0009-tick-nohz-fix-wrong-nohz-idle-cpu-state.patch

From 20260203000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Shubhang Kaushik <shubhang@os.amperecomputing.com>
Date: Tue, 03 Feb 2026 16:49:03 -0800
Subject: [PATCH] tick/nohz: Fix wrong NOHZ idle CPU state

Under CONFIG_NO_HZ_FULL, the scheduler tick can get stopped earlier via
tick_nohz_full_stop_tick() before the CPU subsequently enters the idle
path. In this case, tick_nohz_idle_stop_tick() observes TS_FLAG_STOPPED
already set and skips nohz_balance_enter_idle() because the !was_stopped
condition assumes tick-stop and idle-entry are coupled.
This leaves a tickless idle CPU absent from nohz.idle_cpus_mask, making
it invisible to NOHZ idle load balancing while periodic balancing is
also suppressed.

The patch fixes this by decoupling tick-stop transition accounting from
scheduler bookkeeping. idle_jiffies remains updated only on the
tick-stop transition, while nohz_balance_enter_idle() is invoked
whenever a CPU enters idle with the tick already stopped, relying on its
existing idempotent guard to avoid duplicate registration.

Signed-off-by: Shubhang Kaushik <shubhang@os.amperecomputing.com>
Signed-off-by: Adam Li <adamli@os.amperecomputing.com>
Reviewed-by: Christoph Lameter (Ampere) <cl@gentwo.org>
Link: https://lore.kernel.org/lkml/20260203-fix-nohz-idle-v1-1-ad05a5872080@os.amperecomputing.com/
---
 kernel/time/tick-sched.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index d1f27df1e60e..000000000000 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1159,8 +1159,9 @@ void tick_nohz_idle_stop_tick(void)
 		ts->idle_sleeps++;
 		ts->idle_expires = expires;

-		if (!was_stopped && tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
-			kcpustat_dyntick_start(ts->idle_entrytime);
+		if (tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
+			if (!was_stopped)
+				kcpustat_dyntick_start(ts->idle_entrytime);
 			nohz_balance_enter_idle(cpu);
 		}
 	} else {
--
2.49.0