linux-kernel - [PATCH 1/2] tick/nohz: Fix wrong NOHZ idle CPU state

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Message-Id: <20250815065115.289337-2-adamli@os.amperecomputing.com>
Date: Fri, 15 Aug 2025 06:51:14 +0000
From: Adam Li <adamli@...amperecomputing.com>
To: anna-maria@...utronix.de,
	frederic@...nel.org,
	mingo@...nel.org,
	tglx@...utronix.de,
	cl@...two.org
Cc: cl@...ux.com,
	linux-kernel@...r.kernel.org,
	patches@...erecomputing.com,
	Adam Li <adamli@...amperecomputing.com>
Subject: [PATCH 1/2] tick/nohz: Fix wrong NOHZ idle CPU state

NOHZ idle load balance is done among CPUs in nohz.idle_cpus_mask.
A CPU is added to nohz.idle_cpus_mask in:
do_idle()
  -> tick_nohz_idle_stop_tick()
     -> nohz_balance_enter_idle()

nohz_balance_enter_idle() is called if:
1) tick is stopped (TS_FLAG_STOPPED is set)
2) and tick was not already stopped before tick_nohz_idle_stop_tick()
   stops the tick (!was_stopped)

When CONFIG_NO_HZ_FULL is set and the CPU is in the nohz_full list
then 'was_stopped' may always be true.
The flag 'TS_FLAG_STOPPED' may be already set in
tick_nohz_full_stop_tick(). So nohz_balance_enter_idle() has no chance
to be called.

As a result, CPU will stay in a 'wrong' state:
1) tick is stopped (TS_FLAG_STOPPED is set)
2) and CPU is not in nohz.idle_cpus_mask
3) and CPU stays idle

Neither the periodic nor the NOHZ idle load balancing can move task
to this CPU. Some CPUs keep idle while others busy.

In nohz_balance_enter_idle(), 'rq->nohz_tick_stopped' is checked to avoid
duplicated nohz.idle_cpus_mask setting. So for nohz_balance_enter_idle()
there is no need to check the '!was_stopped' condition.

This patch will add the CPU to nohz.idle_cpus_mask as expected.

Signed-off-by: Adam Li <adamli@...amperecomputing.com>
Reviewed-by: Christoph Lameter (Ampere) <cl@...two.org>
---
 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 c527b421c865..b900a120ab54 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1229,8 +1229,9 @@ 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)) {
-			ts->idle_jiffies = ts->last_jiffies;
+		if (tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
+			if (!was_stopped)
+				ts->idle_jiffies = ts->last_jiffies;
 			nohz_balance_enter_idle(cpu);
 		}
 	} else {
-- 
2.34.1