linux-kernel - Re: Interrupts are being dropped under heavy IO on -rt .

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Date:	Fri, 10 Apr 2009 23:59:48 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	jvrao <jvrao@...ux.vnet.ibm.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	rt-users <linux-rt-users@...r.kernel.org>,
	Mike Anderson <andmike@...ux.vnet.ibm.com>,
	"Luis Claudio R. Goncalves" <lgoncalv@...hat.com>,
	linux-driver@...gic.com
Subject: Re: Interrupts are being dropped under heavy IO on -rt .

On Fri, 10 Apr 2009, jvrao wrote:
> Qlogic driver uses MSI-EDGE interrupts by default.
> 
> On -rt kernel under heavy IO load we observed that the driver missed
> interrupts occasionally.
> 
> Missing interrupts caused spurious mailbox timeouts and path failovers.
> 
> Suspecting MSI interrupts, we disabled MSI and forcing the driver to use APIC
> interrupts (Patch attached).
> 
> With this change, our IO tests ran for extended period of time without any
> issues.
> 
> It would be interesting to see if the MSI infrastructure itself has issues on
> realtime, or it is the combination of qla2xxx/MSI.
> 
> Right now we don't have any setup to test out MSI with other adapters. Hence,
> wondering if anyone faced similar issues.

Hmm. Can we get down to the root cause of that problem instead of
applying a bandaid patch ?

If -rt has problems with MSI edge interrupts then we really need to
figure out why. If the qlogic driver has problems with the different
timings of -rt then we need to fix that one.

Thanks,

	tglx
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