netdev - Re: inaccurate packet scheduling

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Message-ID: <20130108133038.GB1621@minipsycho.orion>
Date:	Tue, 8 Jan 2013 14:30:38 +0100
From:	Jiri Pirko <jiri@...nulli.us>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	edumazet@...gle.com, netdev@...r.kernel.org, kuznet@....inr.ac.ru,
	jhs@...atatu.com
Subject: Re: inaccurate packet scheduling

Wed, Jan 02, 2013 at 05:34:42PM CET, eric.dumazet@...il.com wrote:
>On Wed, 2013-01-02 at 16:26 +0100, Jiri Pirko wrote:
>> Tue, Dec 18, 2012 at 05:51:43PM CET, erdnetdev@...il.com wrote:
>> >On Tue, 2012-12-18 at 08:26 -0800, Stephen Hemminger wrote:
>> >
>> >> Check kernel log for messages about clock. It could be that on the
>> >> machines with issues TSC is not usable for kernel clock.
>> >> Also turn off TSO since it screws up any form of rate control.
>> >
>> >Yes, but we should fix it eventually. I'll take a look.
>> 
>> Hi Eric, did you have a chance to look at this? Or should I give it a try?
>
>I took a look, and TBF does :
>
>if (qdisc_pkt_len(skb) > q->max_size)
>    return qdisc_reshape_fail(skb, sch);
>
>We have several choices :
>
>1) Add a one time warning
>
>2) cap dev->gso_max_size at Qdisc setup time
>
>3) Re-segment the packet instead of dropping it (if GSO packet), and
>call the expensive qdisc_tree_decrease_qlen() function.
>
>4) remove max_size limitation 
>

To my untrained eye, 2) or 4) look like a way to go. Not sure though.
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