netdev - Re: [PATCH RFC] pkt_sched: enable QFQ to support TSO/GSO

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Message-ID: <20121030072415.17199f9a@nehalam.linuxnetplumber.net>
Date:	Tue, 30 Oct 2012 07:24:15 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Paolo Valente <paolo.valente@...more.it>
Cc:	jhs@...atatu.com, davem@...emloft.net,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	rizzo@....unipi.it, fchecconi@...il.com
Subject: Re: [PATCH RFC] pkt_sched: enable QFQ to support TSO/GSO

On Tue, 30 Oct 2012 07:00:56 +0100
Paolo Valente <paolo.valente@...more.it> wrote:

> Hi,
> if the max packet size for some class (configured through tc) is
> violated by the actual size of the packets of that class, then QFQ
> would not schedule classes correctly, and the data structures
> implementing the bucket lists may get corrupted. This problem occurs
> with TSO/GSO even if the max packet size is set to the MTU, and is,
> e.g., the cause of the failure reported in [1]. Two patches have been
> proposed to solve this problem in [2], one of them is a preliminary
> version of this patch.
> 
> This patch addresses the above issues by: 1) setting QFQ parameters to
> proper values for supporting TSO/GSO (in particular, setting the
> maximum possible packet size to 64KB), 2) automatically increasing the
> max packet size for a class, lmax, when a packet with a larger size
> than the current value of lmax arrives.
> 
> The drawback of the first point is that the maximum weight for a class
> is now limited to 4096, which is equal to 1/16 of the maximum weight
> sum.
> 
> Finally, this patch also forcibly caps the timestamps of a class if
> they are too high to be stored in the bucket list. This capping, taken
> from QFQ+ [3], handles the unfrequent case described in the comment to
> the function slot_insert.
> 
> [1] http://marc.info/?l=linux-netdev&m=134968777902077&w=2
> [2] http://marc.info/?l=linux-netdev&m=135096573507936&w=2
> [3] http://marc.info/?l=linux-netdev&m=134902691421670&w=2
> 
> Signed-off-by: Paolo Valente <paolo.valente@...more.it>
> Tested-by: Cong Wang <amwang@...hat.com>

Acked-by: Stephen Hemminger <shemminger@...tta.com>
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