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 08:37:07 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jesper Dangaard Brouer <brouer@...hat.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, Jan 28, 2016 at 4:45 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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.
>
skbs are enqueued on a CPU queue one at at time through
enqueue_to_backlog. It would be nice to do that as a batch of skbs.

>