On Mon, May 17, 2021 at 10:32:59AM +0200, Greg Kurz wrote: > On Wed, 12 May 2021 17:05:53 +0100 > Stefan Hajnoczi <stefa...@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. > > Anyway, I guess you can drop the patches from your tree.
Okay, I have dropped them. Stefan
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