Re: [PATCH net-next 5/5] netdev-genl: Support setting per-NAPI config values

Samiullah Khawaja
2024-09-03T21:58:27Z

On Tue, Sep 3, 2024 at 12:40 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Tue, 3 Sep 2024 12:04:52 -0700 Samiullah Khawaja wrote:
> > Do we need a queue to napi association to set/persist napi
> > configurations?
>
> I'm afraid zero-copy schemes will make multiple queues per NAPI more
> and more common, so pretending the NAPI params (related to polling)
> are pre queue will soon become highly problematic.
Agreed.
>
> > Can a new index param be added to the netif_napi_add
> > and persist the configurations in napi_storage.
>
> That'd be my (weak) preference.
>
> > I guess the problem would be the size of napi_storage.
>
> I don't think so, we're talking about 16B per NAPI,
> struct netdev_queue is 320B, struct netdev_rx_queue is 192B.
> NAPI storage is rounding error next to those :S
Oh, I am sorry I was actually referring to the problem of figuring out
the count of the napi_storage array.
>
> > Also wondering if for some use case persistence would be problematic
> > when the napis are recreated, since the new napi instances might not
> > represent the same context? For example If I resize the dev from 16
> > rx/tx to 8 rx/tx queues and the napi index that was used by TX queue,
> > now polls RX queue.
>
> We can clear the config when NAPI is activated (ethtool -L /
> set-channels). That seems like a good idea.
That sounds good.
>
> The distinction between Rx and Tx NAPIs is a bit more tricky, tho.
> When^w If we can dynamically create Rx queues one day, a NAPI may
> start out as a Tx NAPI and become a combined one when Rx queue is
> added to it.
>
> Maybe it's enough to document how rings are distributed to NAPIs?
>
> First set of NAPIs should get allocated to the combined channels,
> then for remaining rx- and tx-only NAPIs they should be interleaved
> starting with rx?
>
> Example, asymmetric config: combined + some extra tx:
>
> combined tx
> [0..#combined-1] [#combined..#combined+#tx-1]
>
> Split rx / tx - interleave:
>
> [0 rx0] [1 tx0] [2 rx1] [3 tx1] [4 rx2] [5 tx2] ...
>
> This would limit the churn when changing channel counts.
I think this is good. The queue-get dump netlink does provide details
of all the queues in a dev. It also provides a napi-id if the driver
has set it (only few drivers set this). So basically a busy poll
application would look at the queue type and apply configurations on
the relevant napi based on the documentation above (if napi-id is not
set on the queue)?