netdev - Re: [PATCH RFC 2/3] virtio_net: fix tx capacity checks using new API

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Date:	Thu, 02 Jun 2011 11:40:26 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc:	Carsten Otte <cotte@...ibm.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	linux390@...ibm.com, Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Shirley Ma <xma@...ibm.com>, lguest@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-s390@...r.kernel.org, kvm@...r.kernel.org,
	Krishna Kumar <krkumar2@...ibm.com>,
	Tom Lendacky <tahm@...ux.vnet.ibm.com>, steved@...ibm.com,
	habanero@...ux.vnet.ibm.com
Subject: Re: [PATCH RFC 2/3] virtio_net: fix tx capacity checks using new API

On Wed, 1 Jun 2011 12:49:54 +0300, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> In the (rare) case where new descriptors are used
> while virtio_net enables vq callback for the TX vq,
> virtio_net uses the number of sg entries in the skb it frees to
> calculate how many descriptors in the ring have just been made
> available.  But this value is an overestimate: with indirect buffers
> each skb only uses one descriptor entry, meaning we may wake the queue
> only to find we still can't transmit anything.

This is a bit misleading.

The value is an overestimate, but so is the requirement for
2+MAX_SKB_FRAGS, *unless* we suddenly drop into direct mode due to OOM.

Thanks,
Rusty.
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