On Mon, 21 Aug 2017 15:35:42 -0700 Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > On Mon, Aug 21, 2017 at 09:25:06PM +0200, Jesper Dangaard Brouer wrote: > > > > Third gotcha(3): You got this far, loaded xdp on both interfaces, and > > notice now that (with default setup) you can RX with 14Mpps but only > > TX with 6.9Mpps (and might have 5% idle cycles). I debugged this via > > perf tracepoint event xdp:xdp_redirect, and found this was due to > > overrunning the xdp TX ring-queue size. > > we should probably fix this somehow. Gotcha-3 (quoted above) is an interesting problem. At first it looks like a driver tuning problem. But it is actually an inherent property of XDP, as there is no-queue or push-back flow control with XDP, there is no way to handle TX queue overrun. My proposed solution: I want to provide a facility for userspace to load another eBPF program (at the tracepoint xdp_redirect), which can "see" the issue occurring. This allows a XDP/BPF developer to implement their own reaction/mitigation flow-control (e.g. via a map shared with the XDP program). > Once tx-ing netdev added to devmap we can enable xdp on it automatically? I think you are referring to Gotcha-2 here: Second gotcha(2): you cannot TX out a device, unless it also have a xdp bpf program attached. (This is an implicit dependency, as the driver code need to setup XDP resources before it can ndo_xdp_xmit). Yes, we should work on improving this situation. Auto enabling XDP when a netdev is added to a devmap is a good solution. Currently this is tied to loading an XDP bpf_prog. Do you propose loading a dummy bpf_prog on the netdev? (then we need to handle 1. not replacing existing bpf_prog, 2. on take-down don't remove "later" loaded bpf_prog). -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
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