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@xxxxxxxxxx: rcutorture.c fixes, storm fix by introduction of > local_timers_pending() for tick_nohz_next_event() ] > > [ junxiao.chang@xxxxxxxxx: Ensure ktimersd gets woken up even if a > softirq is currently served. ] > > Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx> [rcutorture] > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx> 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.
