linux-kernel - Re: [RFC] net: add TCP fraglist GRO support

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Date: Tue, 23 Apr 2024 17:18:17 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: David Ahern <dsahern@...nel.org>
Cc: Felix Fietkau <nbd@....name>, netdev@...r.kernel.org, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] net: add TCP fraglist GRO support

On Tue, Apr 23, 2024 at 5:03 PM David Ahern <dsahern@...nel.org> wrote:
>
> On 4/23/24 4:15 AM, Eric Dumazet wrote:
> > I think we should push hard to not use frag_list in drivers :/
>
> why is that? I noticed significant gains for local delivery after adding
> frag_list support for H/W GRO. Fewer skbs going up the stack is
> essential for high throughput and reducing CPU load.

Felix case is about forwarding, not local delivery (which is fine)

>
> >
> > And GRO itself could avoid building frag_list skbs
> > in hosts where forwarding is enabled.
>
> But if the egress device supports SG and the driver understands
> frag_list, walking the frag_list should be cheaper than multiple skbs
> traversing the forwarding path.

I do not count any relevant (modern) driver supporting NETIF_F_FRAGLIST

>
> >
> > (Note that we also can increase MAX_SKB_FRAGS to 45 these days)
>
> Using 45 frags has other side effects and not something that can be done
> universally (hence why it is a config option).
>
> 45 frags is for Big TCP at 4kB and that is ~ 3 skbs at the default
> setting of 17 which means an skb chain 2 deep. 1 skb going up the stack
> vs 3 skbs - that is a big difference.

45 frags can also be for non BIG TCP, allowing ~64KB GRO packets
without frag_list.

45*1448 = 65160

Max number of frags per skb is orthogonal to (BIG) TCP.