timer->flags & TIMER_DEFERRABLE)) {
                  if (tick_nohz_full_cpu(base->cpu))
                          wake_up_nohz_cpu(base->cpu);
                  return;
          }
          [...]
  }

From what I grok out of get_nohz_timer_target(), under
timers_migration_enabled we should migrate the timer to an non-idle CPU
(or at the very least a non-isolated CPU) *before* enqueuing the
timer. Without timers_migration_enabled (or if TIMER_PINNED), I don't see
anything that could migrate the timer elsewhere, so:

Why bother kicking a NOHZ CPU for a deferrable timer if it is the next
expiring one? Per the definition:

 * @TIMER_DEFERRABLE: A deferrable timer will work normally when the
 * system is busy, but will not cause a CPU to come out of idle just
 * to service it; instead, the timer will be serviced when the CPU
 * eventually wakes up with a subsequent non-deferrable timer.

I tried to find some discussion over this in LKML, but found nothing.
v3 of the patch did *not* kick a CPU for a deferrable timer, but v4 (the
one that ended up merged) did (see below). Patch in question is:

  a683f390b93f ("timers: Forward the wheel clock whenever possible")

Thanks

====
v3
====
@@ -520,23 +522,27 @@ static void internal_add_timer(struct ti
        __internal_add_timer(base, timer);

        /*
-	 * Check whether the other CPU is in dynticks mode and needs
-	 * to be triggered to reevaluate the timer wheel.  We are
-	 * protected against the other CPU fiddling with the timer by
-	 * holding the timer base lock. This also makes sure that a
-	 * CPU on the way to stop its tick can not evaluate the timer
-	 * wheel.
-	 *
-	 * Spare the IPI for deferrable timers on idle targets though.
-	 * The next busy ticks will take care of it. Except full dynticks
-	 * require special care against races with idle_cpu(), lets deal
-	 * with that later.
-	 */
-	if (IS_ENABLED(CONFIG_NO_HZ_COMMON) && base->nohz_active) {
-		if (!(timer->flags & TIMER_DEFERRABLE) ||
-		    tick_nohz_full_cpu(base->cpu))
-			wake_up_nohz_cpu(base->cpu);
-	}
+	 * We might have to IPI the remote CPU if the base is idle and the
+	 * timer is not deferrable. If the other cpu is on the way to idle
+	 * then it can't set base->is_idle as we hold base lock.
+	 */
+	if (!IS_ENABLED(CONFIG_NO_HZ_COMMON) || !base->is_idle ||
+	    (timer->flags & TIMER_DEFERRABLE))
+		return;
+
+	/* Check whether this is the new first expiring timer */
+	if (time_after_eq(timer->expires, base->next_expiry))
+		return;
+	base->next_expiry = timer->expires;
+
+	/*
+	 * Check whether the other CPU is in dynticks mode and needs to be
+	 * triggered to reevaluate the timer wheel.  We are protected against
+	 * the other CPU fiddling with the timer by holding the timer base
+	 * lock.
+	 */
+	if (tick_nohz_full_cpu(base->cpu))
+		wake_up_nohz_cpu(base->cpu);
 }
====
v4
====
@@ -519,24 +521,37 @@ static void internal_add_timer(struct ti
 {
        __internal_add_timer(base, timer);

+	if (!IS_ENABLED(CONFIG_NO_HZ_COMMON) || !base->nohz_active)
+		return;
+
        /*
-	 * Check whether the other CPU is in dynticks mode and needs
-	 * to be triggered to reevaluate the timer wheel.  We are
-	 * protected against the other CPU fiddling with the timer by
-	 * holding the timer base lock. This also makes sure that a
-	 * CPU on the way to stop its tick can not evaluate the timer
-	 * wheel.
-	 *
-	 * Spare the IPI for deferrable timers on idle targets though.
-	 * The next busy ticks will take care of it. Except full dynticks
-	 * require special care against races with idle_cpu(), lets deal
-	 * with that later.
-	 */
-	if (IS_ENABLED(CONFIG_NO_HZ_COMMON) && base->nohz_active) {
-		if (!(timer->flags & TIMER_DEFERRABLE) ||
-		    tick_nohz_full_cpu(base->cpu))
+	 * This wants some optimizing similar to the below, but we do that
+	 * when we switch from push to pull for deferrable timers.
+	 */
+	if (timer->flags & TIMER_DEFERRABLE) {
+		if (tick_nohz_full_cpu(base->cpu))
                        wake_up_nohz_cpu(base->cpu);
+		return;
        }
+
+	/*
+	 * We might have to IPI the remote CPU if the base is idle and the
+	 * timer is not deferrable. If the other cpu is on the way to idle
+	 * then it can't set base->is_idle as we hold base lock.
+	 */
+	if (!base->is_idle)
+		return;
+
+	/* Check whether this is the new first expiring timer */
+	if (time_after_eq(timer->expires, base->next_expiry))
+		return;
+
+	/*
+	 * Set the next expiry time and kick the cpu so it can reevaluate the
+	 * wheel
+	 */
+	base->next_expiry = timer->expires;
+	wake_up_nohz_cpu(base->cpu);
 }