Public bug reported: SRU Justification
[Impact] Customers have degraded network performance on Hyper-V/Azure This is a request to pick up a patch from the upstream, the patch fixes an issue with Ubuntu as a hyper-v and Azure guest. This patch need to get picked up for 20.04, 18.04. The link to the upstream patch follows: https://git.kernel.org/pub/scm/linux/kernel/git/hyperv/linux.git/commit/?h=hyperv- fixes&id=7c9ff3deeee61b253715dcf968a6307af148c9b2 Description of issue and solution: The vmbus module uses a rotational algorithm to assign target CPUs to a device's channels. Depending on the timing of different device's channel offers, different channels of a device may be assigned to the same CPU. For example on a VM with 2 CPUs, if NIC A and B's channels are offered in the following order, NIC A will have both channels on CPU0, and NIC B will have both channels on CPU1 -- see below. This kind of assignment causes RSS load that is spreading across different channels to end up on the same CPU. Timing of channel offers: NIC A channel 0 NIC B channel 0 NIC A channel 1 NIC B channel 1 VMBUS ID 14: Class_ID = {f8615163-df3e-46c5-913f-f2d2f965ed0e} - Synthetic network adapter Device_ID = {cab064cd-1f31-47d5-a8b4-9d57e320cccd} Sysfs path: /sys/bus/vmbus/devices/cab064cd-1f31-47d5-a8b4-9d57e320cccd Rel_ID=14, target_cpu=0 Rel_ID=17, target_cpu=0 VMBUS ID 16: Class_ID = {f8615163-df3e-46c5-913f-f2d2f965ed0e} - Synthetic network adapter Device_ID = {244225ca-743e-4020-a17d-d7baa13d6cea} Sysfs path: /sys/bus/vmbus/devices/244225ca-743e-4020-a17d-d7baa13d6cea Rel_ID=16, target_cpu=1 Rel_ID=18, target_cpu=1 Update the vmbus CPU assignment algorithm to avoid duplicate CPU assignments within a device. The new algorithm iterates num_online_cpus + 1 times. The existing rotational algorithm to find "next NUMA & CPU" is still here. But if the resulting CPU is already used by the same device, it will try the next CPU. In the last iteration, it assigns the channel to the next available CPU like the existing algorithm. This is not normally expected, because during device probe, we limit the number of channels of a device to be <= number of online CPUs. [Test Plan] This could be tough to test as the patch fixes a race condition. [Where problems could occur] Network performance issues could persist. [Other Info] SF:#00315347 ** Affects: linux (Ubuntu) Importance: Undecided Status: New -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1937078 Title: Drivers: hv: vmbus: Fix duplicate CPU assignments within a device To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1937078/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
