Comment 0 for bug 1847367

Nova version: 18.2.3
Release: Rocky
Compute node OS: CentOS 7.3
Compute node kernel: 3.10.0-327.13.1.el7.x86_64

In https://bugs.launchpad.net/nova/+bug/1570631 and commit https://review.opendev.org/#/c/332660/, a bug was fixed by making the assumption that the kernel version should also dictate the max number of queues on the tap interface when setting hw:vif_multiqueue_enabled=True. It was decided that 3.x kernels have a max queue count of 8. Unfortunately not all distributions follow this, and CentOS/RHEL has supported up to 256 queues since at least 7.2 even with a 3.x kernel.

The result of this is that a 20 core VM created in Mitaka will have 20 queues enabled (because the limit of 8 had not been added). The very same host after being upgraded to Rocky will instead only give 8 queues to the VM even though the kernel supports 256.

Could a workaround option be implemented to disable this check, or manually define the max queue count?

Snippet of drivers/net/tun.c from CentOS 7.2 kernel source code
/* MAX_TAP_QUEUES 256 is chosen to allow rx/tx queues to be equal
 * to max number of VCPUs in guest. */
#define MAX_TAP_QUEUES 256
#define MAX_TAP_FLOWS 4096

Snippet from the 3.10.0 kernel code from https://elixir.bootlin.com/linux/v3.10/source/drivers/net/tun.c:
/* DEFAULT_MAX_NUM_RSS_QUEUES were choosed to let the rx/tx queues allocated for
 * the netdevice to be fit in one page. So we can make sure the success of
 * memory allocation. TODO: increase the limit. */
#define MAX_TAP_QUEUES DEFAULT_MAX_NUM_RSS_QUEUES
#define MAX_TAP_FLOWS 4096

In the above example, DEFAULT_MAX_NUM_RSS_QUEUES is set to 8.