linux-kernel - Re: [PATCH] irq_work: Avoid unnecessary "IRQ work" interrupts

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Message-ID: <20240828095415.43iwHYdM@linutronix.de>
Date: Wed, 28 Aug 2024 11:54:15 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: "Brandt, Oliver - Lenze" <oliver.brandt@...ze.com>
Cc: "tglx@...utronix.de" <tglx@...utronix.de>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] irq_work: Avoid unnecessary "IRQ work" interrupts

On 2024-08-28 11:37:20 [+0200], To Brandt, Oliver - Lenze wrote:
> > Fixes: b4c6f86ec2f6 ('irq_work: Handle some irq_work in a per-CPU thread on PREEMPT_RT')
> > Signed-off-by: Oliver Brandt <oliver.brandt@...ze.com>
> > ---
> >  kernel/irq_work.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/irq_work.c b/kernel/irq_work.c
> > index 2f4fb336dda1..df08b7dde7d5 100644
> > --- a/kernel/irq_work.c
> > +++ b/kernel/irq_work.c
> > @@ -108,7 +108,7 @@ static void __irq_work_queue_local(struct irq_work *work)
> >                 return;
> >  
> >         /* If the work is "lazy", handle it from next tick if any */
> > -       if (!lazy_work || tick_nohz_tick_stopped())
> > +       if (!(lazy_work || rt_lazy_work) || tick_nohz_tick_stopped())
> >                 irq_work_raise(work);
> 
> Looking at this I *think* rt_lazy_work was needed earlier due to
> different code but not anymore. Couldn't you just remove rt_lazy_work
> and set lazy_work in the RT path? That should work.

Actually no. If we merge rt_lazy_work into lazy_work then we would have
the behaviour you have here. But we need irq_work_raise() in order to
	irq_work_run();
	-> wake_irq_workd();
	  -> wake_up_process(irq_workd);

If we don't irq_work_raise() here then it will be delayed until the next
tick and we didn't want that if my memory serves me.

> >  }
> >  

Sebastian