netdev - Re: [PATCH v3 net-next 2/2] xdp: add multi-buff support for xdp running in generic mode

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Date: Sat, 9 Dec 2023 20:23:09 +0100
From: Lorenzo Bianconi <lorenzo@...nel.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Jesper Dangaard Brouer <hawk@...nel.org>, aleksander.lobakin@...el.com,
	netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
	pabeni@...hat.com, lorenzo.bianconi@...hat.com, bpf@...r.kernel.org,
	toke@...hat.com, willemdebruijn.kernel@...il.com,
	jasowang@...hat.com, sdf@...gle.com
Subject: Re: [PATCH v3 net-next 2/2] xdp: add multi-buff support for xdp
 running in generic mode

> On Wed, 6 Dec 2023 13:41:49 +0100 Jesper Dangaard Brouer wrote:
> > BUT then I realized that PP have a weakness, which is the return/free
> > path that need to take a normal spin_lock, as that can be called from
> > any CPU (unlike the RX/alloc case).  Thus, I fear that making multiple
> > devices share a page_pool via softnet_data, increase the chance of lock
> > contention when packets are "freed" returned/recycled.
> 
> I was thinking we can add a pcpu CPU ID to page pool so that
> napi_pp_put_page() has a chance to realize that its on the "right CPU"
> and feed the cache directly.

Are we going to use these page_pools just for virtual devices (e.g. veth) or
even for hw NICs? If we do not bound the page_pool to a netdevice I think we
can't rely on it to DMA map/unmap the buffer, right?
Moreover, are we going to rework page_pool stats first? It seems a bit weird to
have a percpu struct with a percpu pointer in it, right?

Regards,
Lorenzo

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