linux-kernel - Re: Notes from LPC PCI/MSI BoF session

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Message-ID: <20080925155343.GC2997@colo.lackof.org>
Date:	Thu, 25 Sep 2008 09:53:43 -0600
From:	Grant Grundler <grundler@...isc-linux.org>
To:	David Miller <davem@...emloft.net>
Cc:	grundler@...isc-linux.org, jbarnes@...tuousgeek.org,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Notes from LPC PCI/MSI BoF session

On Tue, Sep 23, 2008 at 11:47:05PM -0700, David Miller wrote:
> From: Grant Grundler <grundler@...isc-linux.org>
> Date: Tue, 23 Sep 2008 23:51:16 -0600
> 
> > Dave Miller (and others) have clearly stated they don't want to see
> > CPU affinity handled in the device drivers and want irqbalanced
> > to handle interrupt distribution. The problem with this is irqbalanced
> > needs to know how each device driver is binding multiple MSI to it's queues.
> > Some devices could prefer several MSI go to the same processor and
> > others want each MSI bound to a different "node" (NUMA).
> > 
> > Without any additional API, this means the device driver has to
> > update irqbalanced for each device it supports. We thought pci_ids.h
> > was a PITA...that would be trivial compared to maintaining this.
> 
> We just need a consistent naming scheme for the IRQs to disseminate
> this information to irqbalanced, then there is one change to irqbalanced
> rather than one for each and every driver as you seem to suggest.

That's sort of what I proposed at the end of my email:
| A second solution I thought of later might be for the device driver to
| export (sysfs?) to irqbalanced which MSIs the driver instance owns and
| how many "domains" those MSIs can serve.  irqbalanced can then write
| back into the same (sysfs?) the mapping of MSI to domains and update
| the smp_affinity mask for each of those MSI.

> Anything that's complicated and takes more than a paragraph or two
> to describe is not what we want.

I agree. The discussion will need more though.

thanks,
grant

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