When testing softirq based hrtimers on an ARM32 board, with high
resolution mode and nohz are both inactive, softirq based hrtimers
failed to trigger when moved away from an offline CPU. The flowpath
is as follows.

CPU0				CPU1
				softirq based hrtimers are queued
				offline CPU1
				move hrtimers to CPU0 in hrtimers_cpu_dying()
				send IPI to CPU0 to retrigger next event
'softirq_expires_next' is KTIME_MAX
call retrigger_next_event()
highres and nohz is inactive,just return
'softirq_expires_next' is not updated
hrtimer softirq is never triggered

Some softirq based hrtimers are queued on CPU1. Then we offline CPU1.
hrtimers_cpu_dying() moves hrtimers from CPU1 to CPU0, and then it send
a IPI to CPU0 to let CPU0 call retrigger_next_event(). But high
resolution mode and nohz are both inactive. So retrigger_next_event()
just returned. 'softirq_expires_next' is never updated and remains
KTIME_MAX. So hrtimer softirq is never raised.

To fix this issue, we remove the 'hrtimer_hres_active' and
'tick_nohz_active' check in retrigger_next_event(). It is not a fast
path so we don't care about rare extra cost.

Fixes: 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from outgoing CPU earlier")
Co-developed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
---
v1 -> v2:
Change the commit subject and also change code as suggested by Frederic
link to v1: https://lore.kernel.org/all/20250708101727.166892-1-wangxiongfeng2@huawei.com/
---
 kernel/time/hrtimer.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 30899a8cc52c..e8c479329282 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -787,10 +787,10 @@ static void retrigger_next_event(void *arg)
 	 * of the next expiring timer is enough. The return from the SMP
 	 * function call will take care of the reprogramming in case the
 	 * CPU was in a NOHZ idle sleep.
+	 *
+	 * In periodic low resolution mode, the next softirq expiration
+	 * must also be updated.
 	 */
-	if (!hrtimer_hres_active(base) && !tick_nohz_active)
-		return;
-
 	raw_spin_lock(&base->lock);
 	hrtimer_update_base(base);
 	if (hrtimer_hres_active(base))
@@ -2295,11 +2295,6 @@ int hrtimers_cpu_dying(unsigned int dying_cpu)
 				     &new_base->clock_base[i]);
 	}
 
-	/*
-	 * The migration might have changed the first expiring softirq
-	 * timer on this CPU. Update it.
-	 */
-	__hrtimer_get_next_event(new_base, HRTIMER_ACTIVE_SOFT);
 	/* Tell the other CPU to retrigger the next event */
 	smp_call_function_single(ncpu, retrigger_next_event, NULL, 0);
 
-- 
2.20.1

The following commit has been merged into the timers/urgent branch of tip:

Commit-ID:     e895f8e29119c8c966ea794af9e9100b10becb88
Gitweb:        https://git.kernel.org/tip/e895f8e29119c8c966ea794af9e9100b10becb88
Author:        Xiongfeng Wang <wangxiongfeng2@huawei.com>
AuthorDate:    Tue, 05 Aug 2025 16:10:25 +08:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Tue, 09 Sep 2025 14:05:16 +02:00

hrtimers: Unconditionally update target CPU base after offline timer migration

When testing softirq based hrtimers on an ARM32 board, with high resolution
mode and NOHZ inactive, softirq based hrtimers fail to expire after being
moved away from an offline CPU:

CPU0				CPU1
				hrtimer_start(..., HRTIMER_MODE_SOFT);
cpu_down(CPU1)			...
				hrtimers_cpu_dying()
				  // Migrate timers to CPU0
				  smp_call_function_single(CPU0, returgger_next_event);
  retrigger_next_event()
    if (!highres && !nohz)
        return;

As retrigger_next_event() is a NOOP when both high resolution timers and
NOHZ are inactive CPU0's hrtimer_cpu_base::softirq_expires_next is not
updated and the migrated softirq timers never expire unless there is a
softirq based hrtimer queued on CPU0 later.

Fix this by removing the hrtimer_hres_active() and tick_nohz_active() check
in retrigger_next_event(), which enforces a full update of the CPU base.
As this is not a fast path the extra cost does not matter.

[ tglx: Massaged change log ]

Fixes: 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from outgoing CPU earlier")
Co-developed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250805081025.54235-1-wangxiongfeng2@huawei.com
---
 kernel/time/hrtimer.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 30899a8..e8c4793 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -787,10 +787,10 @@ static void retrigger_next_event(void *arg)
 	 * of the next expiring timer is enough. The return from the SMP
 	 * function call will take care of the reprogramming in case the
 	 * CPU was in a NOHZ idle sleep.
+	 *
+	 * In periodic low resolution mode, the next softirq expiration
+	 * must also be updated.
 	 */
-	if (!hrtimer_hres_active(base) && !tick_nohz_active)
-		return;
-
 	raw_spin_lock(&base->lock);
 	hrtimer_update_base(base);
 	if (hrtimer_hres_active(base))
@@ -2295,11 +2295,6 @@ int hrtimers_cpu_dying(unsigned int dying_cpu)
 				     &new_base->clock_base[i]);
 	}
 
-	/*
-	 * The migration might have changed the first expiring softirq
-	 * timer on this CPU. Update it.
-	 */
-	__hrtimer_get_next_event(new_base, HRTIMER_ACTIVE_SOFT);
 	/* Tell the other CPU to retrigger the next event */
 	smp_call_function_single(ncpu, retrigger_next_event, NULL, 0);