In migration resume phase, all unmasked msix vectors need to be
setup when loading the VF state. However, the setup operation would
take longer if the VM has more VFs and each VF has more unmasked
vectors.
The hot spot is kvm_irqchip_commit_routes, it'll scan and update
all irqfds that are already assigned each invocation, so more
vectors means need more time to process them.
vfio_pci_load_config
vfio_msix_enable
msix_set_vector_notifiers
for (vector = 0; vector < dev->msix_entries_nr; vector++) {
vfio_msix_vector_do_use
vfio_add_kvm_msi_virq
kvm_irqchip_commit_routes <-- expensive
}
We can reduce the cost by only committing once outside the loop.
The routes are cached in kvm_state, we commit them first and then
bind irqfd for each vector.
The test VM has 128 vcpus and 8 VF (each one has 65 vectors),
we measure the cost of the vfio_msix_enable for each VF, and
we can see 90+% costs can be reduce.
VF Count of irqfds[*] Original With this patch
1st 65 8 2
2nd 130 15 2
3rd 195 22 2
4th 260 24 3
5th 325 36 2
6th 390 44 3
7th 455 51 3
8th 520 58 4
Total 258ms 21ms
[*] Count of irqfds
How many irqfds that already assigned and need to process in this
round.
The optimization can be applied to msi type too.
Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>
Link: https://lore.kernel.org/r/20220326060226.1892-6-longpeng2@huawei.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
dc580d51f7