Date: Tue, 17 Nov 2020 08:20:03 +0000 From: bugzilla-noreply@freebsd.org To: bugs@FreeBSD.org Subject: [Bug 251208] bridge(4) combines multiple packets together when traffic load is high Message-ID: <bug-251208-227@https.bugs.freebsd.org/bugzilla/>
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D251208 Bug ID: 251208 Summary: bridge(4) combines multiple packets together when traffic load is high Product: Base System Version: 12.1-RELEASE Hardware: amd64 OS: Any Status: New Severity: Affects Many People Priority: --- Component: kern Assignee: bugs@FreeBSD.org Reporter: buhrow@nfbcal.org When running a bridge between em(4) and Xen PV virtual hosts, the virtual hosts see a lot of oversized packets when inbound traffic is heavy from the physical interface (em0) to the virtual interface of the bridge (xnb0.domid) Here are some of the messages from the Xen VM that illustrate the problem: Nov 16 23:53:45 test-new /netbsd: xennet0: discarding oversize frame (len=3D 1518) this happens over and over again, with two different frame sizes: 1518 and 1586. The MTU on all interfaces is 1500. Output of ifconfig bridge0 is as follows: bridge0: flags=3D8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu = 1500 ether 02:05:35:15:6f:00 id 00:00:00:00:00:00 priority 32768 hellotime 2 fwddelay 15 maxage 20 holdcnt 6 proto rstp maxaddr 2000 timeout 1200 root id 00:00:00:00:00:00 priority 32768 ifcost 0 port 0 member: xnb34.0 flags=3D143<LEARNING,DISCOVER,AUTOEDGE,AUTOPTP> ifmaxaddr 0 port 6 priority 128 path cost 2000000 member: xnb33.0 flags=3D143<LEARNING,DISCOVER,AUTOEDGE,AUTOPTP> ifmaxaddr 0 port 5 priority 128 path cost 2000000 member: em0 flags=3D143<LEARNING,DISCOVER,AUTOEDGE,AUTOPTP> ifmaxaddr 0 port 1 priority 128 path cost 2000000 groups: bridge nd6 options=3D9<PERFORMNUD,IFDISABLED> All the packets the vm is complaining about originate on a host on = the other side of a Cisco switch which reports no giant packets on any of its interfaces. In addition, a tcpdump session on the bridge itself shows no giant packets coming into it from any of its members. What I think is happening is that packets are coming in at such a rate, approximately 7500 pps, that the output side of the bridge is grabbing 2 packets when it processes the output queue and sendning them out as 1. Additional notes: This doesn't seem to affect hvm guests with Xen. I don't know if that's because the emulated ethernet chips in Qemu silently discard the oversized frames or if it's because the output routines used from the bridge(4) driver are different. I'd guess the former, but I'm not familiar enough with the bridge(4) code in FreeBSD to say for sure. to reproduce, set up a Xen PV guest on a bridge paired with an em(4) interface and send it as much traffic as you can. I'm driving the inbound interface at about 100 mbits/sec, the maximum available on this particular switch port. Also, in case it isn't obvious, the FreeBSD installation in question, is runing as the dom0 for this Xen installation. It's also running xen-4.14.0, with the same version of the xentools instaled. -thanks -Brian --=20 You are receiving this mail because: You are the assignee for the bug.=
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