netdev - Re: [PATCH net-next] net: Make gro complete function to return void

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Date: Wed, 31 May 2023 06:38:43 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: David Ahern <dsahern@...nel.org>
Cc: Jakub Kicinski <kuba@...nel.org>, Parav Pandit <parav@...dia.com>, davem@...emloft.net, 
	pabeni@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: Make gro complete function to return void

On Wed, May 31, 2023 at 12:36 AM David Ahern <dsahern@...nel.org> wrote:
>
> On 5/30/23 1:39 PM, Jakub Kicinski wrote:
> > On Tue, 30 May 2023 17:48:22 +0200 Eric Dumazet wrote:
> >>> tcp_gro_complete seems fairly trivial. Any reason not to make it an
> >>> inline and avoid another function call in the datapath?
> >>
> >> Probably, although it is a regular function call, not an indirect one.
> >>
> >> In the grand total of driver rx napi + GRO cost, saving a few cycles
> >> per GRO completed packet is quite small.
> >
> > IOW please make sure you include the performance analysis quantifying
> > the win, if you want to make this a static inline. Or let us know if
> > the patch is good as is, I'm keeping it in pw for now.
>
> I am not suggesting holding up this patch; just constantly looking for
> these little savings here and there to keep lowering the overhead.
>
> 100G, 1500 MTU, line rate is 8.3M pps so GRO wise that would be ~180k
> fewer function calls.

Here with 4K MTU, this is called 67k per second

An __skb_put() instead of skb_put() in a driver (eg mlx5e_build_linear_skb())
would have 45x more impact, and would still be noise.