linux-kernel - Re: lockdep and threaded IRQs (was: ...)

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Date:	Mon, 02 Mar 2009 22:41:46 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	David Brownell <david-b@...bell.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>, me@...ipebalbi.com,
	linux-kernel@...r.kernel.org, linux-input@...r.kernel.org,
	felipe.balbi@...ia.com, dmitry.torokhov@...il.com,
	sameo@...nedhand.com
Subject: Re: lockdep and threaded IRQs (was: ...)

On Mon, 2009-03-02 at 13:37 -0800, David Brownell wrote:

> > How so?, its the natural extension of that work.
> 
> Not the work to shrink the amount of time IRQ latencies
> by shrinking the amount of time IRQs are disabled by
> IRQ handlers.

Ugh, that's done by pushing work out of the hardirq context, not by
doing silly things like enabling irqs from hardirq context.

> > > > we should simply always disable interrupts for 
> > > > interrupt handlers.
> > > 
> > > That would be why you have refused to fix the bug
> > > in lockdep, whereby it forcibly enables that flag?
> > > 
> > > I've been wondering for some months now why you've
> > > left that bug unfixed.
> > 
> > Because running irq handlers with irqs enabled it plain silly.
> 
> Not if you have hardware-prioritized IRQs, which are
> fairly common in some environments ... handling an IRQ
> for high priority device A needn't interfere with the
> handler for lower priority device B, and the system
> overall can work better.
> 
> Not if you need to shrink IRQ latencies by minimizing
> irqs-off critical sections everywhere ... IRQ handlers
> being common offenders for keeping IRQs off too long.
> 
> Not when IRQs can be disabled selectively around the
> real critical sections ... so drivers can leave IRQs
> enabled except in those brief sections.

Yeah, and who gets to debug the stack overflow?

Hardware with irq prio levels can do a prio mask and use a stack per
prio level.

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