Re: [patch 0/2] Run interrupt handlers always with interrupts disabled

[Jamie Lokier <jamie@shareable.org>]

Nicolas Pitre wrote:
> On Fri, 26 Mar 2010, Peter Zijlstra wrote:
> 
> > On Fri, 2010-03-26 at 09:59 +0000, Alan Cox wrote:
> > > > As long as it's rare (which it is) i dont see a problem: you can enable 
> > > > interrupts in the handler by using local_irq_enable(), like the IDE PIO 
> > > > drivers do. That way it's documented a bit better as well, because it shows 
> > > > the precise source of the latency, with a big comment explaining it, etc.
> > > 
> > > I don't think it's as rare as you think particularly in embedded, and the
> > > moment you start explicitly using local_irq_enable() you've simply moved
> > > the underlying problem back and made it far harder to grep for.
> > 
> > We've got local_irq_enable_in_hardirq() which should be used and can
> > easily be grep'ed for.
> > 
> > But yes, I would much prefer to simply convert these known slow handlers
> > to threaded interrupts.
> 
> Can't do that.  The smc91x has a very small internal buffer which has to 
> be emptied using PIO.  Threaded interrupts simply have too high 
> latencies for overruns not to occur.  That's why the RX path is entirely 
> done in hardirq context while the TX path is done in softirq context.

Although I wouldn't be surprised to find threaded interrupts are too
slow on certain hardware, is that _fundamental_ to threaded
interrupts, or is it just that our implementation doesn't have the
funky hot path straight direct from hardirq -> running high priority
RT irq thread when it exceeds previously running priority?

In other words, can we swizzle threaded irqs into something more
resembling software-implemented hard irq priorities, while cunningly
updating the kernel state just enough to look like it's a thread?

-- Jamie