From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: [PATCH] xen-netfront: transmit fully GSO-sized packets Date: Thu, 26 Mar 2015 05:05:37 -0700 Message-ID: <1427371537.25985.136.camel@edumazet-glaptop2.roam.corp.google.com> References: <1427368406-28084-1-git-send-email-jonathan.davies@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, xen-devel@lists.xenproject.org, Konrad Rzeszutek Wilk , Boris Ostrovsky , David Vrabel , Wei Liu To: Jonathan Davies Return-path: Received: from mail-ig0-f181.google.com ([209.85.213.181]:37259 "EHLO mail-ig0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752456AbbCZMFj (ORCPT ); Thu, 26 Mar 2015 08:05:39 -0400 Received: by igcxg11 with SMTP id xg11so51658776igc.0 for ; Thu, 26 Mar 2015 05:05:39 -0700 (PDT) In-Reply-To: <1427368406-28084-1-git-send-email-jonathan.davies@citrix.com> Sender: netdev-owner@vger.kernel.org List-ID: On Thu, 2015-03-26 at 11:13 +0000, Jonathan Davies wrote: > xen-netfront limits transmitted skbs to be at most 44 segments in size. However, > GSO permits up to 65536 bytes, which means a maximum of 45 segments of 1448 > bytes each. This slight reduction in the size of packets means a slight loss in > efficiency. > > Since c/s 9ecd1a75d, xen-netfront sets gso_max_size to > XEN_NETIF_MAX_TX_SIZE - MAX_TCP_HEADER, > where XEN_NETIF_MAX_TX_SIZE is 65535 bytes. > > The calculation used by tcp_tso_autosize (and also tcp_xmit_size_goal since c/s > 6c09fa09d) in determining when to split an skb into two is > sk->sk_gso_max_size - 1 - MAX_TCP_HEADER. > > So the maximum permitted size of an skb is calculated to be > (XEN_NETIF_MAX_TX_SIZE - MAX_TCP_HEADER) - 1 - MAX_TCP_HEADER. > > Intuitively, this looks like the wrong formula -- we don't need two TCP headers. > Instead, there is no need to deviate from the default gso_max_size of 65536 as > this already accommodates the size of the header. > > Currently, the largest skb transmitted by netfront is 63712 bytes (44 segments > of 1448 bytes each), as observed via tcpdump. This patch makes netfront send > skbs of up to 65160 bytes (45 segments of 1448 bytes each). > > Fixes: 9ecd1a75d977 ("xen-netfront: reduce gso_max_size to account for max TCP header") > Signed-off-by: Jonathan Davies > --- > drivers/net/xen-netfront.c | 2 -- > 1 file changed, 2 deletions(-) > > diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c > index e9b960f..fb6e978 100644 > --- a/drivers/net/xen-netfront.c > +++ b/drivers/net/xen-netfront.c > @@ -1279,8 +1279,6 @@ static struct net_device *xennet_create_dev(struct xenbus_device *dev) > netdev->ethtool_ops = &xennet_ethtool_ops; > SET_NETDEV_DEV(netdev, &dev->dev); > > - netif_set_gso_max_size(netdev, XEN_NETIF_MAX_TX_SIZE - MAX_TCP_HEADER); > - > np->netdev = netdev; > > netif_carrier_off(netdev); Hmm, this partially reverts commit 9ecd1a75d977e2e8c48139c7d3efed183f898d94 Why xennet_change_mtu() is not changed by your patch ?