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

[Joe Damato]

On Wed, Sep 04, 2024 at 04:54:17PM -0700, Jakub Kicinski wrote:
> On Wed, 4 Sep 2024 16:40:41 -0700 Stanislav Fomichev wrote:
> > > I think what you are proposing seems fine; I'm just working out the
> > > implementation details and making sure I understand before sending
> > > another revision.  
> > 
> > What if instead of an extra storage index in UAPI, we make napi_id persistent?
> > Then we can keep using napi_id as a user-facing number for the configuration.
> > 
> > Having a stable napi_id would also be super useful for the epoll setup so you
> > don't have to match old/invalid ids to the new ones on device reset.
> 
> that'd be nice, initially I thought that we have some drivers that have
> multiple instances of NAPI enabled for a single "index", but I don't
> see such drivers now.
> 
> > In the code, we can keep the same idea with napi_storage in netdev and
> > ask drivers to provide storage id, but keep that id internal.
> > 
> > The only complication with that is napi_hash_add/napi_hash_del that
> > happen in netif_napi_add_weight. So for the devices that allocate
> > new napi before removing the old ones (most devices?), we'd have to add
> > some new netif_napi_takeover(old_napi, new_napi) to remove the
> > old napi_id from the hash and reuse it in the new one.
> > 
> > So for mlx5, the flow would look like the following:
> > 
> > - mlx5e_safe_switch_params
> >   - mlx5e_open_channels
> >     - netif_napi_add(new_napi)
> >       - adds napi with 'ephemeral' napi id
> >   - mlx5e_switch_priv_channels
> >     - mlx5e_deactivate_priv_channels
> >       - napi_disable(old_napi)
> >       - netif_napi_del(old_napi) - this frees the old napi_id
> >   - mlx5e_activate_priv_channels
> >     - mlx5e_activate_channels
> >       - mlx5e_activate_channel
> >         - netif_napi_takeover(old_napi is gone, so probably take id from napi_storage?)
> > 	  - if napi is not hashed - safe to reuse?
> > 	- napi_enable
> > 
> > This is a bit ugly because we still have random napi ids during reset, but
> > is not super complicated implementation-wise. We can eventually improve
> > the above by splitting netif_napi_add_weight into two steps: allocate and
> > activate (to do the napi_id allocation & hashing). Thoughts?
> 
> The "takeover" would be problematic for drivers which free old NAPI
> before allocating new one (bnxt?). But splitting the two steps sounds
> pretty clean. We can add a helper to mark NAPI as "driver will
> explicitly list/hash later", and have the driver call a new helper
> which takes storage ID and lists the NAPI in the hash.

It sounds like you all are suggesting that napi_id is moved into the
napi_storage array, as well? That way NAPI IDs persist even if the
NAPI structs themselves are recreated?

I think that's interesting and I'm open to supporting that. I wrote
up an RFC that moves stuff in the direction of napi_storage and
modifies 3 drivers but:
  - is incorrect because it breaks the persistence thing we are
    talking about, and
  - it doesn't do the two step take-over thing described above to
    inherit NAPI IDs (as far as I understand ?)

I'm going to send the RFC anyway because I think it'll be easier to
pick up the discussion on code that is hopefully closer to where we
want to land.

I hope that is OK.

- Joe