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Date: Thu, 25 Mar 2010 01:36:52 +0100
From: Andi Kleen <andi@firstfloor.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Andi Kleen <andi@firstfloor.org>, x86@kernel.org,
       linux-kernel@vger.kernel.org, jesse.brandeburg@intel.com
Subject: Re: [PATCH] Prevent nested interrupts when the IRQ stack is near overflowing v2
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On Thu, Mar 25, 2010 at 12:08:23AM +0100, Thomas Gleixner wrote:
> On Wed, 24 Mar 2010, Thomas Gleixner wrote:
> 
> > On Wed, 24 Mar 2010, Andi Kleen wrote:
> > 
> > > Prevent nested interrupts when the IRQ stack is near overflowing v2
> > > 
> > > Interrupts can always nest when they don't run with IRQF_DISABLED.
> > > 
> > > When a lot of interrupts hit the same vector on the same
> > > CPU nested interrupts can overflow the irq stack and cause hangs.
> That's utter nonsense. An interrupt storm on the same vector does not
> cause irq nesting. The irq code prevents reentering a handler and in

Sorry it's the same CPU, not the same vector.  Yes the reference
to same vector was misleading.

"
Multiple vectors on a multi port NIC pointing to the same CPU, 
all hitting the irq stack until it overflows.
"

> case of MSI-X it just disables the IRQ when it comes again while the
> first irq on that vector is still in progress. So the maximum nesting
> is two up to handle_edge_irq() where it disables the IRQ and returns
> right away.

Real maximum nesting is all IRQs running with interrupts on pointing
to the same CPU. Enough from multiple busy IRQ sources and you go boom.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.