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

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Message-Id: <20140827.134510.2172564669938048576.davem@davemloft.net>
Date:	Wed, 27 Aug 2014 13:45:10 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	hannes@...essinduktion.org
Cc:	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: Hannes Frederic Sowa <hannes@...essinduktion.org>
Date: Wed, 27 Aug 2014 14:31:12 +0200

> On Mo, 2014-08-25 at 16:34 -0700, David Miller wrote:
>> Given Jesper's performance numbers, it's not the way to go.
>> 
>> Instead, go with a signalling scheme via new boolean skb->xmit_more.
>> 
>> This has several advantages:
>> 
>> 1) Nearly trivial driver support, just protect the tail pointer
>>    update with the skb->xmit_more check.
> 
> 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?
> 
> We can also add a new function pointer and call that instead of
> ndo_start_xmit. I think only the callq *%rax hurts performance.

I don't think we will have any problems here, the caller will always
be the entity which analyzes the upcoming set of SKBs to submit and
tag them properly.

I really do not want to add a new OP and I even more so do not want to
adjust the ndo_start_xmit() signature.  It's effect is far reaching,
and for absolutely no gain as far as I can see.

Thanks.
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