hrtimers: Force migrate away hrtimers queued after CPUHP_AP_HRTIMERS_DYING

hrtimers are migrated away from the dying CPU to any online target at the CPUHP_AP_HRTIMERS_DYING stage in order not to delay bandwidth timers handling tasks involved in the CPU hotplug forward progress.

However wake ups can still be performed by the outgoing CPU after CPUHP_AP_HRTIMERS_DYING. Those can result again in bandwidth timers being armed. Depending on several considerations (crystal ball power management based election, earliest timer already enqueued, timer migration enabled or not), the target may eventually be the current CPU even if offline. If that happens, the timer is eventually ignored.

The most notable example is RCU which had to deal with each and every of those wake-ups by deferring them to an online CPU, along with related workarounds:

_ e787644caf76 (rcu: Defer RCU kthreads wakeup when CPU is dying) _ 9139f93209d1 (rcu/nocb: Fix RT throttling hrtimer armed from offline CPU) _ f7345ccc62a4 (rcu/nocb: Fix rcuog wake-up from offline softirq)

The problem isn't confined to RCU though as the stop machine kthread (which runs CPUHP_AP_HRTIMERS_DYING) reports its completion at the end of its work through cpu_stop_signal_done() and performs a wake up that eventually arms the deadline server timer:

           WARNING: CPU: 94 PID: 588 at kernel/time/hrtimer.c:1086 hrtimer_start_range_ns+0x289/0x2d0
           CPU: 94 UID: 0 PID: 588 Comm: migration/94 Not tainted
           Stopper: multi_cpu_stop+0x0/0x120 <- stop_machine_cpuslocked+0x66/0xc0
           RIP: 0010:hrtimer_start_range_ns+0x289/0x2d0
           Call Trace:
            <TASK>
            ? hrtimer_start_range_ns
            start_dl_timer
            enqueue_dl_entity
            dl_server_start
            enqueue_task_fair
            enqueue_task
            ttwu_do_activate
            try_to_wake_up
            complete
            cpu_stopper_thread
            smpboot_thread_fn
            kthread
            ret_from_fork
            ret_from_fork_asm
            </TASK>

Instead of providing yet another bandaid to work around the situation, fix it from hrtimers infrastructure instead: always migrate away a timer to an online target whenever it is enqueued from an offline CPU.

This will also allow to revert all the above RCU disgraceful hacks.

Reported-by Vlad Poenaru <[email protected]>

Reported-by Usama Arif <[email protected]>

Fixes 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from outgoing CPU earlier")

Closes [email protected]

Signed-off-by Frederic Weisbecker <[email protected]>

Signed-off-by Paul E. McKenney <[email protected]>

---
 include/linux/hrtimer_defs.h |  1 +
 kernel/time/hrtimer.c        | 92 +++++++++++++++++++++++++++++-------
 2 files changed, 75 insertions(+), 18 deletions(-)

include/linux/hrtimer_defs.h +1 -0

index c3b4b7ed7c16..84a5045f80f3 100644
@@ -125,6 +125,7 @@ struct hrtimer_cpu_base {
 	ktime_t				softirq_expires_next;
 	struct hrtimer			*softirq_next_timer;
 	struct hrtimer_clock_base	clock_base[HRTIMER_MAX_CLOCK_BASES];
+	call_single_data_t		csd;
 } ____cacheline_aligned;
 
 

kernel/time/hrtimer.c +74 -18

index 030426c8c944..082d4b687fa1 100644
@@ -58,6 +58,8 @@
 #define HRTIMER_ACTIVE_SOFT	(HRTIMER_ACTIVE_HARD << MASK_SHIFT)
 #define HRTIMER_ACTIVE_ALL	(HRTIMER_ACTIVE_SOFT | HRTIMER_ACTIVE_HARD)
 
+static void retrigger_next_event(void *arg);
+
 /*
  * The timer bases:
  *

@@ -111,7 +113,8 @@ DEFINE_PER_CPU(struct hrtimer_cpu_base, hrtimer_bases) =
 			.clockid = CLOCK_TAI,
 			.get_time = &ktime_get_clocktai,
 		},
-	}
+	},
+	.csd = CSD_INIT(retrigger_next_event, NULL)
 };
 
 static const int hrtimer_clock_to_base_table[MAX_CLOCKS] = {

@@ -124,6 +127,14 @@ static const int hrtimer_clock_to_base_table[MAX_CLOCKS] = {
 	[CLOCK_TAI]		= HRTIMER_BASE_TAI,
 };
 
+static inline bool hrtimer_base_is_online(struct hrtimer_cpu_base *base)
+{
+	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
+		return true;
+	else
+		return likely(base->online);
+}
+
 /*
  * Functions and macros which are different for UP/SMP systems are kept in a
  * single place

@@ -183,27 +194,58 @@ struct hrtimer_clock_base *lock_hrtimer_base(const struct hrtimer *timer,
 }
 
 /*
- * We do not migrate the timer when it is expiring before the next
- * event on the target cpu. When high resolution is enabled, we cannot
- * reprogram the target cpu hardware and we would cause it to fire
- * late. To keep it simple, we handle the high resolution enabled and
- * disabled case similar.
+ * Check if the elected target is suitable considering its next
+ * event and the hotplug state of the current CPU.
+ *
+ * If the elected target is remote and its next event is after the timer
+ * to queue, then a remote reprogram is necessary. However there is no
+ * guarantee the IPI handling the operation would arrive in time to meet
+ * the high resolution deadline. In this case the local CPU becomes a
+ * preferred target, unless it is offline.
+ *
+ * High and low resolution modes are handled the same way for simplicity.
  *
  * Called with cpu_base->lock of target cpu held.
  */
-static int
-hrtimer_check_target(struct hrtimer *timer, struct hrtimer_clock_base *new_base)
+static bool
+hrtimer_suitable_target(struct hrtimer *timer,
+			struct hrtimer_clock_base *new_base,
+			struct hrtimer_cpu_base *new_cpu_base,
+			struct hrtimer_cpu_base *this_cpu_base)
 {
 	ktime_t expires;
 
+	/*
+	 * The local CPU clockevent can be reprogrammed. Also get_target_base()
+	 * guarantees it is online.
+	 */
+	if (new_cpu_base == this_cpu_base)
+		return true;
+
+	/*
+	 * The offline local CPU can't be the default target if the
+	 * next remote target event is after this timer. Keep the
+	 * elected new base. An IPI will we issued to reprogram
+	 * it as a last resort.
+	 */
+	if (!hrtimer_base_is_online(this_cpu_base))
+		return true;
+
 	expires = ktime_sub(hrtimer_get_expires(timer), new_base->offset);
-	return expires < new_base->cpu_base->expires_next;
+
+	return expires >= new_base->cpu_base->expires_next;
 }
 
 static inline
 struct hrtimer_cpu_base *get_target_base(struct hrtimer_cpu_base *base,
 					 int pinned)
 {
+	if (!hrtimer_base_is_online(base)) {
+		int cpu = cpumask_any_and(cpu_online_mask, housekeeping_cpumask(HK_TYPE_TIMER));
+
+		return &per_cpu(hrtimer_bases, cpu);
+	}
+
 #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
 	if (static_branch_likely(&timers_migration_enabled) && !pinned)
 		return &per_cpu(hrtimer_bases, get_nohz_timer_target());

@@ -254,8 +296,8 @@ switch_hrtimer_base(struct hrtimer *timer, struct hrtimer_clock_base *base,
 		raw_spin_unlock(&base->cpu_base->lock);
 		raw_spin_lock(&new_base->cpu_base->lock);
 
-		if (new_cpu_base != this_cpu_base &&
-		    hrtimer_check_target(timer, new_base)) {
+		if (!hrtimer_suitable_target(timer, new_base, new_cpu_base,
+					     this_cpu_base)) {
 			raw_spin_unlock(&new_base->cpu_base->lock);
 			raw_spin_lock(&base->cpu_base->lock);
 			new_cpu_base = this_cpu_base;

@@ -264,8 +306,8 @@ switch_hrtimer_base(struct hrtimer *timer, struct hrtimer_clock_base *base,
 		}
 		WRITE_ONCE(timer->base, new_base);
 	} else {
-		if (new_cpu_base != this_cpu_base &&
-		    hrtimer_check_target(timer, new_base)) {
+		if (!hrtimer_suitable_target(timer, new_base,  new_cpu_base,
+					     this_cpu_base)) {
 			new_cpu_base = this_cpu_base;
 			goto again;
 		}

@@ -716,8 +758,6 @@ static inline int hrtimer_is_hres_enabled(void)
 	return hrtimer_hres_enabled;
 }
 
-static void retrigger_next_event(void *arg);
-
 /*
  * Switch to high resolution mode
  */

@@ -1206,6 +1246,7 @@ static int __hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim,
 				    u64 delta_ns, const enum hrtimer_mode mode,
 				    struct hrtimer_clock_base *base)
 {
+	struct hrtimer_cpu_base *this_cpu_base = this_cpu_ptr(&hrtimer_bases);
 	struct hrtimer_clock_base *new_base;
 	bool force_local, first;
 

@@ -1217,9 +1258,15 @@ static int __hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim,
 	 * and enforce reprogramming after it is queued no matter whether
 	 * it is the new first expiring timer again or not.
 	 */
-	force_local = base->cpu_base == this_cpu_ptr(&hrtimer_bases);
+	force_local = base->cpu_base == this_cpu_base;
 	force_local &= base->cpu_base->next_timer == timer;
 
+	/*
+	 * Don't force local queuing if this enqueue happens on a unplugged
+	 * CPU after hrtimer_cpu_dying() has been invoked.
+	 */
+	force_local &= this_cpu_base->online;
+
 	/*
 	 * Remove an active timer from the queue. In case it is not queued
 	 * on the current CPU, make sure that remove_hrtimer() updates the

@@ -1249,8 +1296,17 @@ static int __hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim,
 	}
 
 	first = enqueue_hrtimer(timer, new_base, mode);
-	if (!force_local)
-		return first;
+	if (!force_local) {
+		if (hrtimer_base_is_online(this_cpu_base))
+			return first;
+
+		if (first) {
+			struct hrtimer_cpu_base *new_cpu_base = new_base->cpu_base;
+
+			smp_call_function_single_async(new_cpu_base->cpu, &new_cpu_base->csd);
+		}
+		return 0;
+	}
 
 	/*
 	 * Timer was forced to stay on the current CPU to avoid

Signature · 1 line

2.46.0

Quoted 66 lines · Frederic Weisbecker

hrtimers are migrated away from the dying CPU to any online target at
the CPUHP_AP_HRTIMERS_DYING stage in order not to delay bandwidth timers
handling tasks involved in the CPU hotplug forward progress.

However wake ups can still be performed by the outgoing CPU after CPUHP_AP_HRTIMERS_DYING. Those can result again in bandwidth timers being armed. Depending on several considerations (crystal ball power management based election, earliest timer already enqueued, timer
migration enabled or not), the target may eventually be the current CPU even if offline. If that happens, the timer is eventually ignored.

The most notable example is RCU which had to deal with each and every of
those wake-ups by deferring them to an online CPU, along with related
workarounds:

_ e787644caf76 (rcu: Defer RCU kthreads wakeup when CPU is dying) _ 9139f93209d1 (rcu/nocb: Fix RT throttling hrtimer armed from offline CPU)
_ f7345ccc62a4 (rcu/nocb: Fix rcuog wake-up from offline softirq)

The problem isn't confined to RCU though as the stop machine kthread
(which runs CPUHP_AP_HRTIMERS_DYING) reports its completion at the end
of its work through cpu_stop_signal_done() and performs a wake up that
eventually arms the deadline server timer:

           WARNING: CPU: 94 PID: 588 at kernel/time/hrtimer.c:1086
hrtimer_start_range_ns+0x289/0x2d0
           CPU: 94 UID: 0 PID: 588 Comm: migration/94 Not tainted
           Stopper: multi_cpu_stop+0x0/0x120 <-
stop_machine_cpuslocked+0x66/0xc0
           RIP: 0010:hrtimer_start_range_ns+0x289/0x2d0
           Call Trace:
            <TASK>

            ? hrtimer_start_range_ns
            start_dl_timer
            enqueue_dl_entity
            dl_server_start
            enqueue_task_fair
            enqueue_task
            ttwu_do_activate
            try_to_wake_up
            complete
            cpu_stopper_thread
            smpboot_thread_fn
            kthread
            ret_from_fork
            ret_from_fork_asm

            </TASK>

Instead of providing yet another bandaid to work around the situation,
fix it from hrtimers infrastructure instead: always migrate away a timer to an online target whenever it is enqueued from an offline CPU.

This will also allow to revert all the above RCU disgraceful hacks.

Quoted 7 lines

Reported-by Vlad Poenaru <[email protected]>

Reported-by Usama Arif <[email protected]>

Fixes 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from

outgoing CPU earlier")

Closes [email protected]

Signed-off-by Frederic Weisbecker <[email protected]>

Signed-off-by Paul E. McKenney <[email protected]>

This passes over-holiday testing rcutorture, so, perhaps redundantly:

Tested-by Paul E. McKenney <[email protected]>

Quoted 75 lines · Paul E. McKenney

Quoted 66 lines · Frederic Weisbecker

hrtimers are migrated away from the dying CPU to any online target at
the CPUHP_AP_HRTIMERS_DYING stage in order not to delay bandwidth timers
handling tasks involved in the CPU hotplug forward progress.

However wake ups can still be performed by the outgoing CPU after CPUHP_AP_HRTIMERS_DYING. Those can result again in bandwidth timers being armed. Depending on several considerations (crystal ball power management based election, earliest timer already enqueued, timer
migration enabled or not), the target may eventually be the current CPU even if offline. If that happens, the timer is eventually ignored.

The most notable example is RCU which had to deal with each and every of
those wake-ups by deferring them to an online CPU, along with related
workarounds:

_ e787644caf76 (rcu: Defer RCU kthreads wakeup when CPU is dying) _ 9139f93209d1 (rcu/nocb: Fix RT throttling hrtimer armed from offline CPU)
_ f7345ccc62a4 (rcu/nocb: Fix rcuog wake-up from offline softirq)

The problem isn't confined to RCU though as the stop machine kthread
(which runs CPUHP_AP_HRTIMERS_DYING) reports its completion at the end
of its work through cpu_stop_signal_done() and performs a wake up that
eventually arms the deadline server timer:

           WARNING: CPU: 94 PID: 588 at kernel/time/hrtimer.c:1086
hrtimer_start_range_ns+0x289/0x2d0
           CPU: 94 UID: 0 PID: 588 Comm: migration/94 Not tainted
           Stopper: multi_cpu_stop+0x0/0x120 <-
stop_machine_cpuslocked+0x66/0xc0
           RIP: 0010:hrtimer_start_range_ns+0x289/0x2d0
           Call Trace:
            <TASK>

            ? hrtimer_start_range_ns
            start_dl_timer
            enqueue_dl_entity
            dl_server_start
            enqueue_task_fair
            enqueue_task
            ttwu_do_activate
            try_to_wake_up
            complete
            cpu_stopper_thread
            smpboot_thread_fn
            kthread
            ret_from_fork
            ret_from_fork_asm

            </TASK>

Instead of providing yet another bandaid to work around the situation,
fix it from hrtimers infrastructure instead: always migrate away a timer to an online target whenever it is enqueued from an offline CPU.

This will also allow to revert all the above RCU disgraceful hacks.

Quoted 7 lines

Reported-by Vlad Poenaru <[email protected]>

Reported-by Usama Arif <[email protected]>

Fixes 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from

outgoing CPU earlier")

Closes [email protected]

Signed-off-by Frederic Weisbecker <[email protected]>

Signed-off-by Paul E. McKenney <[email protected]>

Quoted 3 lines

This passes over-holiday testing rcutorture, so, perhaps redundantly:

Tested-by Paul E. McKenney <[email protected]>

Hi,

I encountered the same issue even after applying this patch. Below are the details of the warning and call trace.

migration/3: ------------[ cut here ]------------ migration/3: WARNING: CPU: 3 PID: 42 at kernel/time/hrtimer.c:1125 enqueue_hrtimer+0x7c/0xec
migration/3: CPU: 3 UID: 0 PID: 42 Comm: migration/3 Tainted: G
OE 6.12.18-android16-0-g59cb5a849beb-4k #1
0b440e43fa7b24aaa3b7e6e5d2b938948e0cacdb
migration/3: Stopper: multi_cpu_stop+0x0/0x184 <- stop_machine_cpuslocked+0xc0/0x15c

It's not the first time I get such a report on an out of tree kernel. The problem is I don't know if the tainted modules are involved. But something is probably making an offline CPU visible within
the hierarchy on get_nohz_timer_target(). And that new warning made that visible.

Can you try this and tell us if the warning fires?

Thanks.

include/linux/sched/nohz.h +1 -0

index 6d67e9a5af6b..f49512628269 100644
@@ -9,6 +9,7 @@
  #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
  extern void nohz_balance_enter_idle(int cpu);
  extern int get_nohz_timer_target(void);
+extern void assert_domain_online(void);
  #else
  static inline void nohz_balance_enter_idle(int cpu) { }
  #endif

kernel/cpu.c +2 -0

index 07455d25329c..98c8f8408403 100644
@@ -13,6 +13,7 @@
  #include <linux/sched/isolation.h>
  #include <linux/sched/task.h>
  #include <linux/sched/smt.h>
+#include <linux/sched/nohz.h>
  #include <linux/unistd.h>
  #include <linux/cpu.h>
  #include <linux/oom.h>

@@ -1277,6 +1278,7 @@ static int take_cpu_down(void *_param)
         if (err < 0)
                 return err;
 
+       assert_domain_online();
         /*
          * Must be called from CPUHP_TEARDOWN_CPU, which means, as we

are going
         * down, that the current state is CPUHP_TEARDOWN_CPU - 1.

kernel/sched/core.c +14 -0

index 175a5a7ac107..88157b1645cc 100644
@@ -1163,6 +1163,20 @@ void resched_cpu(int cpu)
 
  #ifdef CONFIG_SMP
  #ifdef CONFIG_NO_HZ_COMMON
+void assert_domain_online(void)
+{
+       int cpu = smp_processor_id();
+       int i;
+       struct sched_domain *sd;
+
+       guard(rcu)();
+
+       for_each_domain(cpu, sd) {
+               for_each_cpu(i, sched_domain_span(sd)) {
+                       WARN_ON_ONCE(cpu_is_offline(i));
+               }
+       }
+}
  /*
   * In the semi idle case, use the nearest busy CPU for migrating

timers
  * from an idle CPU.  This is good for power-savings.

Hi Walter Chang,

Le Wed, Mar 26, 2025 at 05:46:38AM +0000, Walter Chang (張維哲) a écrit :

Quoted 103 lines · Paul E. McKenney

Quoted 91 lines

Quoted 81 lines · Frederic Weisbecker

hrtimers are migrated away from the dying CPU to any online target
at
the CPUHP_AP_HRTIMERS_DYING stage in order not to delay bandwidth
timers
handling tasks involved in the CPU hotplug forward progress.

However wake ups can still be performed by the outgoing CPU after
CPUHP_AP_HRTIMERS_DYING. Those can result again in bandwidth timers
being armed. Depending on several considerations (crystal ball power management based election, earliest timer already enqueued,
timer
migration enabled or not), the target may eventually be the current
CPU even if offline. If that happens, the timer is eventually ignored.

The most notable example is RCU which had to deal with each and every of
those wake-ups by deferring them to an online CPU, along with related
workarounds:

_ e787644caf76 (rcu: Defer RCU kthreads wakeup when CPU is dying)
_ 9139f93209d1 (rcu/nocb: Fix RT throttling hrtimer armed from offline CPU)
_ f7345ccc62a4 (rcu/nocb: Fix rcuog wake-up from offline softirq)

The problem isn't confined to RCU though as the stop machine kthread
(which runs CPUHP_AP_HRTIMERS_DYING) reports its completion at the
end
of its work through cpu_stop_signal_done() and performs a wake up
that
eventually arms the deadline server timer:

           WARNING: CPU: 94 PID: 588 at
kernel/time/hrtimer.c:1086
hrtimer_start_range_ns+0x289/0x2d0
           CPU: 94 UID: 0 PID: 588 Comm: migration/94 Not
tainted
           Stopper: multi_cpu_stop+0x0/0x120 <-
stop_machine_cpuslocked+0x66/0xc0
           RIP: 0010:hrtimer_start_range_ns+0x289/0x2d0
           Call Trace:
            <TASK>

            ? hrtimer_start_range_ns
            start_dl_timer
            enqueue_dl_entity
            dl_server_start
            enqueue_task_fair
            enqueue_task
            ttwu_do_activate
            try_to_wake_up
            complete
            cpu_stopper_thread
            smpboot_thread_fn
            kthread
            ret_from_fork
            ret_from_fork_asm

            </TASK>

Instead of providing yet another bandaid to work around the situation,
fix it from hrtimers infrastructure instead: always migrate away a
timer to an online target whenever it is enqueued from an offline
CPU.

This will also allow to revert all the above RCU disgraceful hacks.

Quoted 7 lines

Reported-by Vlad Poenaru <[email protected]>

Reported-by Usama Arif <[email protected]>

Fixes 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from

outgoing CPU earlier")

Closes [email protected]

Signed-off-by Frederic Weisbecker <[email protected]>

Signed-off-by Paul E. McKenney <[email protected]>

Quoted 4 lines

This passes over-holiday testing rcutorture, so, perhaps redundantly:

Tested-by Paul E. McKenney <[email protected]>

Hi,

I encountered the same issue even after applying this patch. Below are the details of the warning and call trace.

Quoted 9 lines

migration/3: ------------[ cut here ]------------ migration/3: WARNING: CPU: 3 PID: 42 at kernel/time/hrtimer.c:1125 enqueue_hrtimer+0x7c/0xec
migration/3: CPU: 3 UID: 0 PID: 42 Comm: migration/3 Tainted:
G
OE      6.12.18-android16-0-g59cb5a849beb-4k #1 0b440e43fa7b24aaa3b7e6e5d2b938948e0cacdb
migration/3: Stopper: multi_cpu_stop+0x0/0x184 <- stop_machine_cpuslocked+0xc0/0x15c

It's not the first time I get such a report on an out of tree kernel. The problem is I don't know if the tainted modules are involved. But something is probably making an offline CPU visible within
the hierarchy on get_nohz_timer_target(). And that new warning made that visible.

Can you try this and tell us if the warning fires?

Thanks.

include/linux/sched/nohz.h +1 -0

index 6d67e9a5af6b..f49512628269 100644
@@ -9,6 +9,7 @@
  #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
  extern void nohz_balance_enter_idle(int cpu);
  extern int get_nohz_timer_target(void);
+extern void assert_domain_online(void);
  #else
  static inline void nohz_balance_enter_idle(int cpu) { }
  #endif

kernel/cpu.c +1 -0

index 07455d25329c..98c8f8408403 100644
@@ -13,6 +13,7 @@
  #include <linux/sched/isolation.h>
  #include <linux/sched/task.h>
  #include <linux/sched/smt.h>
+#include <linux/sched/nohz.h>
  #include <linux/unistd.h>
  #include <linux/cpu.h>
  #include <linux/oom.h>

@@ -1277,6 +1278,7 @@ static int take_cpu_down(void *_param)
  	if (err < 0)
  		return err;

kernel/cpu.c +1 -0

+	assert_domain_online();

  /*
  * Must be called from CPUHP_TEARDOWN_CPU, which means, as we are going
  * down, that the current state is CPUHP_TEARDOWN_CPU - 1.

kernel/sched/core.c +0 -0

index 175a5a7ac107..88157b1645cc 100644
@@ -1163,6 +1163,20 @@ void resched_cpu(int cpu)

 #ifdef CONFIG_SMP
 #ifdef CONFIG_NO_HZ_COMMON

kernel/sched/core.c +14 -0

+void assert_domain_online(void)
+{
+	int cpu = smp_processor_id();
+	int i;
+	struct sched_domain *sd;
+
+	guard(rcu)();
+
+	for_each_domain(cpu, sd) {
+		for_each_cpu(i, sched_domain_span(sd)) {
+			WARN_ON_ONCE(cpu_is_offline(i));
+		}
+	}
+}

 /*
  * In the semi idle case, use the nearest busy CPU for migrating timers
  * from an idle CPU.  This is good for power-savings.

Hi,

By review the get_nohz_timer_target(), it's probably making an offline CPU visible at timer candidates, maybe this patch could fix it?

[PATCH] sched/core: Exclude offline CPUs from the timer candidates

The timer target is chosen from the HK_TYPE_KERNEL_NOISE. However,the candidate may be an offline CPU,
so exclude offline CPUs and choose only from online CPUs.

Signed-off-by kuyo chang <[email protected]>

---
 kernel/sched/core.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

kernel/sched/core.c +1 -1

index cfaca3040b2f..efcc2576e622 100644
@@ -1182,7 +1182,7 @@ int get_nohz_timer_target(void)
        struct sched_domain *sd;
        const struct cpumask *hk_mask;
 
-       if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE)) {
+       if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE) &&
 cpu_online(cpu)) {
                if (!idle_cpu(cpu))
                        return cpu;

                default_cpu = cpu;