Re: [RFC 0/8] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci

["Michael S. Tsirkin" <mst@redhat.com>]

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
>