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

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Message-ID: <20100326120228.GC19308@shareable.org>
Date:	Fri, 26 Mar 2010 12:02:28 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Nicolas Pitre <nico@...xnic.net>,
	Linus Torvalds <torvalds@...l.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Andi Kleen <andi@...stfloor.org>,
	David Miller <davem@...emloft.net>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Russell King <rmk@....linux.org.uk>
Subject: Re: [patch 0/2] Run interrupt handlers always with interrupts disabled

Peter Zijlstra wrote:
> On Fri, 2010-03-26 at 10:20 +0100, Ingo Molnar wrote:
> > * Russell King <rmk@....linux.org.uk> wrote:
> > 
> > > On Fri, Mar 26, 2010 at 12:06:44AM -0000, Thomas Gleixner wrote:
> > > > The following patch series removes the IRQF_DISABLED functionality
> > > > from the core interrupt code and runs all interrupt handlers with
> > > > interrupts disabled.
> > > 
> > > As was covered in previous discussions, what about drivers such as SMC91x 
> > > which take a long time to retrieve packets from the hardware? Always running 
> > > handlers with IRQs disabled will kill things such as serial on these 
> > > platforms.
> > 
> > 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.
> 
> Or alternatively, use threaded interrupts for such slow hardware.

What is the latency of threaded interrupts these days, compared with
non-threaded interrupts?

Slow hardware is quite sensitive to increases in latency.  Obviously
not a problem for the sources of latency: it's a problem for the irq
which is _sensitive_ to latency caused by the other one.  That is
typically a serial port or something.

But the benefit of kernel-settable interrupt priorities (i.e. due to
the threads) may be worth it even for serial ports.  I would love to
see some measurements comparing with and without.

-- Jamie
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