On Fri, 20 Dec 2019 09:25:43 +0100 Björn Töpel <bjorn.topel@xxxxxxxxx> wrote: > I've been doing some benchmarking with AF_XDP, and more specific the > bpf_xdp_redirect_map() helper and xdp_do_redirect(). One thing that > puzzles me is that the percpu-variable accesses stands out. > > I did a horrible hack that just accesses a regular global variable, > instead of the percpu struct bpf_redirect_info, and got a performance > boost from 22.7 Mpps to 23.8 Mpps with the rxdrop scenario from > xdpsock. Yes, this an 2 ns overhead, which is annoying in XDP context. (1/22.7-1/23.8)*1000 = 2 ns > Have anyone else seen this? Yes, I see it all the time... > So, my question to the uarch/percpu folks out there: Why are percpu > accesses (%gs segment register) more expensive than regular global > variables in this scenario. I'm also VERY interested in knowing the answer to above question!? (Adding LKML to reach more people) > One way around that is changing BPF_PROG_RUN, and BPF_CALL_x to pass a > context (struct bpf_redirect_info) explicitly, and access that instead > of doing percpu access. That would be a pretty churny patch, and > before doing that it would be nice to understand why percpu stands out > performance-wise. -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
