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

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Message-ID: <1453985153.7627.8.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 28 Jan 2016 04:45:53 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	Tom Herbert <tom@...bertland.com>,
	John Fastabend <john.fastabend@...il.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>,
	Alexei Starovoitov <alexei.starovoitov@...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 10:25 +0100, Jesper Dangaard Brouer wrote:

> Yes, that is exactly what I'm contemplating :-)  That is idea "(1)".
> 
> A natural extension to this work, which I expect Tom will love, is to
> also use the idea for RPS.  Once we have a SKB list in stack/GRO-layer,
> then we could build a local sk_buff_head list for each remote CPU, by
> calling get_rps_cpu().   And then enqueue_list_to_backlog, by a
> skb_queue_splice_tail(&cpu_list, &cpu->sd->input_pkt_queue) call.
> 
> This would amortize the cost of transferring packets to a remote CPU,
> which Eric AFAIK points out is costing approx ~133ns.
> 

Jesper, RPS and RFS already defer sending the IPI and submit batches to
remote cpus.

See commits 

e326bed2f47d0365da5a8faaf8ee93ed2d86325b ("rps: immediate send IPI in
process_backlog()")

88751275b8e867d756e4f86ae92afe0232de129f ("rps: shortcut
net_rps_action()")

And of course all the discussions we had to come up with
0a9627f2649a02bea165cfd529d7bcb625c2fcad ("rps: Receive Packet
Steering")

The current state :

net_rps_action_and_irq_enable() sends the IPI at the end of
net_rx_action() once all NAPI handlers have been called, and therefore
have accumulated packets and cook rps_ipi_list (via calls to
rps_ipi_queued() from enqueue_to_backlog())


Adding another stage in the pipeline would not help.