Re: [RFC net-next 0/6] Cleanup IRQ affinity checks in several drivers

[Joe Damato <jdamato@fastly.com>]

On Wed, Aug 14, 2024 at 08:09:15AM -0700, Jakub Kicinski wrote:
> On Wed, 14 Aug 2024 13:12:08 +0100 Joe Damato wrote:
> > Actually... how about a slightly different approach, which caches
> > the affinity mask in the core?
> 
> I was gonna say :)
> 
> >   0. Extend napi struct to have a struct cpumask * field
> > 
> >   1. extend netif_napi_set_irq to:
> >     a. store the IRQ number in the napi struct (as you suggested)
> >     b. call irq_get_effective_affinity_mask to store the mask in the
> >        napi struct
> >     c. set up generic affinity_notify.notify and
> >        affinity_notify.release callbacks to update the in core mask
> >        when it changes
> 
> This part I'm not an export on.
> 
> >   2. add napi_affinity_no_change which now takes a napi_struct
> > 
> >   3. cleanup all 5 drivers:
> >     a. add calls to netif_napi_set_irq for all 5 (I think no RTNL
> >        is needed, so I think this would be straight forward?)
> >     b. remove all affinity_mask caching code in 4 of 5 drivers
> >     c. update all 5 drivers to call napi_affinity_no_change in poll
> > 
> > Then ... anyone who adds support for netif_napi_set_irq to their
> > driver in the future gets automatic support in-core for
> > caching/updating of the mask? And in the future netdev-genl could
> > dump the mask since its in-core?
> > 
> > I'll mess around with that locally to see how it looks, but let me
> > know if that sounds like a better overall approach.

I ended up going with the approach laid out above; moving the IRQ
affinity mask updating code into the core (which adds that ability
to gve/mlx4/mlx5... it seems mlx4/5 cached but didn't have notifiers
setup to update the cached copy?) and adding calls to
netif_napi_set_irq in i40e/iavf and deleting their custom notifier
code.

It's almost ready for rfcv2; I think this approach is probably
better ?

> Could we even handle this directly as part of __napi_poll(),
> once the driver gives core all of the relevant pieces of information ?

I had been thinking the same thing, too, but it seems like at least
one driver (mlx5) counts the number of affinity changes to export as
a stat, so moving all of this to core would break that.

So, I may avoid attempting that for this series.

I'm still messing around with this but will send an rfcv2 in a bit.