Re: [PATCH v2 0/4] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci

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

On Mon, May 17, 2021 at 10:32:59AM +0200, Greg Kurz wrote:
> On Wed, 12 May 2021 17:05:53 +0100
> Stefan Hajnoczi <stefanha@redhat.com> wrote:
> 
> > On Fri, May 07, 2021 at 06:59:01PM +0200, 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):
> > > 
> > >               virtio-scsi      virtio-blk
> > > 
> > > 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 simply changes the device models to extend the transaction
> > > to all virtqueueues, like already done in the past in the generic
> > > code with 710fccf80d78 ("virtio: improve virtio devices initialization
> > > time").
> > > 
> > > Only virtio-scsi and virtio-blk are covered here, but a similar change
> > > might also be beneficial to other device types such as host-scsi-pci,
> > > vhost-user-scsi-pci and vhost-user-blk-pci.
> > > 
> > >               virtio-scsi      virtio-blk
> > > 
> > > 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
> > > 
> > > This should fix https://bugzilla.redhat.com/show_bug.cgi?id=1927108
> > > which reported the issue for virtio-scsi-pci.
> > > 
> > > Changes since v1:
> > > - Add some comments (Stefan)
> > > - Drop optimization on the error path in patch 2 (Stefan)
> > > 
> > > Changes since RFC:
> > > 
> > > As suggested by Stefan, splimplify the code by directly beginning and
> > > committing the memory transaction from the device model, without all
> > > the virtio specific proxying code and no changes needed in the memory
> > > subsystem.
> > > 
> > > Greg Kurz (4):
> > >   virtio-blk: Fix rollback path in virtio_blk_data_plane_start()
> > >   virtio-blk: Configure all host notifiers in a single MR transaction
> > >   virtio-scsi: Set host notifiers and callbacks separately
> > >   virtio-scsi: Configure all host notifiers in a single MR transaction
> > > 
> > >  hw/block/dataplane/virtio-blk.c | 45 ++++++++++++++++++++-
> > >  hw/scsi/virtio-scsi-dataplane.c | 72 ++++++++++++++++++++++++---------
> > >  2 files changed, 97 insertions(+), 20 deletions(-)
> > > 
> > > -- 
> > > 2.26.3
> > > 
> > 
> > Thanks, applied to my block tree:
> > https://gitlab.com/stefanha/qemu/commits/block
> > 
> 
> Hi Stefan,
> 
> It seems that Michael already merged the previous version of this
> patch set with its latest PR.
> 
> https://gitlab.com/qemu-project/qemu/-/commit/6005ee07c380cbde44292f5f6c96e7daa70f4f7d
> 
> It is thus missing the v1->v2 changes. Basically some comments to
> clarify the optimization we're doing with the MR transaction and
> the removal of the optimization on an error path.
> 
> The optimization on the error path isn't needed indeed but it
> doesn't hurt. No need to change that now that the patches are
> upstream.
> 
> I can post a follow-up patch to add the missing comments though.
> While here, I'd even add these comments in the generic
> virtio_device_*_ioeventfd_impl() calls as well, since they already
> have the very same optimization.

Yes, please post patches on top.

> Anyway, I guess you can drop the patches from your tree.
> 
> Cheers,
> 
> --
> Greg
> 
> > Stefan
>