FAILED: patch "[PATCH] tick/nohz: Reevaluate the tick for kicks flushed outside hardirq" failed to apply to 5.15-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x e389ae428847a2e6a3b698b5967df68ccf90361c
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107061158-failed-5.15-e389ae428847@kernel.org' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From e389ae428847a2e6a3b698b5967df68ccf90361c Mon Sep 17 00:00:00 2001
From: Peng Wang <peng_wang@linux.alibaba.com>
Date: Fri, 23 Oct 2026 22:51:00 +0800
Subject: [PATCH] tick/nohz: Reevaluate the tick for kicks flushed outside
 hardirq

The nohz full kick relies on interrupt exit to reevaluate the
tick. However, a remote kick can also be flushed from task context without
an interrupt 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 interrupt 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@kernel.org>
Co-developed-by: He Rongguang <herongguang@linux.alibaba.com>
Signed-off-by: He Rongguang <herongguang@linux.alibaba.com>
Signed-off-by: Peng Wang <peng_wang@linux.alibaba.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20261023145101.8217-1-peng_wang@linux.alibaba.com
---
 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 6c3fea3867139..baa3ac9b48f88 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) =
-- 
2.53.0