netdev - Re: [PATCH 0/2] Get rid of ndo_xmit

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Message-Id: <20140827.134650.1202884223919773168.davem@davemloft.net>
Date:	Wed, 27 Aug 2014 13:46:50 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	hannes@...essinduktion.org, netdev@...r.kernel.org,
	therbert@...gle.com, jhs@...atatu.com, edumazet@...gle.com,
	jeffrey.t.kirsher@...el.com, rusty@...tcorp.com.au,
	dborkman@...hat.com, brouer@...hat.com
Subject: Re: [PATCH 0/2] Get rid of ndo_xmit_flush

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 27 Aug 2014 06:23:58 -0700

> On Wed, 2014-08-27 at 14:31 +0200, Hannes Frederic Sowa wrote:
> 
>> One thing one should keep in mind is, that there must be a skb available
>> to trigger the flush, maybe this will hurt us one day.
>> 
>> Thinking more about it should we go with a coccinelle script and
>> replace/extend ndo_start_xmit with an additional argument?
> 
> This will be a pain for backports and things like that.

Agreed.

> skb->xmit_more is a bit annoying, because it consumes one bit in all
> skbs, while it could be one byte per cpu as ndo_start_xmit() is called
> while BH are disabled.
> 
> It also forces a cache line dirtying, that will hurt qdisc like HTB
> where skbs can be cooked/enqueued by remote cpus.

The xmit_more value sits in the same cache line as the queue number,
which also gets dirtied in the TX fast path.
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