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

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Date:	Tue, 02 Sep 2014 00:31:09 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	David Miller <davem@...emloft.net>
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,
	john.r.fastabend@...el.com
Subject: Re: [PATCH 0/2] Get rid of ndo_xmit_flush

On Mo, 2014-09-01 at 14:56 -0700, David Miller wrote:
> From: Hannes Frederic Sowa <hannes@...essinduktion.org>
> Date: Mon, 01 Sep 2014 22:05:42 +0200
> 
> > But as soon as we try to make Qdiscs absolutely lockless, we don't have
> > any guard that we don't concurrently dequeue skbs from it and suddenly
> > one Qdisc dequeue processing entity couldn't notify the driver that the
> > end of the batching was reached. I think this could become a problem
> > depending on how much of the locking is removed?
> 
> I am certain that batching will require taking the device transmit
> lock over the ->ndo_start_xmit() invocations, and therefore the
> deferral decisions must atomically be made inside of that context.

I'll stop to throw doubt into this discussion now. ;)

I am afraid that TX-lock is per nic-queue and it won't work out that
easy because skbs for different queues can reside in a Qdisc.

Bye,
Hannes


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