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

[Greg Kurz <groug@kaod.org>]

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.

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