netdev - Re: [PATCH RFC net-next] pktgen: introduce 'rx' mode

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Date:	Wed, 29 Apr 2015 15:56:51 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Thomas Graf <tgraf@...g.ch>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH RFC net-next] pktgen: introduce 'rx' mode

On Wed, 2015-04-29 at 15:38 -0700, Alexei Starovoitov wrote:

> I'm not making and not suggesting any new rules.
> ip_rcv is doing this skb_share_check() not because of pktgen rx,
> but because there can be taps and deliver_skb() as you said.
> gro has a different interface and this pktgen cannot benchmark it.
> rps/rfs is not benchmarkable but this approach either.
> To me this is all fine. I'm not trying to do a universal
> benchmarking tool. This one is dumb and simple and primarily
> oriented to benchmark changes to netif_receive_skb and ingress
> qdisc only. I'm not suggesting to use it everywhere.
> I already mentioned in cover letter:
> "The profile dump looks as expected for RX of UDP packets
> without local socket except presence of __skb_clone."
> Clearly I'm not suggesting to use pktgen rx to optimize IP stack
> and not suggesting at all that stack should assume users!=1
> when skb hits netif_receive_skb. Today at the beginning of
> netif_receive_skb we know that users==1 without checking.
> I'm not changing that assumption.
> Just like pktgen xmit path is cheating little bit while
> benchmarking TX, I'm cheating a little bit with users!=1 on RX.

Look, you don't have to write all this, just fix the bug I mentioned to
you about RPS / RFS. (details in Documentation/networking/scaling.txt )

Simply read the code in enqueue_to_backlog(), this obviously needs to
own skb->next/prev

So pktgen in RX mode MUST deliver skb with skb->users = 1, there is no
way around it.


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