linux-kernel - Re: Get rid of IRQF_DISABLED - (was [PATCH] genirq: warn about IRQF_SHARED|IRQF

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Date:	Mon, 30 Nov 2009 22:31:20 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Nicolas Pitre <nico@...vell.com>,
	Eric Miao <eric.y.miao@...il.com>,
	John Stultz <johnstul@...ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Remy Bohmer <linux@...mer.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrea Gallo <andrea.gallo@...ricsson.com>,
	Jamie Lokier <jamie@...reable.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Get rid of IRQF_DISABLED - (was [PATCH] genirq: warn about
 IRQF_SHARED|IRQF_DISABLED)

On Tue, 1 Dec 2009, Benjamin Herrenschmidt wrote:
> Well, thing is, in cases where we have a sane PIC, the PIC itself is
> perfectly good at keeping the source and anything of the same priority
> or lower masked while we handle an irq.

Unfortunately the majority of PICs does not fall into that category.
 
> So disabling local CPU IRQs will basically add an unnecessary blocking
> of both timer interrupts and perfmon interrupts while doing so.
> 
> Yes, all driver interrupt handlers -should- be only running short amount
> of code in their handlers but you know how it is. The drift introduced
> on timer and perfmon events can be a problem, the later might even make
> it difficult to figure out what an -interrupt- is taking more time than
> it should.

The timer problem only affects the old style tick/jiffies driven
hardware where you have no continous clock source for keeping track of
time. Even x86 managed to do something about that recently :)

Are the perf events on power generally coming through the standard irq
handler code path and/or sensitive to local_irq_disable() ?

> I would suggest we timestamp the handlers in the core btw and warn if
> they take too long so we get a chance to track down the bad guys.

The hassle is to find a time which we think is appropriate as a
threshold which is of course depending on the cpu power of a
system. Also I wonder whether we'd need to make such a warning thing
aware of irq nesting.

Thanks,

	tglx
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