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

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Message-ID: <20090608052247.GA19826@gondor.apana.org.au>
Date:	Mon, 8 Jun 2009 15:22:47 +1000
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH 2/4] virtio_net: return NETDEV_TX_BUSY instead of
	queueing an extra skb.

On Wed, Jun 03, 2009 at 12:47:04PM +0930, Rusty Russell wrote:
> 
> We could figure out if we can take the worst-case packet, and underutilize
> our queue.  And fix the other *67* drivers.

Most of those are for debugging purposes, i.e., they'll never
happen unless the driver is buggy.

> Of course that doesn't even work, because we return NETDEV_TX_BUSY from dev.c!

If and when your driver becomes part of the core and it has to
feed into other drivers, then you can use this argument :)

> "Hi, core netdevs here.  Don't use NETDEV_TX_BUSY.   Yeah, we can't figure out
> how to avoid it either.  But y'know, just hack something together".

No you've misunderstood my complaint.  I'm not trying to get you
to replace NETDEV_TX_BUSY by the equally abhorrent queue in the
driver, I'm saying that you should stop the queue before you get
a packet that overflows by looking at the amount of free queue
space after transmitting each packet.

For most drivers this is easy to do.  What's so different about
virtio-net that makes this impossible?

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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