[PATCH v3] 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 v3:
- Add WARN_ON_ONCE() checks for a non-nohz_full CPU and for a
  task-context callback still marked idle, as requested by Frederic.

v2: https://lore.kernel.org/all/20261022175500.15940-1-peng_wang@xxxxxxxxxxxxxxxxx/

 kernel/time/tick-sched.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 6c3fea386713..baa3ac9b48f8 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -400,9 +400,21 @@ 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() */
+	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
+
+	if (WARN_ON_ONCE(!tick_nohz_full_cpu(smp_processor_id())))
+		return;
+
+	/* Task-context flushes may not be followed by an IRQ exit. */
+	if (!in_hardirq()) {
+		if (WARN_ON_ONCE(tick_sched_flag_test(ts, TS_FLAG_INIDLE)))
+			return;
+		tick_nohz_full_update_tick(ts);
+	}
 }
 
 static DEFINE_PER_CPU(struct irq_work, nohz_full_kick_work) =

base-commit: 70dd4ee72eb2d5793c62e71c14e3aba78e5a4e56
-- 
2.47.3