[ovs-dev] [PATCH v4 0/5] XDP offload using flow API provider

William Tu u9012063 at gmail.com
Thu Feb 4 17:36:21 UTC 2021


Hi Toshiaki,

Thanks for the patch. I've been testing it for a couple days.
I liked it a lot! The compile and build process all work without any issues.

On Thu, Jul 30, 2020 at 7:55 PM Toshiaki Makita
<toshiaki.makita1 at gmail.com> wrote:
>
> This patch adds an XDP-based flow cache using the OVS netdev-offload
> flow API provider.  When an OVS device with XDP offload enabled,
> packets first are processed in the XDP flow cache (with parse, and
> table lookup implemented in eBPF) and if hits, the action processing
> are also done in the context of XDP, which has the minimum overhead.
>
> This provider is based on top of William's recently posted patch for
> custom XDP load.  When a custom XDP is loaded, the provider detects if
> the program supports classifier, and if supported it starts offloading
> flows to the XDP program.
>
> The patches are derived from xdp_flow[1], which is a mechanism similar to
> this but implemented in kernel.
>
>
> * Motivation
>
> While userspace datapath using netdev-afxdp or netdev-dpdk shows good
> performance, there are use cases where packets better to be processed in
> kernel, for example, TCP/IP connections, or container to container
> connections.  Current solution is to use tap device or af_packet with
> extra kernel-to/from-userspace overhead.  But with XDP, a better solution
> is to steer packets earlier in the XDP program, and decides to send to
> userspace datapath or stay in kernel.
>
> One problem with current netdev-afxdp is that it forwards all packets to
> userspace, The first patch from William (netdev-afxdp: Enable loading XDP
> program.) only provides the interface to load XDP program, howerver users
> usually don't know how to write their own XDP program.
>
> XDP also supports HW-offload so it may be possible to offload flows to
> HW through this provider in the future, although not currently.
> The reason is that map-in-map is required for our program to support
> classifier with subtables in XDP, but map-in-map is not offloadable.
> If map-in-map becomes offloadable, HW-offload of our program may also
> be possible.

I think it's too far away for XDP to be offloaded into HW and meet OVS's
feature requirements.
There is a research prototype here, FYI.
https://www.usenix.org/conference/osdi20/presentation/brunella

>
>
> * How to use
>
> 1. Install clang/llvm >= 9, libbpf >= 0.0.6 (included in kernel 5.5), and
>    kernel >= 5.3.
>
> 2. make with --enable-afxdp --enable-xdp-offload
> --enable-bpf will generate XDP program "bpf/flowtable_afxdp.o".  Note that

typo: I think you mean --enable-xdp-offload

> the BPF object will not be installed anywhere by "make install" at this point.
>
> 3. Load custom XDP program
> E.g.
> $ ovs-vsctl add-port ovsbr0 veth0 -- set int veth0 options:xdp-mode=native \
>   options:xdp-obj="/path/to/ovs/bpf/flowtable_afxdp.o"
> $ ovs-vsctl add-port ovsbr0 veth1 -- set int veth1 options:xdp-mode=native \
>   options:xdp-obj="/path/to/ovs/bpf/flowtable_afxdp.o"
>
> 4. Enable XDP_REDIRECT
> If you use veth devices, make sure to load some (possibly dummy) programs
> on the peers of veth devices. This patch set includes a program which
> does nothing but returns XDP_PASS. You can use it for the veth peer like
> this:
> $ ip link set veth1 xdpdrv object /path/to/ovs/bpf/xdp_noop.o section xdp

I'd suggest not using "veth1" as an example, because in (3) above, people
might think "veth1" is already attached to ovsbr0.
IIUC, here your "veth1" should be the device at the peer inside
another namespace.

>
> Some HW NIC drivers require as many queues as cores on its system. Tweak
> queues using "ethtool -L".
>
> 5. Enable hw-offload
> $ ovs-vsctl set Open_vSwitch . other_config:offload-driver=linux_xdp
> $ ovs-vsctl set Open_vSwitch . other_config:hw-offload=true
> This will starts offloading flows to the XDP program.
>
> You should be able to see some maps installed, including "debug_stats".
> $ bpftool map
>
> If packets are successfully redirected by the XDP program,
> debug_stats[2] will be counted.
> $ bpftool map dump id <ID of debug_stats>
>
> Currently only very limited keys and output actions are supported.
> For example NORMAL action entry and IP based matching work with current
> key support. VLAN actions used by port tag/trunks are also supported.
>

I don't know if this is too much to ask for.
I wonder if you, or we can work together, to add at least a tunnel
support, ex: vxlan?
The current version is a good prototype for people to test an L2/L3
XDP offload switch,
but without a good use case, it's hard to attract more people to
contribute or use it.

>From a maintenance point of view, can we add a test case to avoid regression?
For example, something like "make check-afxdp".
We can have "make check-xdp-offload". I can also help adding it.

>
> * Performance
>
> Tested 2 cases. 1) i40e to veth, 2) i40e to i40e.
> Test 1 Measured drop rate at veth interface with redirect action from
> physical interface (i40e 25G NIC, XXV 710) to veth. The CPU is Xeon
> Silver 4114 (2.20 GHz).
>                                                                XDP_DROP
>                     +------+                      +-------+    +-------+
>  pktgen -- wire --> | eth0 | -- NORMAL ACTION --> | veth0 |----| veth2 |
>                     +------+                      +-------+    +-------+
>
> Test 2 uses i40e instead of veth, and measured tx packet rate at output
> device.
>

Thanks for the performance results. I tested using two 25Gb ConnectX-6
cards, using mlx5. I got pretty similar performance results as yours.
But in general, I'm more worried about whether a feature can be implemented
in XDP than its performance.

> Single-flow performance test results:
>
> 1) i40e-veth
>
>   a) no-zerocopy in i40e
>
>     - xdp   3.7 Mpps
>     - afxdp 980 kpps
>
>   b) zerocopy in i40e (veth does not have zc)
>
>     - xdp   1.9 Mpps
>     - afxdp 980 Kpps
>
> 2) i40e-i40e
>
>   a) no-zerocopy
>
>     - xdp   3.5 Mpps
>     - afxdp 1.5 Mpps
>
>   b) zerocopy
>
>     - xdp   2.0 Mpps
>     - afxdp 4.4 Mpps
>
> ** xdp is better when zc is disabled. The reason of poor performance on zc
>    is that xdp_frame requires packet memory allocation and memcpy on
>    XDP_REDIRECT to other devices iff zc is enabled.
>
> ** afxdp with zc is better than xdp without zc, but afxdp is using 2 cores
>    in this case, one is pmd and the other is softirq. When pmd and softirq
>    were running on the same core, the performance was extremely poor as
>    pmd consumes cpus. I also tested afxdp-nonpmd to run softirq and
>    userspace processing on the same core, but the result was lower than
>    (pmd results) / 2.
>    With nonpmd, xdp performance was the same as xdp with pmd. This means
>    xdp only uses one core (for softirq only). Even with pmd, we need only
>    one pmd for xdp even when we want to use more cores for multi-flow.
>

Since XDP is evolving, it would be helpful to point out some future work and
current limitations. As far as I know
1) No broadcast/multicast support in XDP. Patch below:
https://lwn.net/Articles/817582/
2) No large packet support (ex: TSO)
3) No HW checksum offload
Will continue reviewing the following individual patches.
Regards,
William


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