netdev - Re: [PATCH netnext] mpls: Reduce skb re-allocations due to skb

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Message-ID: <20240819093441.GC11472@kernel.org>
Date: Mon, 19 Aug 2024 10:34:41 +0100
From: Simon Horman <horms@...nel.org>
To: Christoph Paasch <cpaasch@...le.com>
Cc: netdev <netdev@...r.kernel.org>, Eric Dumazet <edumazet@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>, Jakub Kicinski <kuba@...nel.org>,
	David Miller <davem@...emloft.net>, Roopa Prabhu <roopa@...dia.com>,
	Craig Taylor <cmtaylor@...le.com>
Subject: Re: [PATCH netnext] mpls: Reduce skb re-allocations due to skb_cow()

On Fri, Aug 16, 2024 at 03:20:03PM -0700, Christoph Paasch wrote:
> Hello!
> 
> > On Aug 16, 2024, at 4:18 AM, Simon Horman <horms@...nel.org> wrote:
> > 
> > On Thu, Aug 15, 2024 at 09:12:01AM -0700, Christoph Paasch wrote:
> >> mpls_xmit() needs to prepend the MPLS-labels to the packet. That implies
> >> one needs to make sure there is enough space for it in the headers.
> >> 
> >> Calling skb_cow() implies however that one wants to change even the
> >> playload part of the packet (which is not true for MPLS). Thus, call
> >> skb_cow_head() instead, which is what other tunnelling protocols do.
> >> 
> >> Running a server with this comm it entirely removed the calls to
> >> pskb_expand_head() from the callstack in mpls_xmit() thus having
> >> significant CPU-reduction, especially at peak times.
> > 
> > Hi Christoph and Craig,
> > 
> > Including some performance data here would be nice.
> 
> Getting exact production performance data is going to be a major challenge. Not a technical challenge, but rather logistically, ...

Understood :)