| Message ID | 20210325150735.1098387-1-groug@kaod.org (mailing list archive) |
|---|---|
| Headers | show |
| Series | virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci | expand |
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? So I guess we'll need that in 6.0 or revert the # of vqs change for now ... > 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 >
On Thu, Mar 25, 2021 at 01:05:16PM -0400, Michael S. Tsirkin 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? > So I guess we'll need that in 6.0 or revert the # of vqs > change for now ... Commit 9445e1e15e66c19e42bea942ba810db28052cd05 ("virtio-blk-pci: default num_queues to -smp N") was already released in QEMU 5.2.0. It is not a QEMU 6.0 regression. I'll review this series on Monday. Stefan
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 > > >
On Thu, 25 Mar 2021 17:43:10 +0000 Stefan Hajnoczi <stefanha@redhat.com> wrote: > On Thu, Mar 25, 2021 at 01:05:16PM -0400, Michael S. Tsirkin 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? > > So I guess we'll need that in 6.0 or revert the # of vqs > > change for now ... > > Commit 9445e1e15e66c19e42bea942ba810db28052cd05 ("virtio-blk-pci: > default num_queues to -smp N") was already released in QEMU 5.2.0. It is > not a QEMU 6.0 regression. > Oh you're right, I've just checked and QEMU 5.2.0 has the same problem. > I'll review this series on Monday. > Thanks ! > Stefan
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 Excellent numbers! I wonder if the code can be simplified since memory_region_transaction_begin/end() supports nesting. Why not call them directly from the device model instead of introducing callbacks in core virtio and virtio-pci code? Also, do you think there are other opportunities to have a long transaction to batch up machine init, device hotplug, etc? It's not clear to me when transactions must be ended. Clearly it's necessary to end the transaction if we need to do something that depends on the MemoryRegion, eventfd, etc being updated. But most of the time there is no immediate need to end the transaction and more code could share the same transaction before we go back to the event loop or vcpu thread. Stefan
On Mon, 29 Mar 2021 18:35:16 +0100 Stefan Hajnoczi <stefanha@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 > > Excellent numbers! > > I wonder if the code can be simplified since > memory_region_transaction_begin/end() supports nesting. Why not call > them directly from the device model instead of introducing callbacks in > core virtio and virtio-pci code? > It seems a bit awkward that the device model should assume a memory transaction is needed to setup host notifiers, which are ioeventfds under the hood but the device doesn't know that. > Also, do you think there are other opportunities to have a long > transaction to batch up machine init, device hotplug, etc? It's not > clear to me when transactions must be ended. Clearly it's necessary to The transaction *must* be ended before calling virtio_bus_cleanup_host_notifier() because address_space_add_del_ioeventfds(), called when finishing the transaction, needs the "to-be-closed" eventfds to be still open, otherwise the KVM_IOEVENTFD ioctl() might fail with EBADF. See this change in patch 3: @@ -315,6 +338,10 @@ static void virtio_bus_unset_and_cleanup_host_notifiers(VirtioBusState *bus, for (i = 0; i < nvqs; i++) { virtio_bus_set_host_notifier(bus, i + n_offset, false); + } + /* Let address_space_update_ioeventfds() run before closing ioeventfds */ + virtio_bus_set_host_notifier_commit(bus); + for (i = 0; i < nvqs; i++) { virtio_bus_cleanup_host_notifier(bus, i + n_offset); } } Maybe I should provide more details why we're doing that ? > end the transaction if we need to do something that depends on the > MemoryRegion, eventfd, etc being updated. But most of the time there is > no immediate need to end the transaction and more code could share the > same transaction before we go back to the event loop or vcpu thread. > I can't tell for all scenarios that involve memory transactions but it seems this is definitely not the case for ioeventfds : the rest of the code expects the transaction to be complete. > Stefan Thanks for the review ! Cheers, -- Greg
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(-)