netdev - Re: inaccurate packet scheduling

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Date:	Tue, 29 Jan 2013 08:13:03 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jiri Pirko <jiri@...nulli.us>
Cc:	edumazet@...gle.com, netdev@...r.kernel.org, kuznet@....inr.ac.ru,
	jhs@...atatu.com
Subject: Re: inaccurate packet scheduling

On Tue, 2013-01-29 at 13:23 +0100, Jiri Pirko wrote:

> part of the commit message says:
> <quote>
> The bits per second on the wire is still 5200Mb/s with new HTB
> because qdisc accounts for packet length using skb->len, which
> is smaller than total bytes on the wire if GSO is used.  But
> that is for another patch regardless of how time is accounted.	
> </quote>
> I believe that is a similar problem like ours. But looks like this
> "another patch" never got in.
> 

Hmm, I thought I addressed this in 

commit 1def9238d4aa2146924994aa4b7dc861f03b9362
Author: Eric Dumazet <edumazet@...gle.com>
Date:   Thu Jan 10 12:36:42 2013 +0000

    net_sched: more precise pkt_len computation
    
    One long standing problem with TSO/GSO/GRO packets is that skb->len
    doesn't represent a precise amount of bytes on wire.
    
    Headers are only accounted for the first segment.
    For TCP, thats typically 66 bytes per 1448 bytes segment missing,
    an error of 4.5 % for normal MSS value.
    
    As consequences :
    
    1) TBF/CBQ/HTB/NETEM/... can send more bytes than the assigned limits.
    2) Device stats are slightly under estimated as well.
    
    Fix this by taking account of headers in qdisc_skb_cb(skb)->pkt_len
    computation.
    
    Packet schedulers should use qdisc pkt_len instead of skb->len for their
    bandwidth limitations, and TSO enabled devices drivers could use pkt_len
    if their statistics are not hardware assisted, and if they don't scratch
    skb->cb[] first word.
    
    Both egress and ingress paths work, thanks to commit fda55eca5a
    (net: introduce skb_transport_header_was_set()) : If GRO built
    a GSO packet, it also set the transport header for us.


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