[PATCH v8 08/25] timer: Rework idle logic

From: Anna-Maria Behnsen
Date: Wed Oct 04 2023 - 08:35:53 EST


From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

To improve readability of the code, split base->idle calculation and
expires calculation into separate parts.

Thereby the following subtle change happens if the next event is just one
jiffy ahead and the tick was already stopped: Originally base->is_idle
remains true in this situation. Now base->is_idle turns to false. This may
spare an IPI if a timer is enqueued remotely to an idle CPU that is going
to tick on the next jiffy.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Anna-Maria Behnsen <anna-maria@xxxxxxxxxxxxx>
Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
v4: Change condition to force 0 delta and update commit message (Frederic)
---
kernel/time/timer.c | 29 ++++++++++++++---------------
1 file changed, 14 insertions(+), 15 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index dc58c479d35a..18f8aac9b19a 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1943,21 +1943,20 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
*/
__forward_timer_base(base, basej);

- if (time_before_eq(nextevt, basej)) {
- expires = basem;
- base->is_idle = false;
- } else {
- if (base->timers_pending)
- expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
- /*
- * If we expect to sleep more than a tick, mark the base idle.
- * Also the tick is stopped so any added timer must forward
- * the base clk itself to keep granularity small. This idle
- * logic is only maintained for the BASE_STD base, deferrable
- * timers may still see large granularity skew (by design).
- */
- if ((expires - basem) > TICK_NSEC)
- base->is_idle = true;
+ /*
+ * Base is idle if the next event is more than a tick away. Also
+ * the tick is stopped so any added timer must forward the base clk
+ * itself to keep granularity small. This idle logic is only
+ * maintained for the BASE_STD base, deferrable timers may still
+ * see large granularity skew (by design).
+ */
+ base->is_idle = time_after(nextevt, basej + 1);
+
+ if (base->timers_pending) {
+ /* If we missed a tick already, force 0 delta */
+ if (time_before(nextevt, basej))
+ nextevt = basej;
+ expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
}
raw_spin_unlock(&base->lock);

--
2.39.2