[PATCH v2 0/3] softirq: Use a dedicated thread for timer wakeups on PREEMPT_RT.

[PATCH v2 0/3] softirq: Use a dedicated thread for timer wakeups on PREEMPT_RT.

[Sebastian Andrzej Siewior]

Hi,

the following was in the PREEMPT_RT queue since last softirq rework. The
result is that timer wake ups (hrtimer, timer_list) happens in hardirq
processing them requires to wake ksoftirqd. ksoftirqd runs at SCHED_OTHER so it
will compete for resources with all other tasks in the system, potentially
delayed the processing further.

The idea was to let the timers be processed by a dedicated thread
running at low SCHED_FIFO priority.
While looking at it again, it might make sense to have the
pending_softirq flag per-thread to avoid threads with higher priority
picking up softirqs from low-priority threads. This isn't yet a problem
because adding softirqs for processing happens only from threaded
interrupts. So the low-priority thread will wait until the high-priority
thread is done. And the high-priority thread will PI-boost the
low-priority thread until it is done. It would only make sense to make
the flags per-thread once the BH lock is gone.

The patch is limited to PREEMPT_RT. The ksoftirqd bullets from above
apply also to !PREEMPT_RT +threadirqs. Would it make sense to restrict
it to force_irqthreads() instead?

v1…v2: https://lore.kernel.org/all/20241004103842.131014-1-bigeasy@linutronix.de/
 Frederick's comments:
 - Use __raise_softirq_irqoff() to raise the softirq for !PREEMPT_RT. Also a
   lockdep test to ensure that this is always invoked from an IRQ.
 - Make raise_ktimers_thread() only OR the flag and nothing else to
   align with __raise_softirq_irqoff(). The wake happens on return from
   interrupt anyway.
 - A comment in timersd_setup() and interrupt.h
 - local_pending_timers() => local_timers_pending().

Sebastian

[PATCH v2 1/3] hrtimer: Use __raise_softirq_irqoff() to raise the softirq.

[Sebastian Andrzej Siewior]

As an optimisation use __raise_softirq_irqoff() to raise the softirq.
This is always called from an interrupt handler so it can be reduced to
just or set softirq flag and let softirq be invoked on return from
interrupt.

Use __raise_softirq_irqoff() to raise the softirq.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/time/hrtimer.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index cddcd08ea827f..5402e0f242178 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1811,7 +1811,7 @@ void hrtimer_interrupt(struct clock_event_device *dev)
 	if (!ktime_before(now, cpu_base->softirq_expires_next)) {
 		cpu_base->softirq_expires_next = KTIME_MAX;
 		cpu_base->softirq_activated = 1;
-		raise_softirq_irqoff(HRTIMER_SOFTIRQ);
+		__raise_softirq_irqoff(HRTIMER_SOFTIRQ);
 	}
 
 	__hrtimer_run_queues(cpu_base, now, flags, HRTIMER_ACTIVE_HARD);
@@ -1906,7 +1906,7 @@ void hrtimer_run_queues(void)
 	if (!ktime_before(now, cpu_base->softirq_expires_next)) {
 		cpu_base->softirq_expires_next = KTIME_MAX;
 		cpu_base->softirq_activated = 1;
-		raise_softirq_irqoff(HRTIMER_SOFTIRQ);
+		__raise_softirq_irqoff(HRTIMER_SOFTIRQ);
 	}
 
 	__hrtimer_run_queues(cpu_base, now, flags, HRTIMER_ACTIVE_HARD);
-- 
2.45.2

Re: [PATCH v2 1/3] hrtimer: Use __raise_softirq_irqoff() to raise the softirq.

[Frederic Weisbecker]

Le Thu, Oct 24, 2024 at 04:55:49PM +0200, Sebastian Andrzej Siewior a écrit :
> As an optimisation use __raise_softirq_irqoff() to raise the softirq.
> This is always called from an interrupt handler so it can be reduced to
> just or set softirq flag and let softirq be invoked on return from
> interrupt.
> 
> Use __raise_softirq_irqoff() to raise the softirq.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

[PATCH v2 2/3] timers: Use __raise_softirq_irqoff() to raise the softirq.

[Sebastian Andrzej Siewior]

As an optimisation use __raise_softirq_irqoff() to raise the softirq.
This is always called from an interrupt handler, interrupts are already
disabled so it can be reduced to just or set softirq flag and let
softirq be invoked on return from interrupt.

Use __raise_softirq_irqoff() to raise the softirq.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/time/timer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 0fc9d066a7be4..1759de934284c 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -2499,7 +2499,7 @@ static void run_local_timers(void)
 		 */
 		if (time_after_eq(jiffies, READ_ONCE(base->next_expiry)) ||
 		    (i == BASE_DEF && tmigr_requires_handle_remote())) {
-			raise_softirq(TIMER_SOFTIRQ);
+			__raise_softirq_irqoff(TIMER_SOFTIRQ);
 			return;
 		}
 	}
-- 
2.45.2

Re: [PATCH v2 2/3] timers: Use __raise_softirq_irqoff() to raise the softirq.

[Frederic Weisbecker]

Le Thu, Oct 24, 2024 at 04:55:50PM +0200, Sebastian Andrzej Siewior a écrit :
> As an optimisation use __raise_softirq_irqoff() to raise the softirq.
> This is always called from an interrupt handler, interrupts are already
> disabled so it can be reduced to just or set softirq flag and let
> softirq be invoked on return from interrupt.
> 
> Use __raise_softirq_irqoff() to raise the softirq.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

[PATCH v2 3/3] softirq: Use a dedicated thread for timer wakeups on PREEMPT_RT.

[Sebastian Andrzej Siewior]

A timer/ hrtimer softirq is raised in-IRQ context. With threaded
interrupts enabled or on PREEMPT_RT this leads to waking the ksoftirqd
for the processing of the softirq. ksoftirqd runs as SCHED_OTHER which
means it will compete with other tasks for CPU ressources.
This can introduce long delays for timer processing on heavy loaded
systems and is not desired.

Split the TIMER_SOFTIRQ and HRTIMER_SOFTIRQ processing into a dedicated
timers thread and let it run at the lowest SCHED_FIFO priority.
Wake-ups for RT tasks happen from hardirq context so only timer_list timers
and hrtimers for "regular" tasks are processed here. The higher priority
ensures that wakeups are performed before scheduling SCHED_OTHER tasks.

Using a dedicated variable to store the pending softirq bits values
ensure that the timer are not accidentally picked up by ksoftirqd and
other threaded interrupts.
It shouldn't be picked up by ksoftirqd since it runs at lower priority.
However if ksoftirqd is already running while a timer fires, then
ksoftird will be PI-boosted due to the BH-lock to ktimer's priority.
Ideally we try to avoid having ksoftirqd running.

The timer thread can pick up pending softirqs from ksoftirqd but only
if the softirq load is high. It is not be desired that the picked up
softirqs are processed at SCHED_FIFO priority under high softirq load
but this can already happen by a PI-boost by a force-threaded interrupt.

[ frederic@kernel.org: rcutorture.c fixes, storm fix by introduction of
  local_timers_pending() for tick_nohz_next_event() ]

[ junxiao.chang@intel.com: Ensure ktimersd gets woken up even if a
  softirq is currently served. ]

Reviewed-by: Paul E. McKenney <paulmck@kernel.org> [rcutorture]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/interrupt.h | 44 +++++++++++++++++++++++++
 kernel/rcu/rcutorture.c   |  6 ++++
 kernel/softirq.c          | 69 ++++++++++++++++++++++++++++++++++++++-
 kernel/time/hrtimer.c     |  4 +--
 kernel/time/tick-sched.c  |  2 +-
 kernel/time/timer.c       |  2 +-
 6 files changed, 122 insertions(+), 5 deletions(-)

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 457151f9f263d..9637af78087f3 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -616,6 +616,50 @@ extern void __raise_softirq_irqoff(unsigned int nr);
 extern void raise_softirq_irqoff(unsigned int nr);
 extern void raise_softirq(unsigned int nr);
 
+/*
+ * Handle timers in a dedicated thread at a low SCHED_FIFO priority instead in
+ * ksoftirqd as to be prefred over SCHED_NORMAL tasks.
+ */
+#ifdef CONFIG_PREEMPT_RT
+DECLARE_PER_CPU(struct task_struct *, timersd);
+DECLARE_PER_CPU(unsigned long, pending_timer_softirq);
+
+void raise_ktimers_thread(unsigned int nr);
+
+static inline void raise_timer_softirq(void)
+{
+	raise_ktimers_thread(TIMER_SOFTIRQ);
+}
+
+static inline void raise_hrtimer_softirq(void)
+{
+	raise_ktimers_thread(HRTIMER_SOFTIRQ);
+}
+
+static inline unsigned int local_timers_pending(void)
+{
+	return __this_cpu_read(pending_timer_softirq);
+}
+
+#else
+static inline void raise_timer_softirq(void)
+{
+	lockdep_assert_in_irq();
+	__raise_softirq_irqoff(TIMER_SOFTIRQ);
+}
+
+static inline void raise_hrtimer_softirq(void)
+{
+	lockdep_assert_in_irq();
+	__raise_softirq_irqoff(HRTIMER_SOFTIRQ);
+}
+
+static inline unsigned int local_timers_pending(void)
+{
+	return local_softirq_pending();
+}
+#endif
+
 DECLARE_PER_CPU(struct task_struct *, ksoftirqd);
 
 static inline struct task_struct *this_cpu_ksoftirqd(void)
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index bb75dbf5c800c..609687fd742d5 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -2440,6 +2440,12 @@ static int rcutorture_booster_init(unsigned int cpu)
 		WARN_ON_ONCE(!t);
 		sp.sched_priority = 2;
 		sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
+#ifdef CONFIG_PREEMPT_RT
+		t = per_cpu(timersd, cpu);
+		WARN_ON_ONCE(!t);
+		sp.sched_priority = 2;
+		sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
+#endif
 	}
 
 	/* Don't allow time recalculation while creating a new task. */
diff --git a/kernel/softirq.c b/kernel/softirq.c
index d082e7840f880..b452206cf93b2 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -624,6 +624,24 @@ static inline void tick_irq_exit(void)
 #endif
 }
 
+#ifdef CONFIG_PREEMPT_RT
+DEFINE_PER_CPU(struct task_struct *, timersd);
+DEFINE_PER_CPU(unsigned long, pending_timer_softirq);
+
+static void wake_timersd(void)
+{
+	struct task_struct *tsk = __this_cpu_read(timersd);
+
+	if (tsk)
+		wake_up_process(tsk);
+}
+
+#else
+
+static inline void wake_timersd(void) { }
+
+#endif
+
 static inline void __irq_exit_rcu(void)
 {
 #ifndef __ARCH_IRQ_EXIT_IRQS_DISABLED
@@ -636,6 +654,10 @@ static inline void __irq_exit_rcu(void)
 	if (!in_interrupt() && local_softirq_pending())
 		invoke_softirq();
 
+	if (IS_ENABLED(CONFIG_PREEMPT_RT) && local_timers_pending() &&
+	    !(in_nmi() | in_hardirq()))
+		wake_timersd();
+
 	tick_irq_exit();
 }
 
@@ -971,12 +993,57 @@ static struct smp_hotplug_thread softirq_threads = {
 	.thread_comm		= "ksoftirqd/%u",
 };
 
+#ifdef CONFIG_PREEMPT_RT
+static void timersd_setup(unsigned int cpu)
+{
+	/* Above SCHED_NORMAL to handle timers before regular tasks. */
+	sched_set_fifo_low(current);
+}
+
+static int timersd_should_run(unsigned int cpu)
+{
+	return local_timers_pending();
+}
+
+void raise_ktimers_thread(unsigned int nr)
+{
+	lockdep_assert_in_irq();
+	trace_softirq_raise(nr);
+	__this_cpu_or(pending_timer_softirq, 1 << nr);
+}
+
+static void run_timersd(unsigned int cpu)
+{
+	unsigned int timer_si;
+
+	ksoftirqd_run_begin();
+
+	timer_si = local_timers_pending();
+	__this_cpu_write(pending_timer_softirq, 0);
+	or_softirq_pending(timer_si);
+
+	__do_softirq();
+
+	ksoftirqd_run_end();
+}
+
+static struct smp_hotplug_thread timer_threads = {
+	.store			= &timersd,
+	.setup			= timersd_setup,
+	.thread_should_run	= timersd_should_run,
+	.thread_fn		= run_timersd,
+	.thread_comm		= "ktimers/%u",
+};
+#endif
+
 static __init int spawn_ksoftirqd(void)
 {
 	cpuhp_setup_state_nocalls(CPUHP_SOFTIRQ_DEAD, "softirq:dead", NULL,
 				  takeover_tasklets);
 	BUG_ON(smpboot_register_percpu_thread(&softirq_threads));
-
+#ifdef CONFIG_PREEMPT_RT
+	BUG_ON(smpboot_register_percpu_thread(&timer_threads));
+#endif
 	return 0;
 }
 early_initcall(spawn_ksoftirqd);
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 5402e0f242178..133d49f703d93 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1811,7 +1811,7 @@ void hrtimer_interrupt(struct clock_event_device *dev)
 	if (!ktime_before(now, cpu_base->softirq_expires_next)) {
 		cpu_base->softirq_expires_next = KTIME_MAX;
 		cpu_base->softirq_activated = 1;
-		__raise_softirq_irqoff(HRTIMER_SOFTIRQ);
+		raise_hrtimer_softirq();
 	}
 
 	__hrtimer_run_queues(cpu_base, now, flags, HRTIMER_ACTIVE_HARD);
@@ -1906,7 +1906,7 @@ void hrtimer_run_queues(void)
 	if (!ktime_before(now, cpu_base->softirq_expires_next)) {
 		cpu_base->softirq_expires_next = KTIME_MAX;
 		cpu_base->softirq_activated = 1;
-		__raise_softirq_irqoff(HRTIMER_SOFTIRQ);
+		raise_hrtimer_softirq();
 	}
 
 	__hrtimer_run_queues(cpu_base, now, flags, HRTIMER_ACTIVE_HARD);
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 753a184c70907..976a212cca2e8 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -859,7 +859,7 @@ static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
 
 static inline bool local_timer_softirq_pending(void)
 {
-	return local_softirq_pending() & BIT(TIMER_SOFTIRQ);
+	return local_timers_pending() & BIT(TIMER_SOFTIRQ);
 }
 
 /*
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 1759de934284c..79f0dc73ac436 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -2499,7 +2499,7 @@ static void run_local_timers(void)
 		 */
 		if (time_after_eq(jiffies, READ_ONCE(base->next_expiry)) ||
 		    (i == BASE_DEF && tmigr_requires_handle_remote())) {
-			__raise_softirq_irqoff(TIMER_SOFTIRQ);
+			raise_timer_softirq();
 			return;
 		}
 	}
-- 
2.45.2

Re: [PATCH v2 3/3] softirq: Use a dedicated thread for timer wakeups on PREEMPT_RT.

[Frederic Weisbecker]

Le Thu, Oct 24, 2024 at 04:55:51PM +0200, Sebastian Andrzej Siewior a écrit :
> A timer/ hrtimer softirq is raised in-IRQ context. With threaded
> interrupts enabled or on PREEMPT_RT this leads to waking the ksoftirqd
> for the processing of the softirq. ksoftirqd runs as SCHED_OTHER which
> means it will compete with other tasks for CPU ressources.
> This can introduce long delays for timer processing on heavy loaded
> systems and is not desired.
> 
> Split the TIMER_SOFTIRQ and HRTIMER_SOFTIRQ processing into a dedicated
> timers thread and let it run at the lowest SCHED_FIFO priority.
> Wake-ups for RT tasks happen from hardirq context so only timer_list timers
> and hrtimers for "regular" tasks are processed here. The higher priority
> ensures that wakeups are performed before scheduling SCHED_OTHER tasks.
> 
> Using a dedicated variable to store the pending softirq bits values
> ensure that the timer are not accidentally picked up by ksoftirqd and
> other threaded interrupts.
> It shouldn't be picked up by ksoftirqd since it runs at lower priority.
> However if ksoftirqd is already running while a timer fires, then
> ksoftird will be PI-boosted due to the BH-lock to ktimer's priority.
> Ideally we try to avoid having ksoftirqd running.
> 
> The timer thread can pick up pending softirqs from ksoftirqd but only
> if the softirq load is high. It is not be desired that the picked up
> softirqs are processed at SCHED_FIFO priority under high softirq load
> but this can already happen by a PI-boost by a force-threaded interrupt.
> 
> [ frederic@kernel.org: rcutorture.c fixes, storm fix by introduction of
>   local_timers_pending() for tick_nohz_next_event() ]
> 
> [ junxiao.chang@intel.com: Ensure ktimersd gets woken up even if a
>   softirq is currently served. ]
> 
> Reviewed-by: Paul E. McKenney <paulmck@kernel.org> [rcutorture]
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

Just a few nits:


> ---
>  include/linux/interrupt.h | 44 +++++++++++++++++++++++++
>  kernel/rcu/rcutorture.c   |  6 ++++
>  kernel/softirq.c          | 69 ++++++++++++++++++++++++++++++++++++++-
>  kernel/time/hrtimer.c     |  4 +--
>  kernel/time/tick-sched.c  |  2 +-
>  kernel/time/timer.c       |  2 +-
>  6 files changed, 122 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
> index 457151f9f263d..9637af78087f3 100644
> --- a/include/linux/interrupt.h
> +++ b/include/linux/interrupt.h
> @@ -616,6 +616,50 @@ extern void __raise_softirq_irqoff(unsigned int nr);
>  extern void raise_softirq_irqoff(unsigned int nr);
>  extern void raise_softirq(unsigned int nr);
>  
> +/*
> + * Handle timers in a dedicated thread at a low SCHED_FIFO priority instead in
> + * ksoftirqd as to be prefred over SCHED_NORMAL tasks.
> + */

This doesn't parse. How about, inspired by your changelog:

"""
Wake-ups for RT tasks happen from hardirq context so only timer_list timers
and hrtimers for SCHED_OTHER tasks are processed from softirq. As they are
raised from hardirq, their processing would normally happen from ksoftirqd
which runs as SCHED_OTHER and compete with other tasks. Moving timers softirqs
to a low-prio SCHED_FIFO kthread instead ensures that wakeups from timers are
performed before scheduling the target SCHED_OTHER tasks.
"""

Thanks.

Re: [PATCH v2 3/3] softirq: Use a dedicated thread for timer wakeups on PREEMPT_RT.

[Sebastian Andrzej Siewior]

On 2024-10-28 15:01:55 [+0100], Frederic Weisbecker wrote:
> > diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
> > index 457151f9f263d..9637af78087f3 100644
> > --- a/include/linux/interrupt.h
> > +++ b/include/linux/interrupt.h
> > @@ -616,6 +616,50 @@ extern void __raise_softirq_irqoff(unsigned int nr);
> >  extern void raise_softirq_irqoff(unsigned int nr);
> >  extern void raise_softirq(unsigned int nr);
> >  
> > +/*
> > + * Handle timers in a dedicated thread at a low SCHED_FIFO priority instead in
> > + * ksoftirqd as to be prefred over SCHED_NORMAL tasks.
> > + */
> 
> This doesn't parse. How about, inspired by your changelog:
…

What about this essay instead:

| With forced-threaded interrupts enabled a raised softirq is deferred to
| ksoftirqd unless it can be handled within the threaded interrupt. This
| affects timer_list timers and hrtimers which are explicitly marked with
| HRTIMER_MODE_SOFT.
| With PREEMPT_RT enabled more hrtimers are moved to softirq for processing
| which includes all timers which are not explicitly marked HRTIMER_MODE_HARD.
| Userspace controlled timers (like the clock_nanosleep() interface) is divided
| into two categories: Tasks with elevated scheduling policy including
| SCHED_{FIFO|RR|DL} and the remaining scheduling policy. The tasks with the
| elevated scheduling policy are woken up directly from the HARDIRQ while all
| other wake ups are delayed to so softirq and so to ksoftirqd.
|
| The ksoftirqd runs at SCHED_OTHER policy at which it should remain since it
| handles the softirq in an overloaded situation (not handled everything
| within its last run).
| If the timers are handled at SCHED_OTHER priority then they competes with all
| other SCHED_OTHER tasks for CPU resources are possibly delayed.
| Moving timers softirqs to a low priority SCHED_FIFO thread instead ensures
| that timer are performed before scheduling any SCHED_OTHER thread.

And with this piece of text I convinced myself to also enable this in
the forced-threaded case.

> Thanks.

Sebastian

Re: [PATCH v2 3/3] softirq: Use a dedicated thread for timer wakeups on PREEMPT_RT.

[Frederic Weisbecker]

Le Tue, Oct 29, 2024 at 02:52:31PM +0100, Sebastian Andrzej Siewior a écrit :
> On 2024-10-28 15:01:55 [+0100], Frederic Weisbecker wrote:
> > > diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
> > > index 457151f9f263d..9637af78087f3 100644
> > > --- a/include/linux/interrupt.h
> > > +++ b/include/linux/interrupt.h
> > > @@ -616,6 +616,50 @@ extern void __raise_softirq_irqoff(unsigned int nr);
> > >  extern void raise_softirq_irqoff(unsigned int nr);
> > >  extern void raise_softirq(unsigned int nr);
> > >  
> > > +/*
> > > + * Handle timers in a dedicated thread at a low SCHED_FIFO priority instead in
> > > + * ksoftirqd as to be prefred over SCHED_NORMAL tasks.
> > > + */
> > 
> > This doesn't parse. How about, inspired by your changelog:
> …
> 
> What about this essay instead:
> 
> | With forced-threaded interrupts enabled a raised softirq is deferred to
> | ksoftirqd unless it can be handled within the threaded interrupt. This
> | affects timer_list timers and hrtimers which are explicitly marked with
> | HRTIMER_MODE_SOFT.
> | With PREEMPT_RT enabled more hrtimers are moved to softirq for processing
> | which includes all timers which are not explicitly marked HRTIMER_MODE_HARD.
> | Userspace controlled timers (like the clock_nanosleep() interface) is divided
> | into two categories: Tasks with elevated scheduling policy including
> | SCHED_{FIFO|RR|DL} and the remaining scheduling policy. The tasks with the
> | elevated scheduling policy are woken up directly from the HARDIRQ while all
> | other wake ups are delayed to so softirq and so to ksoftirqd.

First "so" is intended?

> |
> | The ksoftirqd runs at SCHED_OTHER policy at which it should remain since it
> | handles the softirq in an overloaded situation (not handled everything
> | within its last run).
> | If the timers are handled at SCHED_OTHER priority then they competes with all
> | other SCHED_OTHER tasks for CPU resources are possibly delayed.
> | Moving timers softirqs to a low priority SCHED_FIFO thread instead ensures
> | that timer are performed before scheduling any SCHED_OTHER thread.

Works for me!

> And with this piece of text I convinced myself to also enable this in
> the forced-threaded case.

Yes, that makes sense.

Thanks.

> > Thanks.
> 
> Sebastian

Re: [PATCH v2 3/3] softirq: Use a dedicated thread for timer wakeups on PREEMPT_RT.

[Sebastian Andrzej Siewior]

On 2024-10-29 22:47:21 [+0100], Frederic Weisbecker wrote:
> Le Tue, Oct 29, 2024 at 02:52:31PM +0100, Sebastian Andrzej Siewior a écrit :
> > On 2024-10-28 15:01:55 [+0100], Frederic Weisbecker wrote:
> > > > diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
> > > > index 457151f9f263d..9637af78087f3 100644
> > > > --- a/include/linux/interrupt.h
> > > > +++ b/include/linux/interrupt.h
> > > > @@ -616,6 +616,50 @@ extern void __raise_softirq_irqoff(unsigned int nr);
> > > >  extern void raise_softirq_irqoff(unsigned int nr);
> > > >  extern void raise_softirq(unsigned int nr);
> > > >  
> > > > +/*
> > > > + * Handle timers in a dedicated thread at a low SCHED_FIFO priority instead in
> > > > + * ksoftirqd as to be prefred over SCHED_NORMAL tasks.
> > > > + */
> > > 
> > > This doesn't parse. How about, inspired by your changelog:
> > …
> > 
> > What about this essay instead:
> > 
> > | With forced-threaded interrupts enabled a raised softirq is deferred to
> > | ksoftirqd unless it can be handled within the threaded interrupt. This
> > | affects timer_list timers and hrtimers which are explicitly marked with
> > | HRTIMER_MODE_SOFT.
> > | With PREEMPT_RT enabled more hrtimers are moved to softirq for processing
> > | which includes all timers which are not explicitly marked HRTIMER_MODE_HARD.
> > | Userspace controlled timers (like the clock_nanosleep() interface) is divided
> > | into two categories: Tasks with elevated scheduling policy including
> > | SCHED_{FIFO|RR|DL} and the remaining scheduling policy. The tasks with the
> > | elevated scheduling policy are woken up directly from the HARDIRQ while all
> > | other wake ups are delayed to so softirq and so to ksoftirqd.
> 
> First "so" is intended?

No, it needs to go.

> > |
> > | The ksoftirqd runs at SCHED_OTHER policy at which it should remain since it
> > | handles the softirq in an overloaded situation (not handled everything
> > | within its last run).
> > | If the timers are handled at SCHED_OTHER priority then they competes with all
> > | other SCHED_OTHER tasks for CPU resources are possibly delayed.
> > | Moving timers softirqs to a low priority SCHED_FIFO thread instead ensures
> > | that timer are performed before scheduling any SCHED_OTHER thread.
> 
> Works for me!

Great.

> > And with this piece of text I convinced myself to also enable this in
> > the forced-threaded case.
> 
> Yes, that makes sense.

Good. Then I'm pick to up your tag for that patch. Thank you.

> Thanks.
> 
Sebastian