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

[Greg Kurz <groug@kaod.org>]

On Thu, 25 Mar 2021 13:05:16 -0400
"Michael S. Tsirkin" <mst@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
> > 
>