netdev - Re: [PATCH 2/4] virtio_net: return NETDEV_TX_BUSY instead of queueing an extra skb.

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Message-ID: <20090622073417.GA21698@gondor.apana.org.au>
Date:	Mon, 22 Jun 2009 15:34:17 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Krishna Kumar2 <krkumar2@...ibm.com>
Cc:	David Miller <davem@...emloft.net>,
	Matt Carlson <mcarlson@...adcom.com>, netdev@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 2/4] virtio_net: return NETDEV_TX_BUSY instead of
	queueing an extra skb.

On Mon, Jun 22, 2009 at 11:16:03AM +0530, Krishna Kumar2 wrote:
>
> I was curious about "queueing it in the driver" part: why is this bad? Do
> you
> anticipate any performance problems, or does it break QoS, or something
> else I
> have missed?

Queueing it in the driver is bad because it is no different than
queueing it at the upper layer, which is what will happen when
you return TX_BUSY.

Because we've ripped out the qdisc requeueing logic (which is
horribly complex and only existed because of TX_BUSY), it means
that higher priority packets cannot preempt a packet that is queued
in this way.

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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