softirq: Use a dedicated thread for timer wakeups on PREEMPT_RT.

The timer and hrtimer soft interrupts are raised in hard interrupt
context. With threaded interrupts force enabled or on PREEMPT_RT this leads
to waking the ksoftirqd for the processing of the soft interrupt.

ksoftirqd runs as SCHED_OTHER task which means it will compete with other
tasks for CPU resources.  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.

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. ]

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org> [rcutorture]
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/all/20241106150419.2593080-4-bigeasy@linutronix.de

1 files changed, 2 insertions, 2 deletions

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 5402e0f24217..d9911516e743 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1811,7 +1811,7 @@ retry:
 	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_timer_softirq(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_timer_softirq(HRTIMER_SOFTIRQ);
 	}
 
 	__hrtimer_run_queues(cpu_base, now, flags, HRTIMER_ACTIVE_HARD);