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Re: [RFC 0/8] virtio: Improve boot time of virtio-scsi-pci and virtio-bl
From: |
Greg Kurz |
Subject: |
Re: [RFC 0/8] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci |
Date: |
Tue, 30 Mar 2021 15:15:27 +0200 |
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
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- [RFC 6/8] virtio-blk: Use virtio_bus_set_host_notifiers(), (continued)
- [RFC 6/8] virtio-blk: Use virtio_bus_set_host_notifiers(), Greg Kurz, 2021/03/25
- [RFC 7/8] virtio-scsi: Set host notifiers and callbacks separately, Greg Kurz, 2021/03/25
- [RFC 8/8] virtio-scsi: Use virtio_bus_set_host_notifiers(), Greg Kurz, 2021/03/25
- Re: [RFC 0/8] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci, Michael S. Tsirkin, 2021/03/25
- Re: [RFC 0/8] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci, Stefan Hajnoczi, 2021/03/29
- Re: [RFC 0/8] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci,
Greg Kurz <=