Re: [PATCH 3/3] PCI: vmd: Use managed irq affinities

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Wed, Nov 06, 2019 at 08:33:01PM +0000, Derrick, Jonathan wrote:
> On Thu, 2019-11-07 at 05:27 +0900, Keith Busch wrote:
> > On Wed, Nov 06, 2019 at 08:14:41PM +0000, Derrick, Jonathan wrote:
> > > Yes that problem exists today 
> > 
> > Not really, because we're currently using unamanged interrupts which
> > migrate to online CPUs. It's only the managed ones that don't migrate
> > because they have a unchangeable affinity.
> > 
> > > and this set limits the exposure as it's
> > > a rare case where you have a child NVMe device with fewer than 32
> > > vectors.
> > 
> > I'm deeply skeptical that is the case. Even the P3700 has only 31 IO
> > queues, yielding 31 vectors for IO services, so that already won't work
> > with this scheme.
> > 
> Darn you're right. At one point I had VMD vector 1 using NVMe AQ,
> leaving the 31 remaining VMD vectors for NVMe IO queues. This would
> have lined up perfectly. Had changed it last minute and that does break
> the scheme for P3700....
> 
> > But assuming you wanted to only use devices that have at least 32 IRQ
> > vectors, the nvme driver also allows users to carve those vectors up
> > into fully affinitized sets for different services (read vs. write is
> > the one the block stack supports), which would also break if alignment
> > to the parent device's IRQ setup is required.
> 
> Wasn't aware of this. Fair enough.

Marked the series with "Changes Requested", waiting for a new
version.

Lorenzo