* Re: [PATCH v2] sched/idle: disable tick in idle=poll idle entry [not found] <aQJWWIDMMUxqDxnR@tpad> @ 2025-10-30 16:06 ` Frederic Weisbecker 2025-11-03 11:30 ` Marcelo Tosatti 0 siblings, 1 reply; 3+ messages in thread From: Frederic Weisbecker @ 2025-10-30 16:06 UTC (permalink / raw) To: Marcelo Tosatti; +Cc: linux-kernel, Peter Zijlstra, Rafael J. Wysocki, linux-pm (Adding more people in Cc) Le Wed, Oct 29, 2025 at 03:00:56PM -0300, Marcelo Tosatti a écrit : > > 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(-) > > diff --git a/include/linux/sched.h b/include/linux/sched.h > index cbb7340c5866..1f6938dc20cd 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -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 > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index f1ebf67b48e2..f0b84600084b 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -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) > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c > index c39b089d4f09..428c2d1cbd1b 100644 > --- a/kernel/sched/idle.c > +++ b/kernel/sched/idle.c > @@ -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. > cpu_idle_poll(); > } else { > cpuidle_idle_call(); > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > index c527b421c865..efc3653999dc 100644 > --- a/kernel/time/tick-sched.c > +++ b/kernel/time/tick-sched.c > @@ -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. > irq_work_queue_on(&per_cpu(nohz_full_kick_work, cpu), cpu); > } > > -- Frederic Weisbecker SUSE Labs ^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH v2] sched/idle: disable tick in idle=poll idle entry 2025-10-30 16:06 ` [PATCH v2] sched/idle: disable tick in idle=poll idle entry Frederic Weisbecker @ 2025-11-03 11:30 ` Marcelo Tosatti 2025-12-10 13:32 ` Frederic Weisbecker 0 siblings, 1 reply; 3+ messages in thread From: Marcelo Tosatti @ 2025-11-03 11:30 UTC (permalink / raw) To: Frederic Weisbecker Cc: linux-kernel, Peter Zijlstra, Rafael J. Wysocki, linux-pm On Thu, Oct 30, 2025 at 05:06:49PM +0100, Frederic Weisbecker wrote: > (Adding more people in Cc) > > Le Wed, Oct 29, 2025 at 03:00:56PM -0300, Marcelo Tosatti a écrit : > > > > 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(-) > > > > diff --git a/include/linux/sched.h b/include/linux/sched.h > > index cbb7340c5866..1f6938dc20cd 100644 > > --- a/include/linux/sched.h > > +++ b/include/linux/sched.h > > @@ -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 > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index f1ebf67b48e2..f0b84600084b 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -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) > > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c > > index c39b089d4f09..428c2d1cbd1b 100644 > > --- a/kernel/sched/idle.c > > +++ b/kernel/sched/idle.c > > @@ -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. 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. > > cpu_idle_poll(); > > } else { > > cpuidle_idle_call(); > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > > index c527b421c865..efc3653999dc 100644 > > --- a/kernel/time/tick-sched.c > > +++ b/kernel/time/tick-sched.c > > @@ -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. 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? > Also this needs a big comment. Sure! Thanks. ^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH v2] sched/idle: disable tick in idle=poll idle entry 2025-11-03 11:30 ` Marcelo Tosatti @ 2025-12-10 13:32 ` Frederic Weisbecker 0 siblings, 0 replies; 3+ messages in thread From: Frederic Weisbecker @ 2025-12-10 13:32 UTC (permalink / raw) To: Marcelo Tosatti; +Cc: linux-kernel, Peter Zijlstra, Rafael J. Wysocki, linux-pm Le Mon, Nov 03, 2025 at 08:30:06AM -0300, Marcelo Tosatti a écrit : > On Thu, Oct 30, 2025 at 05:06:49PM +0100, Frederic Weisbecker wrote: > > (Adding more people in Cc) > > > > Le Wed, Oct 29, 2025 at 03:00:56PM -0300, Marcelo Tosatti a écrit : > > > > > > 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(-) > > > > > > diff --git a/include/linux/sched.h b/include/linux/sched.h > > > index cbb7340c5866..1f6938dc20cd 100644 > > > --- a/include/linux/sched.h > > > +++ b/include/linux/sched.h > > > @@ -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 > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > > index f1ebf67b48e2..f0b84600084b 100644 > > > --- a/kernel/sched/core.c > > > +++ b/kernel/sched/core.c > > > @@ -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) > > > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c > > > index c39b089d4f09..428c2d1cbd1b 100644 > > > --- a/kernel/sched/idle.c > > > +++ b/kernel/sched/idle.c > > > @@ -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. > > 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. 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. > > > > cpu_idle_poll(); > > > } else { > > > cpuidle_idle_call(); > > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > > > index c527b421c865..efc3653999dc 100644 > > > --- a/kernel/time/tick-sched.c > > > +++ b/kernel/time/tick-sched.c > > > @@ -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. > > 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? 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. -- Frederic Weisbecker SUSE Labs ^ permalink raw reply [flat|nested] 3+ messages in thread
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2025-10-30 16:06 ` [PATCH v2] sched/idle: disable tick in idle=poll idle entry Frederic Weisbecker
2025-11-03 11:30 ` Marcelo Tosatti
2025-12-10 13:32 ` Frederic Weisbecker
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