netdev - Re: [PATCH net-next] bpf, skb_do_redirect: clear sender

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Date:	Mon, 16 Nov 2015 10:07:56 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Daniel Borkmann <daniel@...earbox.net>
Cc:	Alexei Starovoitov <ast@...mgrid.com>,
	"Devon H. O'Dell" <dho@...tly.com>, davem@...emloft.net,
	edumazet@...gle.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] bpf, skb_do_redirect: clear sender_cpu before
 xmit

On Fri, 2015-10-09 at 19:33 +0200, Daniel Borkmann wrote:
> On 10/09/2015 04:35 AM, Alexei Starovoitov wrote:
> > On 10/8/15 5:50 PM, Devon H. O'Dell wrote:
> >>> with the amount of skb_sender_cpu_clear() all over the code base
> >>> >I wonder whether there is a better solution to all of these.
> >> I think there is. We found that splitting the union of sender_cpu and
> >> napi_id solved the issue for us. In general, I think this is an OK
> >> solution as long as the following hold:
> >>
> >>   * skbs are always allocated via kzalloc
> >>   * out -> out cloned skbs are always cloned on the same CPU
> >>   * an extra four bytes in skbuff isn't a bad thing
> >
> > I'm pretty sure extending sk_buff for this is not acceptable.
> 
> +1, I agree.
> 
> > I was thinking may be we can use sign bit to distinguish between
> > napi_id and sender_cpu.
> > Like:
> >          if ((int)skb->sender_cpu >= 0)
> >                  skb->sender_cpu = - (raw_smp_processor_id() + 1);
> > and inside get_xps_queue() use it only if it's negative.
> > Then we can remove skb_sender_cpu_clear() from everywhere.
> > Adding a check to napi_hash_add() to make sure that napi_id is not
> > negative is probably ok too.
> > Thoughts?
> 
> I think this doesn't make it any more maintainable.
> 
> skb_sender_cpu_clear(), one can at least git-grep to easily find
> out and review call-sites in the code. There are various members
> already used differently depending on the context.

Extending busy polling support for tunnels devices actually requires to
make some changes in this area.

We need to keep skb->napi_id set until skb reaches a socket, but
skb_scrub_packet() currently defeats the thing.

I will leave skb_sender_cpu_clear() in place but it will be empty.



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