linux-kernel - Re: [PATCH 3/4] AHCI: Request multiple MSIs

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Date:	Sun, 20 Jul 2008 01:49:07 -0600
From:	Grant Grundler <grundler@...isc-linux.org>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Grant Grundler <grundler@...isc-linux.org>,
	linux-pci@...r.kernel.org, kaneshige.kenji@...fujitsu.com,
	mingo@...e.hu, tglx@...utronix.de, davem@...emloft.net,
	dan.j.williams@...el.com, Martine.Silbermann@...com,
	benh@...nel.crashing.org, michael@...erman.id.au,
	linux-kernel@...r.kernel.org,
	Matthew Wilcox <willy@...ux.intel.com>
Subject: Re: [PATCH 3/4] AHCI: Request multiple MSIs

On Mon, Jul 07, 2008 at 11:48:03AM -0600, Matthew Wilcox wrote:
> On Mon, Jul 07, 2008 at 10:45:34AM -0600, Grant Grundler wrote:
> > If the system is busy, the readl is the cost of coalescing the
> > interrupts.  I suspect it's cheaper to take one readl than
> > handle 16 individual interrupts.
> 
> 16 would be a maximum imposed by the AHCI spec.  My ICH9 board has 6
> ports, but requests all 16 interrupts
> 
> > I'm just pointing out the only upside of the existing code and not trying
> > to argue against this patch.
> 
> There may well be an upside to the existing code, but it's pretty slim.
> The oprofile shows clearly that ahci_interrupt is the largest consumer of
> time during an iozone run.  The only thing that routine does is read the
> HOST_IRQ_STAT register, acquire the spinlock and loop calling
> ahci_port_intr().
> 
> I don't have a profile for this new code yet.  Hopefully we'll have one
> by the end of the day.

Willy,
where you able to get this profile?
I'm still curious.

> 
> > BTW, one more downside of the regular IRQ is it's possibly shared.
> > Using MSI guaratees exclusive IRQ and avoids spurious readl's
> > when AHCI is not busy but the other device is. This would be worth
> > noting (or as a reminder) in the change log or as a comment in
> > the code.
> 
> AHCI already allocates itself a new MSI if the machine supports MSI.
> This change merely extends AHCI to use multiple MSIs.

ok.

thanks,
grant

> 
> Thanks.
> 
> -- 
> Intel are signing my paycheques ... these opinions are still mine
> "Bill, look, we understand that you're interested in selling us this
> operating system, but compare it to ours.  We can't possibly take such
> a retrograde step."
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