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

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Message-ID: <1409145838.3173.41.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 27 Aug 2014 06:23:58 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	David Miller <davem@...emloft.net>, 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

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.

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.

Also, most drivers will never benefit from this new infra.

Mellanox mlx4 uses the Blue Frame trick to avoid the iowrite() cost.

I am afraid Jesper tests are not complete to truly have a picture of the
extra costs, because he made sure no false sharing was possible (A
single cpu does everything and keeps skb in its cache)

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