[PATCH v2] tick/nohz: Reevaluate the tick for kicks flushed outside hardirq

[Peng Wang]

The nohz full kick relies on IRQ exit to reevaluate the tick. However,
a remote kick can also be flushed from task context without an IRQ
exit guaranteed to follow. If a tick dependency is set or an earlier
timer is queued after the idle-exit reevaluation, the required tick
update can be delayed until another interrupt.

Reevaluate the tick directly from the callback outside hardirq context.
Keep the existing IRQ-exit handling for hardirq callbacks. The offline
flush remains harmless because tick_sched_timer_dying() has already
cleared TS_FLAG_NOHZ.

Fixes: 4b44a21dd640 ("irq_work, smp: Allow irq_work on call_single_queue")
Fixes: a5183862e76f ("tick/nohz: Conditionally restart tick on idle exit")
Suggested-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Co-developed-by: He Rongguang <herongguang@xxxxxxxxxxxxxxxxx>
Signed-off-by: He Rongguang <herongguang@xxxxxxxxxxxxxxxxx>
Signed-off-by: Peng Wang <peng_wang@xxxxxxxxxxxxxxxxx>
---
Changes in v2:
- Reevaluate the tick directly outside hardirq instead of requeueing,
  as suggested by Frederic.
- Use the full update for both dependencies and timer deadlines. Keep
  IRQ-exit handling for hardirq callbacks and the existing NOHZ guard
  for CPU teardown.

v1: https://lore.kernel.org/all/43668cf88d976084e2865ec07dea82b64ad402ba.1792240863.git.peng_wang@xxxxxxxxxxxxxxxxx/

 kernel/time/tick-sched.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 6c3fea386713..ece2c21cec84 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -400,9 +400,13 @@ static bool can_stop_full_tick(int cpu, struct tick_sched *ts)
 	return true;
 }
 
+static void tick_nohz_full_update_tick(struct tick_sched *ts);
+
 static void nohz_full_kick_func(struct irq_work *work)
 {
-	/* Empty, the tick restart happens on tick_nohz_irq_exit() */
+	/* Task-context flushes may not be followed by an IRQ exit. */
+	if (!in_hardirq())
+		tick_nohz_full_update_tick(this_cpu_ptr(&tick_cpu_sched));
 }
 
 static DEFINE_PER_CPU(struct irq_work, nohz_full_kick_work) =

base-commit: 70dd4ee72eb2d5793c62e71c14e3aba78e5a4e56
-- 
2.47.3