On Wed, 26 May 2021 08:35:49 -0700 John Fastabend <john.fastabend@xxxxxxxxx> wrote: > I'll still have a basic question though. I've never invested much time > into the hints because its still not clear to me what the use case is? > What would we put in the hints and do we have any data to show it would be > a performance win. I've documented and measured[1] the performance overhead of the missing checksum for UDP packets when XDP-redirecting into veth (that does XDP_PASS). Full delivery into a socket we can save 8% (54.28 ns / +109Kpps). Lorenzo is working patches outside XDP-hints for this, but it only handle CHECKSUM_UNNECESSARY, and if we need CHECKSUM_COMPLETE then we also need XDP-hints/metadata (for storing skb->csum). [1] https://github.com/xdp-project/xdp-project/blob/master/areas/core/xdp_frame01_checksum.org > If its a simple hash of the headers then how would we use it? Even with a simple/smaller hash you can tune the RSS-hash on parts of the packet you like, see[2]. That would be valuable for doing lookups in BPF-maps. [2] https://github.com/stackpath/rxtxcpu/tree/master/Documentation/case-studies For cpumap redirect I would like to spread packets with this RX-hash, as I can avoid parsing packets headers on RX-CPU. The mlx5 NIC support 64-bit unique flow hash, that you could use as a lookup key in your (Cilium) conntrack table, or a container/sockmap redirect-tracking table. > The map_lookup/updates use IP addrs for keys in Cilium. So I think the > suggestion is to offload the jhash operation? But that requires some > program changes to work. Could someone convince me? Is my explanations enough, or are you still not convinced? > Maybe packet timestamp? I have a concrete use-case that needs packet timestamps for AF_XDP. It is the control system inside a wind-turbine that use a time-triggered Real-Time protocol. I actually both need hardware RX-timestamps and TX-timestamps. A lot it lacking on the TX-side to allow AF_XDP to send down a transmission timestamp, but I have a real-use-case that needs this (before end-of year). I have hardware i210 and i225 chips in my testlab so I can get this working. Thanks you John for engaging and challenging us in our design of XDP-hints, I truly appreciate your feedback! :-) -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
