On Thu, 25 Mar 2021 13:05:16 -0400
"Michael S. Tsirkin" <m...@redhat.com> wrote:
> 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?
This is a regression only if we consider the defaults. Manually setting
num_queues to a high value already affects existing devices.
> So I guess we'll need that in 6.0 or revert the # of vqs
> change for now ...
>
Not sure it is safe to merge these fixes this late... also,
as said above, I've only tested virtio-scsi and virtio-blk
but I believe the vhost-user-* variants might be affected too.
Reverting the # of vqs would really be a pity IMHO. What
about mitigating the effects ? e.g. enforce previous
behavior only if # vcpus > 64 ?
> > 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
> >
>
