netdev - Re: [PATCH 1/10 REV5] [Doc] HOWTO Documentation for batching

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Date:	Fri, 14 Sep 2007 11:37:09 -0700
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	Krishna Kumar <krkumar2@...ibm.com>
Cc:	johnpol@....mipt.ru, herbert@...dor.apana.org.au, hadi@...erus.ca,
	kaber@...sh.net, shemminger@...ux-foundation.org,
	davem@...emloft.net, jagana@...ibm.com, Robert.Olsson@...a.slu.se,
	peter.p.waskiewicz.jr@...el.com, kumarkr@...ux.ibm.com,
	xma@...ibm.com, gaagaan@...il.com, netdev@...r.kernel.org,
	rdreier@...co.com, rick.jones2@...com, mcarlson@...adcom.com,
	jeff@...zik.org, general@...ts.openfabrics.org, mchan@...adcom.com,
	tgraf@...g.ch, sri@...ibm.com
Subject: Re: [PATCH 1/10 REV5] [Doc] HOWTO Documentation for batching

On Fri, 14 Sep 2007 14:31:18 +0530 Krishna Kumar wrote:

> Add Documentation describing batching skb xmit capability.
> 
> Signed-off-by: Krishna Kumar <krkumar2@...ibm.com>
> ---
>  batching_skb_xmit.txt |  107 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 107 insertions(+)
> 
> diff -ruNp org/Documentation/networking/batching_skb_xmit.txt new/Documentation/networking/batching_skb_xmit.txt
> --- org/Documentation/networking/batching_skb_xmit.txt	1970-01-01 05:30:00.000000000 +0530
> +++ new/Documentation/networking/batching_skb_xmit.txt	2007-09-14 10:25:36.000000000 +0530
> @@ -0,0 +1,107 @@
> +
> +Section 4: Nitty gritty details for driver writers
> +--------------------------------------------------
> +
> +	Batching is enabled from core networking stack only from softirq
> +	context (NET_TX_SOFTIRQ), and dev_queue_xmit() doesn't use batching.
> +
> +	This leads to the following situation:
> +		A skb was not sent out as either driver lock was contested or
> +		the device was blocked. When the softirq handler runs, it
> +		moves all skbs from the device queue to the batch list, but
> +		then it too could fail to send due to lock contention. The
> +		next xmit (of a single skb) called from dev_queue_xmit() will
> +		not use batching and try to xmit skb, while previous skbs are
> +		still present in the batch list. This results in the receiver
> +		getting out-of-order packets, and in case of TCP the sender
> +		would have unnecessary retransmissions.
> +
> +	To fix this problem, error cases where driver xmit gets called with a
> +	skb must code as follows:
> +		1. If driver xmit cannot get tx lock, return NETDEV_TX_LOCKED
> +		   as usual. This allows qdisc to requeue the skb.
> +		2. If driver xmit got the lock but failed to send the skb, it
> +		   should return NETDEV_TX_BUSY but before that it should have
> +		   queue'd the skb to the batch list. In this case, the qdisc

                   queued

> +		   does not requeue the skb.

and then
Acked-by: Randy Dunlap <randy.dunlap@...cle.com>

Thanks,
---
~Randy
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