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

["Derrick, Jonathan" <jonathan.derrick@intel.com>]

On Mon, 2019-11-18 at 10:49 +0000, Lorenzo Pieralisi wrote:
> 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

This will need an entirely different strategy to be useful, so can be
dropped for now.

Thank you,
Jon