From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: network performance degradation in virtio_net in 4.12-rc Date: Mon, 22 May 2017 20:42:10 +0300 Message-ID: <20170522203532-mutt-send-email-mst@kernel.org> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jason Wang , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org To: Mikulas Patocka Return-path: Received: from mx1.redhat.com ([209.132.183.28]:3979 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758767AbdEVRmL (ORCPT ); Mon, 22 May 2017 13:42:11 -0400 Content-Disposition: inline In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: On Mon, May 22, 2017 at 10:25:19AM -0400, Mikulas Patocka wrote: > Hi > > I see severe network performance degradation with the kernels 4.12-rc1 and > 4.12-rc2 in the network virtio driver. Download rate drops down to about > 100kB/s. > > I bisected it and it is caused by patch > d85b758f72b05a774045545f24d70980e3e9aac4 ("virtio_net: fix support for > small rings"). When I revert this patch, the problem goes away. > > The host is Debian Jessie with kernel 4.4.62, the guest is Debian Sid with > kernel 4.12-rc. > > Mikulas Thanks for the report! I'm not sure what causes it on this host. I'll try to reproduce. Meanwhile, could you please locate this line of code: + vi->rq[i].min_buf_len = mergeable_min_buf_len(vi, vi->rq[i].vq); and add something like printk(KERN_ERR, "min buf = 0x%x expected 0x%x size 0x%x big %d\n", vi->rq[i].min_buf_len, GOOD_PACKET_LEN, virtqueue_get_vring_size(vi->rq[i].vq), (int)vi->big_packets); after it? Then boot and capture the output. Thanks! -- MST