netdev - Re: Bypass at packet-page level (Was: Optimizing instruction-cache, more packets at each stage)

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Date:	Thu, 28 Jan 2016 05:25:43 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	John Fastabend <john.fastabend@...il.com>,
	Tom Herbert <tom@...bertland.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	David Miller <davem@...emloft.net>,
	Or Gerlitz <gerlitz.or@...il.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Alexander Duyck <alexander.duyck@...il.com>,
	Daniel Borkmann <borkmann@...earbox.net>,
	Marek Majkowski <marek@...udflare.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Florian Westphal <fw@...len.de>,
	Paolo Abeni <pabeni@...hat.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Amir Vadai <amirva@...il.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Vladislav Yasevich <vyasevich@...il.com>
Subject: Re: Bypass at packet-page level (Was: Optimizing instruction-cache,
 more packets at each stage)

On Thu, 2016-01-28 at 10:52 +0100, Jesper Dangaard Brouer wrote:

> I'm still in flux/undecided how long we should delay the first touching
> of pkt-data, which happens when calling eth_type_trans().  Should it
> stay in the driver or not(?).

Some cpus have limited prefetch capabilities.
Sometimes, prefetches need to be spaced, otherwise they are ignored.
A driver author might be tempted to 'optimize' its rx handler for few
cpus.

Also, removing eth_type_trans() from the drivers would require quite
some work, but would be generic and certainly helpful.