linux-kernel - Re: [PATCH v2 3/3] softirq: Use a dedicated thread for timer wakeups on PREEMPT

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Message-ID: <20241029135231.ScfxKhz1@linutronix.de>
Date: Tue, 29 Oct 2024 14:52:31 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 3/3] softirq: Use a dedicated thread for timer wakeups
 on PREEMPT_RT.

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