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

[Shay Drori <shayd@nvidia.com>]

On 14/08/2024 18:19, Joe Damato wrote:
> 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.

several net drivers (mlx5, mlx4, ice, ena and more) are using a feature
called ARFS (rmap)[1], and this feature is using the affinity notifier
mechanism.
Also, affinity notifier infra is supporting only a single notifier per
IRQ.

Hence, your suggestion (1.c) will break the ARFS feature.

[1] see irq_cpu_rmap_add()

>>
>>>    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?)


maybe This is probably due to what I wrote above..


> 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.
>