On Thu, Mar 25, 2021 at 04:07:27PM +0100, Greg Kurz wrote:
> Now that virtio-scsi-pci and virtio-blk-pci map 1 virtqueue per vCPU,
> a serious slow down may be observed on setups with a big enough number
> of vCPUs.
>
> Exemple with a pseries guest on a bi-POWER9 socket system (128 HW threads):
>
> 1 0m20.922s 0m21.346s
> 2 0m21.230s 0m20.350s
> 4 0m21.761s 0m20.997s
> 8 0m22.770s 0m20.051s
> 16 0m22.038s 0m19.994s
> 32 0m22.928s 0m20.803s
> 64 0m26.583s 0m22.953s
> 128 0m41.273s 0m32.333s
> 256 2m4.727s 1m16.924s
> 384 6m5.563s 3m26.186s
>
> Both perf and gprof indicate that QEMU is hogging CPUs when setting up
> the ioeventfds:
>
> 67.88% swapper [kernel.kallsyms] [k] power_pmu_enable
> 9.47% qemu-kvm [kernel.kallsyms] [k] smp_call_function_single
> 8.64% qemu-kvm [kernel.kallsyms] [k] power_pmu_enable
> =>2.79% qemu-kvm qemu-kvm [.] memory_region_ioeventfd_before
> =>2.12% qemu-kvm qemu-kvm [.]
> address_space_update_ioeventfds
> 0.56% kworker/8:0-mm [kernel.kallsyms] [k] smp_call_function_single
>
> address_space_update_ioeventfds() is called when committing an MR
> transaction, i.e. for each ioeventfd with the current code base,
> and it internally loops on all ioventfds:
>
> static void address_space_update_ioeventfds(AddressSpace *as)
> {
> [...]
> FOR_EACH_FLAT_RANGE(fr, view) {
> for (i = 0; i < fr->mr->ioeventfd_nb; ++i) {
>
> This means that the setup of ioeventfds for these devices has
> quadratic time complexity.
>
> This series introduce generic APIs to allow batch creation and deletion
> of ioeventfds, and converts virtio-blk and virtio-scsi to use them. This
> greatly improves the numbers:
>
> 1 0m21.271s 0m22.076s
> 2 0m20.912s 0m19.716s
> 4 0m20.508s 0m19.310s
> 8 0m21.374s 0m20.273s
> 16 0m21.559s 0m21.374s
> 32 0m22.532s 0m21.271s
> 64 0m26.550s 0m22.007s
> 128 0m29.115s 0m27.446s
> 256 0m44.752s 0m41.004s
> 384 1m2.884s 0m58.023s
>
> The series deliberately spans over multiple subsystems for easier
> review and experimenting. It also does some preliminary fixes on
> the way. It is thus posted as an RFC for now, but if the general
> idea is acceptable, I guess a non-RFC could be posted and maybe
> extend the feature to some other devices that might suffer from
> similar scaling issues, e.g. vhost-scsi-pci, vhost-user-scsi-pci
> and vhost-user-blk-pci, even if I haven't checked.
>
> This should fix https://bugzilla.redhat.com/show_bug.cgi?id=1927108
> which reported the issue for virtio-scsi-pci.
Series looks ok from a quick look ...
this is a regression isn't it?
So I guess we'll need that in 6.0 or revert the # of vqs
change for now ...
> Greg Kurz (8):
> memory: Allow eventfd add/del without starting a transaction
> virtio: Introduce virtio_bus_set_host_notifiers()
> virtio: Add API to batch set host notifiers
> virtio-pci: Batch add/del ioeventfds in a single MR transaction
> virtio-blk: Fix rollback path in virtio_blk_data_plane_start()
> virtio-blk: Use virtio_bus_set_host_notifiers()
> virtio-scsi: Set host notifiers and callbacks separately
> virtio-scsi: Use virtio_bus_set_host_notifiers()
>
> hw/virtio/virtio-pci.h | 1 +
> include/exec/memory.h | 48 ++++++++++++++++------
> include/hw/virtio/virtio-bus.h | 7 ++++
> hw/block/dataplane/virtio-blk.c | 26 +++++-------
> hw/scsi/virtio-scsi-dataplane.c | 68 ++++++++++++++++++--------------
> hw/virtio/virtio-bus.c | 70 +++++++++++++++++++++++++++++++++
> hw/virtio/virtio-pci.c | 53 +++++++++++++++++--------
> softmmu/memory.c | 42 ++++++++++++--------
> 8 files changed, 225 insertions(+), 90 deletions(-)
>
> --
> 2.26.3
>
