netdev - Re: [patch net-next v5 10/11] tbf: take into account gso skbs

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Message-ID: <20130308152338.GA1520@minipsycho.orion>
Date:	Fri, 8 Mar 2013 16:23:38 +0100
From:	Jiri Pirko <jiri@...nulli.us>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
	jhs@...atatu.com, kuznet@....inr.ac.ru, j.vimal@...il.com
Subject: Re: [patch net-next v5 10/11] tbf: take into account gso skbs

Tue, Feb 19, 2013 at 06:01:27PM CET, eric.dumazet@...il.com wrote:
>On Tue, 2013-02-19 at 17:46 +0100, Jiri Pirko wrote:
>
>> About the gso_segment, do you see any cons doing that on enqueue path
>> rather than dequeue?
>> 
>
>It would be fine, and could be done in core stack instead of qdisc.
>

So you mean for example in tcp code? the maximum possible size would be
propagated from set qdiscs up to the tcp code?

I'm not sure how exactly do that.

>netif_skb_features() for example has the following (incomplete) check
>
>if (skb_shinfo(skb)->gso_segs > skb->dev->gso_max_segs)
>    features &= ~NETIF_F_GSO_MASK;

Why this is incomplete?

>
>We do have a dev->gso_max_size, but its currently used in TCP stack to
>size the skbs built in tcp_sendmsg().

Where exactly in tcp_sendmsg() this is? I found dev->gso_max_size is copied to
sk_gso_max_size in tcp_v4_connect->sk_setup_caps.

>
>In a forwarding workload, it seems we dont use/check gso_max_size.

Yep, that would require to do the segmentation in enqueue anyway. Maybe
I can implement segmentation in enqueue path first and provide tcp
optimalization after that. What do you think?


Thanks!

Jiri
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