Re: lockdep and threaded IRQs

Peter Zijlstra
2009-03-02T23:07:30Z

On Mon, 2009-03-02 at 14:57 -0800, Andrew Morton wrote:
> On Mon, 02 Mar 2009 14:46:47 -0800 (PST)
> David Miller <davem@davemloft.net> wrote:
>
> > From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Date: Mon, 02 Mar 2009 23:19:31 +0100
> >
> > > I state that every !IRQF_DISABLED usage is a bug, either due to broken
> > > hardware or broken drivers.
> >
> > We'll send you the bill to have everyone's hardware
> > replaced :-)
>
> yes, but with what?
>
> No matter how fast all our interrupt handlers are, running them with
> local interrupts disabled has to worsen the worst-case interrupt
> latency.
>
> I don't see how removing !IRQF_DISABLED improves the kernel - in fact
> there's a latency argument for making !IRQF_DISABLED the default.

On preempt-rt all we do in the hardirq path is mask the interrupt line
and wake up a thread. That's the extreme end of low latency interrupts.

Arguably there is a middle way that works for !-rt.

However, striving to enable interrupts in all interrupt handlers is
asking for stack overruns. Interrupt nesting just isn't really helpful.