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sched/idle: disable tick in idle=poll idle entry

PATCH Superseded Marcelo Tosatti 3 participants 6 messages Oct 29, 2025 – Dec 10, 2025

Subsystems (2)
Scheduler Nohz, dynticks support

v2 of 3 Superseded Latest is v3 →

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author
#1 lore raw
In reply to Marcelo Tosatti
Quoted 9 lines · Marcelo Tosatti

Commit a5183862e76fdc25f36b39c2489b816a5c66e2e5 ("tick/nohz: Conditionally restart tick on idle exit") allows a nohz_full CPU to enter idle and return from it with the scheduler tick disabled (since the tick might be undesired noise).

The idle=poll case still unconditionally restarts the tick when entering idle.

Quoted 6 lines

To reduce the noise for that case as well, stop the tick when entering idle, for the idle=poll case.

Change tick_nohz_full_kick_cpu to set NEED_RESCHED bit, to handle the case where a new timer is added from an interrupt. This breaks out of cpu_idle_poll and rearms the timer if necessary.

Frederic,

As a reminder, this is the original patch and discussion:

https://patchew.org/linux/ZIEqlkIASx2F2DRF@tpad/

Marcelo Tosatti

author
#0 lore raw

PATCH v2 1/1 sched/idle: disable tick in idle=poll idle entry

Commit a5183862e76fdc25f36b39c2489b816a5c66e2e5 ("tick/nohz: Conditionally restart tick on idle exit") allows a nohz_full CPU to enter idle and return from it with the scheduler tick disabled (since the tick might be undesired noise).

The idle=poll case still unconditionally restarts the tick when entering idle.

To reduce the noise for that case as well, stop the tick when entering idle, for the idle=poll case.

Change tick_nohz_full_kick_cpu to set NEED_RESCHED bit, to handle the case where a new timer is added from an interrupt. This breaks out of cpu_idle_poll and rearms the timer if necessary.

---

v2: Handle the case where a new timer is added from an interrupt (Frederic Weisbecker)

 include/linux/sched.h    |    2 ++
 kernel/sched/core.c      |   10 ++++++++++
 kernel/sched/idle.c      |    2 +-
 kernel/time/tick-sched.c |    1 +
 4 files changed, 14 insertions(+), 1 deletion(-)
include/linux/sched.h +2 -0
index cbb7340c5866..1f6938dc20cd 100644
@@ -2428,4 +2428,6 @@ extern void migrate_enable(void);
 
 DEFINE_LOCK_GUARD_0(migrate, migrate_disable(), migrate_enable())
 
+void set_tif_resched_if_polling(int cpu);
+
 #endif
kernel/sched/core.c +10 -0
index f1ebf67b48e2..f0b84600084b 100644
@@ -988,6 +988,11 @@ static bool set_nr_if_polling(struct task_struct *p)
 	return true;
 }
 
+void set_tif_resched_if_polling(int cpu)
+{
+	set_nr_if_polling(cpu_rq(cpu)->idle);
+}
+
 #else
 static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
 {

@@ -999,6 +1004,11 @@ static inline bool set_nr_if_polling(struct task_struct *p)
 {
 	return false;
 }
+
+void set_tif_resched_if_polling(int cpu)
+{
+	set_tsk_need_resched(cpu_rq(cpu)->idle);
+}
 #endif
 
 static bool __wake_q_add(struct wake_q_head *head, struct task_struct *task)
kernel/sched/idle.c +1 -1
index c39b089d4f09..428c2d1cbd1b 100644
@@ -324,7 +324,7 @@ static void do_idle(void)
 		 * idle as we know that the IPI is going to arrive right away.
 		 */
 		if (cpu_idle_force_poll || tick_check_broadcast_expired()) {
-			tick_nohz_idle_restart_tick();
+			tick_nohz_idle_stop_tick();
 			cpu_idle_poll();
 		} else {
 			cpuidle_idle_call();
kernel/time/tick-sched.c +1 -0
index c527b421c865..efc3653999dc 100644
@@ -408,6 +408,7 @@ void tick_nohz_full_kick_cpu(int cpu)
 	if (!tick_nohz_full_cpu(cpu))
 		return;
 
+	set_tif_resched_if_polling(cpu);
 	irq_work_queue_on(&per_cpu(nohz_full_kick_work, cpu), cpu);
 }
maintainer
#2 lore raw

(Adding more people in Cc)

Quoted 70 lines · Marcelo Tosatti

Commit a5183862e76fdc25f36b39c2489b816a5c66e2e5 ("tick/nohz: Conditionally restart tick on idle exit") allows a nohz_full CPU to enter idle and return from it with the scheduler tick disabled (since the tick might be undesired noise).

The idle=poll case still unconditionally restarts the tick when entering idle.

To reduce the noise for that case as well, stop the tick when entering idle, for the idle=poll case.

Change tick_nohz_full_kick_cpu to set NEED_RESCHED bit, to handle the case where a new timer is added from an interrupt. This breaks out of cpu_idle_poll and rearms the timer if necessary.

---

v2: Handle the case where a new timer is added from an interrupt (Frederic Weisbecker)

 include/linux/sched.h    |    2 ++
 kernel/sched/core.c      |   10 ++++++++++
 kernel/sched/idle.c      |    2 +-
 kernel/time/tick-sched.c |    1 +
 4 files changed, 14 insertions(+), 1 deletion(-)
include/linux/sched.h +2 -0
index cbb7340c5866..1f6938dc20cd 100644
@@ -2428,4 +2428,6 @@ extern void migrate_enable(void);
 
 DEFINE_LOCK_GUARD_0(migrate, migrate_disable(), migrate_enable())
 
+void set_tif_resched_if_polling(int cpu);
+
 #endif
kernel/sched/core.c +10 -0
index f1ebf67b48e2..f0b84600084b 100644
@@ -988,6 +988,11 @@ static bool set_nr_if_polling(struct task_struct *p)
 	return true;
 }
 
+void set_tif_resched_if_polling(int cpu)
+{
+	set_nr_if_polling(cpu_rq(cpu)->idle);
+}
+
 #else
 static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
 {

@@ -999,6 +1004,11 @@ static inline bool set_nr_if_polling(struct task_struct *p)
 {
 	return false;
 }
+
+void set_tif_resched_if_polling(int cpu)
+{
+	set_tsk_need_resched(cpu_rq(cpu)->idle);
+}
 #endif
 
 static bool __wake_q_add(struct wake_q_head *head, struct task_struct *task)
kernel/sched/idle.c +0 -0
index c39b089d4f09..428c2d1cbd1b 100644

kernel/sched/idle.c static void do_idle(void) · line 327

Quoted 5 lines
kernel/sched/idle.c +1 -1
@@ -324,7 +324,7 @@ static void do_idle(void)
 		 * idle as we know that the IPI is going to arrive right away.
 		 */
 		if (cpu_idle_force_poll || tick_check_broadcast_expired()) {
-			tick_nohz_idle_restart_tick();
+			tick_nohz_idle_stop_tick();

Shouldn't we simply remove the tick_nohz_idle_restart_tick() line? The nohz_full CPU should have entered here with the tick disabled already.

Also non-nohz_full systems shouldn't care.

kernel/time/tick-sched.c void tick_nohz_full_kick_cpu(int cpu) · line 411

Quoted 12 lines
kernel/sched/idle.c +0 -0
 			cpu_idle_poll();
 		} else {
 			cpuidle_idle_call();
kernel/time/tick-sched.c +1 -0
index c527b421c865..efc3653999dc 100644
@@ -408,6 +408,7 @@ void tick_nohz_full_kick_cpu(int cpu)
 	if (!tick_nohz_full_cpu(cpu))
 		return;
 
+	set_tif_resched_if_polling(cpu);

Perhaps stuff that within wake_up_full_nohz_cpu() and call set_nr_if_polling() directly. Also this needs a big comment.

Thanks.

Quoted 4 lines
kernel/time/tick-sched.c +0 -0
 	irq_work_queue_on(&per_cpu(nohz_full_kick_work, cpu), cpu);
 }
Signature · 2 lines
Frederic Weisbecker
SUSE Labs
#5 lore raw
In reply to Marcelo Tosatti
Quoted 8 lines · Marcelo Tosatti

Commit a5183862e76fdc25f36b39c2489b816a5c66e2e5 ("tick/nohz: Conditionally restart tick on idle exit") allows a nohz_full CPU to enter idle and return from it with the scheduler tick disabled (since the tick might be undesired noise).

The idle=poll case still unconditionally restarts the tick when entering idle.

Quoted 8 lines

To reduce the noise for that case as well, stop the tick when entering idle, for the idle=poll case.

Change tick_nohz_full_kick_cpu to set NEED_RESCHED bit, to handle the case where a new timer is added from an interrupt. This breaks out of cpu_idle_poll and rearms the timer if necessary.

---

ERROR: Missing Signed-off-by: line by nominal patch author 'Marcelo Tosatti <[email protected]>'

You've not started doing kernel development three days ago, right?

author
#3 lore raw
In reply to Frederic Weisbecker
Quoted 82 lines · Frederic Weisbecker

(Adding more people in Cc)

Quoted 70 lines · Marcelo Tosatti

Commit a5183862e76fdc25f36b39c2489b816a5c66e2e5 ("tick/nohz: Conditionally restart tick on idle exit") allows a nohz_full CPU to enter idle and return from it with the scheduler tick disabled (since the tick might be undesired noise).

The idle=poll case still unconditionally restarts the tick when entering idle.

To reduce the noise for that case as well, stop the tick when entering idle, for the idle=poll case.

Change tick_nohz_full_kick_cpu to set NEED_RESCHED bit, to handle the case where a new timer is added from an interrupt. This breaks out of cpu_idle_poll and rearms the timer if necessary.

---

v2: Handle the case where a new timer is added from an interrupt (Frederic Weisbecker)

 include/linux/sched.h    |    2 ++
 kernel/sched/core.c      |   10 ++++++++++
 kernel/sched/idle.c      |    2 +-
 kernel/time/tick-sched.c |    1 +
 4 files changed, 14 insertions(+), 1 deletion(-)
include/linux/sched.h +2 -0
index cbb7340c5866..1f6938dc20cd 100644
@@ -2428,4 +2428,6 @@ extern void migrate_enable(void);
 
 DEFINE_LOCK_GUARD_0(migrate, migrate_disable(), migrate_enable())
 
+void set_tif_resched_if_polling(int cpu);
+
 #endif
kernel/sched/core.c +10 -0
index f1ebf67b48e2..f0b84600084b 100644
@@ -988,6 +988,11 @@ static bool set_nr_if_polling(struct task_struct *p)
 	return true;
 }
 
+void set_tif_resched_if_polling(int cpu)
+{
+	set_nr_if_polling(cpu_rq(cpu)->idle);
+}
+
 #else
 static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
 {

@@ -999,6 +1004,11 @@ static inline bool set_nr_if_polling(struct task_struct *p)
 {
 	return false;
 }
+
+void set_tif_resched_if_polling(int cpu)
+{
+	set_tsk_need_resched(cpu_rq(cpu)->idle);
+}
 #endif
 
 static bool __wake_q_add(struct wake_q_head *head, struct task_struct *task)
kernel/sched/idle.c +0 -0
index c39b089d4f09..428c2d1cbd1b 100644

kernel/sched/idle.c static void do_idle(void) · line 327

Quoted 5 lines
kernel/sched/idle.c +1 -1
@@ -324,7 +324,7 @@ static void do_idle(void)
 		 * idle as we know that the IPI is going to arrive right away.
 		 */
 		if (cpu_idle_force_poll || tick_check_broadcast_expired()) {
-			tick_nohz_idle_restart_tick();
+			tick_nohz_idle_stop_tick();

Shouldn't we simply remove the tick_nohz_idle_restart_tick() line? The nohz_full CPU should have entered here with the tick disabled already.

Quoted 1 line

Also non-nohz_full systems shouldn't care.

With tick_nohz_idle_restart_tick removed:

<idle>-0 [001] d.h2. 51.356672: hrtimer_start: hrtimer=ffff927ae205c418 function=tick_nohz_handler expires=51360062500 softexpires=51360062500 mode=ABS <idle>-0 [001] d.h2. 51.357671: hrtimer_cancel: hrtimer=ffff927ae205c418
<idle>-0 [001] d.h1. 51.357671: hrtimer_expire_entry: hrtimer=ffff927ae205c418 function=tick_nohz_handler now=51360063398 <idle>-0 [001] d.h1. 51.357671: hrtimer_expire_exit: hrtimer=ffff927ae205c418
<idle>-0 [001] d.h2. 51.357671: hrtimer_start: hrtimer=ffff927ae205c418 function=tick_nohz_handler expires=51361062500 softexpires=51361062500 mode=ABS <idle>-0 [001] d.h2. 51.358671: hrtimer_cancel: hrtimer=ffff927ae205c418
<idle>-0 [001] d.h1. 51.358671: hrtimer_expire_entry: hrtimer=ffff927ae205c418 function=tick_nohz_handler now=51361063420 <idle>-0 [001] d.h1. 51.358672: hrtimer_expire_exit: hrtimer=ffff927ae205c418
<idle>-0 [001] d.h2. 51.358672: hrtimer_start: hrtimer=ffff927ae205c418 function=tick_nohz_handler expires=51362062500 softexpires=51362062500 mode=ABS <idle>-0 [001] d.h2. 51.359671: hrtimer_cancel: hrtimer=ffff927ae205c418
<idle>-0 [001] d.h1. 51.359671: hrtimer_expire_entry: hrtimer=ffff927ae205c418 function=tick_nohz_handler now=51362063447 <idle>-0 [001] d.h1. 51.359672: hrtimer_expire_exit: hrtimer=ffff927ae205c418
<idle>-0 [001] d.h2. 51.359672: hrtimer_start: hrtimer=ffff927ae205c418 function=tick_nohz_handler expires=51363062500 softexpires=51363062500 mode=ABS

CPU 1 is idle and isolated.

Quoted 11 lines
Quoted 11 lines
kernel/sched/idle.c +0 -0
 			cpu_idle_poll();
 		} else {
 			cpuidle_idle_call();
kernel/time/tick-sched.c +0 -0
index c527b421c865..efc3653999dc 100644
@@ -408,6 +408,7 @@ void tick_nohz_full_kick_cpu(int cpu)
 	if (!tick_nohz_full_cpu(cpu))
 		return;
Quoted 4 lines

kernel/time/tick-sched.c void tick_nohz_full_kick_cpu(int cpu) · line 411

Quoted 1 line
kernel/time/tick-sched.c +1 -0
+	set_tif_resched_if_polling(cpu);

Perhaps stuff that within wake_up_full_nohz_cpu() and call set_nr_if_polling() directly.

Can't call set_nr_if_polling() directly since if TIF_POLLING_NRFLAG is undefined:

static inline bool set_nr_if_polling(struct task_struct *p)

{
        return false;
}

So the wakeup won't occur. Or am i missing something?

Quoted 1 line

Also this needs a big comment.

Sure!

Thanks.

maintainer
#4 lore raw
Quoted 101 lines · Marcelo Tosatti
Quoted 82 lines · Frederic Weisbecker

(Adding more people in Cc)

Quoted 70 lines · Marcelo Tosatti

Commit a5183862e76fdc25f36b39c2489b816a5c66e2e5 ("tick/nohz: Conditionally restart tick on idle exit") allows a nohz_full CPU to enter idle and return from it with the scheduler tick disabled (since the tick might be undesired noise).

The idle=poll case still unconditionally restarts the tick when entering idle.

To reduce the noise for that case as well, stop the tick when entering idle, for the idle=poll case.

Change tick_nohz_full_kick_cpu to set NEED_RESCHED bit, to handle the case where a new timer is added from an interrupt. This breaks out of cpu_idle_poll and rearms the timer if necessary.

---

v2: Handle the case where a new timer is added from an interrupt (Frederic Weisbecker)

 include/linux/sched.h    |    2 ++
 kernel/sched/core.c      |   10 ++++++++++
 kernel/sched/idle.c      |    2 +-
 kernel/time/tick-sched.c |    1 +
 4 files changed, 14 insertions(+), 1 deletion(-)
include/linux/sched.h +2 -0
index cbb7340c5866..1f6938dc20cd 100644
@@ -2428,4 +2428,6 @@ extern void migrate_enable(void);
 
 DEFINE_LOCK_GUARD_0(migrate, migrate_disable(), migrate_enable())
 
+void set_tif_resched_if_polling(int cpu);
+
 #endif
kernel/sched/core.c +10 -0
index f1ebf67b48e2..f0b84600084b 100644
@@ -988,6 +988,11 @@ static bool set_nr_if_polling(struct task_struct *p)
 	return true;
 }
 
+void set_tif_resched_if_polling(int cpu)
+{
+	set_nr_if_polling(cpu_rq(cpu)->idle);
+}
+
 #else
 static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
 {

@@ -999,6 +1004,11 @@ static inline bool set_nr_if_polling(struct task_struct *p)
 {
 	return false;
 }
+
+void set_tif_resched_if_polling(int cpu)
+{
+	set_tsk_need_resched(cpu_rq(cpu)->idle);
+}
 #endif
 
 static bool __wake_q_add(struct wake_q_head *head, struct task_struct *task)
kernel/sched/idle.c +0 -0
index c39b089d4f09..428c2d1cbd1b 100644

kernel/sched/idle.c static void do_idle(void) · line 327

Quoted 5 lines
kernel/sched/idle.c +1 -1
@@ -324,7 +324,7 @@ static void do_idle(void)
 		 * idle as we know that the IPI is going to arrive right away.
 		 */
 		if (cpu_idle_force_poll || tick_check_broadcast_expired()) {
-			tick_nohz_idle_restart_tick();
+			tick_nohz_idle_stop_tick();

Shouldn't we simply remove the tick_nohz_idle_restart_tick() line? The nohz_full CPU should have entered here with the tick disabled already.

Quoted 1 line

Also non-nohz_full systems shouldn't care.

With tick_nohz_idle_restart_tick removed:

<idle>-0 [001] d.h2. 51.356672: hrtimer_start: hrtimer=ffff927ae205c418 function=tick_nohz_handler expires=51360062500 softexpires=51360062500 mode=ABS <idle>-0 [001] d.h2. 51.357671: hrtimer_cancel: hrtimer=ffff927ae205c418
<idle>-0 [001] d.h1. 51.357671: hrtimer_expire_entry: hrtimer=ffff927ae205c418 function=tick_nohz_handler now=51360063398 <idle>-0 [001] d.h1. 51.357671: hrtimer_expire_exit: hrtimer=ffff927ae205c418
<idle>-0 [001] d.h2. 51.357671: hrtimer_start: hrtimer=ffff927ae205c418 function=tick_nohz_handler expires=51361062500 softexpires=51361062500 mode=ABS <idle>-0 [001] d.h2. 51.358671: hrtimer_cancel: hrtimer=ffff927ae205c418
<idle>-0 [001] d.h1. 51.358671: hrtimer_expire_entry: hrtimer=ffff927ae205c418 function=tick_nohz_handler now=51361063420 <idle>-0 [001] d.h1. 51.358672: hrtimer_expire_exit: hrtimer=ffff927ae205c418
<idle>-0 [001] d.h2. 51.358672: hrtimer_start: hrtimer=ffff927ae205c418 function=tick_nohz_handler expires=51362062500 softexpires=51362062500 mode=ABS <idle>-0 [001] d.h2. 51.359671: hrtimer_cancel: hrtimer=ffff927ae205c418
<idle>-0 [001] d.h1. 51.359671: hrtimer_expire_entry: hrtimer=ffff927ae205c418 function=tick_nohz_handler now=51362063447 <idle>-0 [001] d.h1. 51.359672: hrtimer_expire_exit: hrtimer=ffff927ae205c418
<idle>-0 [001] d.h2. 51.359672: hrtimer_start: hrtimer=ffff927ae205c418 function=tick_nohz_handler expires=51363062500 softexpires=51363062500 mode=ABS

Quoted 1 line

CPU 1 is idle and isolated.

Suprising, somehow the CPU's tick never interrupted a non-idle section. I guess it's possible after boot. Or the CPU had tick dependencies before. I was about to propose stopping the tick right before exiting to userspace but since you're using idle=poll, I guess userspace must be reached as fast as possible and therefore you prefer to stop the tick before the next wake-up rather that after?

Also instead of polling in kernel, why not polling in userspace for events? This sounds like a saner isolation design. Entering/exiting the kernel is always a risk for something going wrong.

Quoted 20 lines
Quoted 12 lines
Quoted 12 lines
kernel/sched/idle.c +0 -0
 			cpu_idle_poll();
 		} else {
 			cpuidle_idle_call();
kernel/time/tick-sched.c +0 -0
index c527b421c865..efc3653999dc 100644
@@ -408,6 +408,7 @@ void tick_nohz_full_kick_cpu(int cpu)
 	if (!tick_nohz_full_cpu(cpu))
 		return;
Quoted 4 lines

kernel/time/tick-sched.c void tick_nohz_full_kick_cpu(int cpu) · line 411

Quoted 1 line
kernel/time/tick-sched.c +1 -0
+	set_tif_resched_if_polling(cpu);

Perhaps stuff that within wake_up_full_nohz_cpu() and call set_nr_if_polling() directly.

Can't call set_nr_if_polling() directly since if TIF_POLLING_NRFLAG is undefined:

Quoted 6 lines

static inline bool set_nr_if_polling(struct task_struct *p)

{
        return false;
}

So the wakeup won't occur. Or am i missing something?

Ok but can you at least move that to wake_up_full_nohz_cpu()? tick_nohz_full_kick_cpu() is more general and doesn't only concern new timers.

Signature · 2 lines
Frederic Weisbecker
SUSE Labs