From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rusty Russell Subject: Re: [PATCH RFC 2/3] virtio_net: fix tx capacity checks using new API Date: Thu, 02 Jun 2011 11:40:26 +0930 Message-ID: <87vcwphtkd.fsf__14783.9183606722$1306987127$gmane$org@rustcorp.com.au> References: <110045d5bf250e6ed9c739f8cc6adbbf54e3cc9c.1306921434.git.mst@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <110045d5bf250e6ed9c739f8cc6adbbf54e3cc9c.1306921434.git.mst@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: "Michael S. Tsirkin" Cc: Krishna Kumar , Carsten Otte , lguest@lists.ozlabs.org, Shirley Ma , kvm@vger.kernel.org, linux-s390@vger.kernel.org, netdev@vger.kernel.org, habanero@linux.vnet.ibm.com, Heiko Carstens , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, steved@us.ibm.com, Christian Borntraeger , Tom Lendacky , Martin Schwidefsky , linux390@de.ibm.com List-Id: virtualization@lists.linuxfoundation.org On Wed, 1 Jun 2011 12:49:54 +0300, "Michael S. Tsirkin" 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.