netdev - Re: [PATCH net-next 05/15] net: page

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Message-ID: <20231025131740.489fdfcf@kernel.org>
Date: Wed, 25 Oct 2023 13:17:40 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Mina Almasry <almasrymina@...gle.com>
Cc: davem@...emloft.net, netdev@...r.kernel.org, edumazet@...gle.com,
 pabeni@...hat.com, hawk@...nel.org, ilias.apalodimas@...aro.org
Subject: Re: [PATCH net-next 05/15] net: page_pool: record pools per netdev

On Wed, 25 Oct 2023 12:56:44 -0700 Mina Almasry wrote:
> > +#if IS_ENABLED(CONFIG_PAGE_POOL)
> > +       /** @page_pools: page pools created for this netdevice */
> > +       struct hlist_head       page_pools;
> > +#endif  
> 
> I wonder if this per netdev field is really necessary. Is it not
> possible to do the same simply looping over the  (global) page_pools
> xarray? Or is that too silly of an idea. I guess on some systems you
> may end up with 100s or 1000s of active or orphaned page pools and
> then globally iterating over the whole page_pools xarray can be really
> slow..

I think we want the per-netdev hlist either way, on netdev
unregistration we need to find its pools to clear the pointers.
At which point we can as well use that to dump the pools.

I don't see a strong reason to use one approach over the other.
Note that other objects like napi and queues (WIP patches) also walk
netdevs and dump sub-objects from them.

> > @@ -48,6 +49,7 @@ struct pp_alloc_cache {
> >   * @pool_size: size of the ptr_ring
> >   * @nid:       NUMA node id to allocate from pages from
> >   * @dev:       device, for DMA pre-mapping purposes
> > + * @netdev:    netdev this pool will serve (leave as NULL if none or multiple)  
> 
> Is this an existing use case (page_pools that serve null or multiple
> netdevs), or a future use case? My understanding is that currently
> page_pools serve at most 1 rx-queue. Spot checking a few drivers that
> seems to be true.

I think I saw one embedded driver for a switch-like device which has
queues servicing all ports, and therefore netdevs.
We'd need some help from people using such devices to figure out what
the right way to represent them is, and what extra bits of
functionality they need.

> I'm guessing 1 is _always_ loopback?

AFAIK, yes. I should probably use LOOPBACK_IFINDEX, to make it clearer.