Re: [PATCH v3 0/8] vdpa: Send all CVQ state load commands in parallel

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

On Wed, Jul 19, 2023 at 08:35:50PM +0800, Hawkins Jiawei wrote:
> 在 2023/7/19 17:11, Michael S. Tsirkin 写道:
> > On Wed, Jul 19, 2023 at 03:53:45PM +0800, Hawkins Jiawei wrote:
> >> This patchset allows QEMU to delay polling and checking the device
> >> used buffer until either the SVQ is full or control commands shadow
> >> buffers are full, instead of polling and checking immediately after
> >> sending each SVQ control command, so that QEMU can send all the SVQ
> >> control commands in parallel, which have better performance improvement.
> >>
> >> I use vp_vdpa device to simulate vdpa device, and create 4094 VLANS in
> >> guest to build a test environment for sending multiple CVQ state load
> >> commands. This patch series can improve latency from 10023 us to
> >> 8697 us for about 4099 CVQ state load commands, about 0.32 us per command.
> >
> > Looks like a tiny improvement.
> > At the same time we have O(n^2) behaviour with memory mappings.
> 
> Hi Michael,
> 
> Thanks for your review.
> 
> I wonder why you say "we have O(n^2) behaviour on memory mappings" here?

it's not specific to virtio - it's related to device init.
generally each device has some memory. during boot bios
enables each individually O(n) where n is # of devices.
memory maps has to be updated and in qemu this update
is at least superlinear with n (more like O(n log n) I think).
This gets up > O(n^2) with n number of devices.

>  From my understanding, QEMU maps two page-size buffers as control
> commands shadow buffers at device startup. These buffers then are used
> to cache SVQ control commands, where QEMU fills them with multiple SVQ control
> commands bytes, flushes them when SVQ descriptors are full or these
> control commands shadow buffers reach their capacity.
> 
> QEMU repeats this process until all CVQ state load commands have been
> sent in loading.
> 
> In this loading process, only control commands shadow buffers
> translation should be relative to memory mappings, which should be
> O(log n) behaviour to my understanding(Please correct me if I am wrong).
> 
> > Not saying we must not do this but I think it's worth
> > checking where the bottleneck is. My guess would be
> > vp_vdpa is not doing things in parallel. Want to try fixing that
> 
> As for "vp_vdpa is not doing things in parallel.", do you mean
> the vp_vdpa device cannot process QEMU's SVQ control commands
> in parallel?
> 
> In this situation, I will try to use real vdpa hardware to
> test the patch series performance.

yea, pls do that.

> > to see how far it can be pushed?
> 
> Currently, I am involved in the "Add virtio-net Control Virtqueue state
> restore support" project in Google Summer of Code now. Because I am
> uncertain about the time it will take to fix that problem in the vp_vdpa
> device, I prefer to complete the gsoc project first.
> 
> Thanks!
> 
> 
> >
> >
> >> Note that this patch should be based on
> >> patch "Vhost-vdpa Shadow Virtqueue VLAN support" at [1].
> >>
> >> [1]. https://lists.gnu.org/archive/html/qemu-devel/2023-07/msg03719.html
> >>
> >> TestStep
> >> ========
> >> 1. regression testing using vp-vdpa device
> >>    - For L0 guest, boot QEMU with two virtio-net-pci net device with
> >> `ctrl_vq`, `ctrl_rx`, `ctrl_rx_extra` features on, command line like:
> >>        -device virtio-net-pci,disable-legacy=on,disable-modern=off,
> >> iommu_platform=on,mq=on,ctrl_vq=on,guest_announce=off,
> >> indirect_desc=off,queue_reset=off,ctrl_rx=on,ctrl_rx_extra=on,...
> >>
> >>    - For L1 guest, apply the patch series and compile the source code,
> >> start QEMU with two vdpa device with svq mode on, enable the `ctrl_vq`,
> >> `ctrl_rx`, `ctrl_rx_extra` features on, command line like:
> >>        -netdev type=vhost-vdpa,x-svq=true,...
> >>        -device virtio-net-pci,mq=on,guest_announce=off,ctrl_vq=on,
> >> ctrl_rx=on,ctrl_rx_extra=on...
> >>
> >>    - For L2 source guest, run the following bash command:
> >> ```bash
> >> #!/bin/sh
> >>
> >> for idx1 in {0..9}
> >> do
> >>    for idx2 in {0..9}
> >>    do
> >>      for idx3 in {0..6}
> >>      do
> >>        ip link add macvlan$idx1$idx2$idx3 link eth0
> >> address 4a:30:10:19:$idx1$idx2:1$idx3 type macvlan mode bridge
> >>        ip link set macvlan$idx1$idx2$idx3 up
> >>      done
> >>    done
> >> done
> >> ```
> >>    - Execute the live migration in L2 source monitor
> >>
> >>    - Result
> >>      * with this series, QEMU should not trigger any error or warning.
> >>
> >>
> >>
> >> 2. perf using vp-vdpa device
> >>    - For L0 guest, boot QEMU with two virtio-net-pci net device with
> >> `ctrl_vq`, `ctrl_vlan` features on, command line like:
> >>        -device virtio-net-pci,disable-legacy=on,disable-modern=off,
> >> iommu_platform=on,mq=on,ctrl_vq=on,guest_announce=off,
> >> indirect_desc=off,queue_reset=off,ctrl_vlan=on,...
> >>
> >>    - For L1 guest, apply the patch series, then apply an addtional
> >> patch to record the load time in microseconds as following:
> >> ```diff
> >> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> >> index 6b958d6363..501b510fd2 100644
> >> --- a/hw/net/vhost_net.c
> >> +++ b/hw/net/vhost_net.c
> >> @@ -295,7 +295,10 @@ static int vhost_net_start_one(struct vhost_net *net,
> >>       }
> >>
> >>       if (net->nc->info->load) {
> >> +        int64_t start_us = g_get_monotonic_time();
> >>           r = net->nc->info->load(net->nc);
> >> +        error_report("vhost_vdpa_net_load() = %ld us",
> >> +                     g_get_monotonic_time() - start_us);
> >>           if (r < 0) {
> >>               goto fail;
> >>           }
> >> ```
> >>
> >>    - For L1 guest, compile the code, and start QEMU with two vdpa device
> >> with svq mode on, enable the `ctrl_vq`, `ctrl_vlan` features on,
> >> command line like:
> >>        -netdev type=vhost-vdpa,x-svq=true,...
> >>        -device virtio-net-pci,mq=on,guest_announce=off,ctrl_vq=on,
> >> ctrl_vlan=on...
> >>
> >>    - For L2 source guest, run the following bash command:
> >> ```bash
> >> #!/bin/sh
> >>
> >> for idx in {1..4094}
> >> do
> >>    ip link add link eth0 name vlan$idx type vlan id $idx
> >> done
> >> ```
> >>
> >>    - wait for some time, then execute the live migration in L2 source monitor
> >>
> >>    - Result
> >>      * with this series, QEMU should not trigger any warning
> >> or error except something like "vhost_vdpa_net_load() = 8697 us"
> >>      * without this series, QEMU should not trigger any warning
> >> or error except something like "vhost_vdpa_net_load() = 10023 us"
> >>
> >> ChangeLog
> >> =========
> >> v3:
> >>    - refactor vhost_svq_poll() to accept cmds_in_flight
> >> suggested by Jason and Eugenio
> >>    - refactor vhost_vdpa_net_cvq_add() to make control commands buffers
> >> is not tied to `s->cvq_cmd_out_buffer` and `s->status`, so we can reuse
> >> it suggested by Eugenio
> >>    - poll and check when SVQ is full or control commands shadow buffers is
> >> full
> >>
> >> v2: https://lore.kernel.org/all/cover.1683371965.git.yin31149@gmail.com/
> >>    - recover accidentally deleted rows
> >>    - remove extra newline
> >>    - refactor `need_poll_len` to `cmds_in_flight`
> >>    - return -EINVAL when vhost_svq_poll() return 0 or check
> >> on buffers written by device fails
> >>    - change the type of `in_cursor`, and refactor the
> >> code for updating cursor
> >>    - return directly when vhost_vdpa_net_load_{mac,mq}()
> >> returns a failure in vhost_vdpa_net_load()
> >>
> >> v1: https://lore.kernel.org/all/cover.1681732982.git.yin31149@gmail.com/
> >>
> >> Hawkins Jiawei (8):
> >>    vhost: Add argument to vhost_svq_poll()
> >>    vdpa: Use iovec for vhost_vdpa_net_cvq_add()
> >>    vhost: Expose vhost_svq_available_slots()
> >>    vdpa: Avoid using vhost_vdpa_net_load_*() outside
> >>      vhost_vdpa_net_load()
> >>    vdpa: Check device ack in vhost_vdpa_net_load_rx_mode()
> >>    vdpa: Move vhost_svq_poll() to the caller of vhost_vdpa_net_cvq_add()
> >>    vdpa: Add cursors to vhost_vdpa_net_loadx()
> >>    vdpa: Send cvq state load commands in parallel
> >>
> >>   hw/virtio/vhost-shadow-virtqueue.c |  38 ++--
> >>   hw/virtio/vhost-shadow-virtqueue.h |   3 +-
> >>   net/vhost-vdpa.c                   | 354 ++++++++++++++++++-----------
> >>   3 files changed, 249 insertions(+), 146 deletions(-)
> >>
> >> --
> >> 2.25.1
> >