linux-kernel - Re: [PATCH 1/1] softirq: Use a dedicated thread for timer wakeups on PREEMPT

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Message-ID: <ZxjLoAINhmGKImHW@pavilion.home>
Date: Wed, 23 Oct 2024 12:10:40 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 1/1] softirq: Use a dedicated thread for timer wakeups on
 PREEMPT_RT.

Le Wed, Oct 23, 2024 at 08:30:14AM +0200, Sebastian Andrzej Siewior a écrit :
> On 2024-10-23 00:27:34 [+0200], Frederic Weisbecker wrote:
> > > Try again without the "ksoftirqd will collect it all" since this won't
> > > happen since the revert I mentioned.
> > 
> > I still don't get it, this makes:
> > 
> > """
> > Once the ksoftirqd is marked as pending (or is running), a softirq which
> > would have been processed at the end of the threaded interrupt, which runs
> > at an elevated priority, is now moved to ksoftirqd which runs at SCHED_OTHER
> > priority and competes with every regular task for CPU resources.
> > """
> > 
> > ksoftirqd raised for timers still doesn't prevent a threaded IRQ from running
> > softirqs, unless it preempts ksoftirqd and waits with PI. So is it what you're
> > trying to solve?
> > 
> > Or is the problem that timer softirqs are executed with SCHED_NORMAL?
> 
> Exactly. It runs at SCHED_NORMAL and competes with everything else. It
> can delay tasks wakes depending on system load.

Ok so that narrows down the problem and it's much clearer, thanks.

> > > > > +void raise_timer_softirq(void)
> > > > > +{
> > > > > +	unsigned long flags;
> > > > > +
> > > > > +	local_irq_save(flags);
> > > > > +	raise_ktimers_thread(TIMER_SOFTIRQ);
> > > > > +	wake_timersd();
> > > > 
> > > > This is supposed to be called from hardirq only, right?
> > > > Can't irq_exit_rcu() take care of it? Why is it different
> > > > from HRTIMER_SOFTIRQ ?
> > > 
> > > Good question. This shouldn't be any different compared to the hrtimer
> > > case. This is only raised in hardirq, so yes, the irq_save can go away
> > > and the wake call, too.
> > 
> > Cool. You can add lockdep_assert_in_irq() within raise_ktimers_thread() for
> > some well deserved relief :-)
> 
> If you want to, sure. I would add them to both raise functions.

Yeah, just in case. Thanks!

> 
> > Thanks.
> 
> Sebastian