Song Liu <liu.song....@gmail.com> writes: > On Wed, May 30, 2018 at 9:45 AM, Toke Høiland-Jørgensen <t...@toke.dk> wrote: >> This adds an example program showing how to sample packets from XDP using >> the perf event buffer. The example userspace program just prints the >> ethernet header for every packet sampled. >> >> Most of the userspace code is borrowed from other examples, most notably >> trace_output. >> >> Note that the example only works when everything runs on CPU0; so >> suitable smp_affinity needs to be set on the device. Some drivers seem >> to reset smp_affinity when loading an XDP program, so it may be >> necessary to change it after starting the example userspace program. > > Why does this only works when everything runs on CPU0? Is this > something we can improve?
Yeah, good question. Basically, the call from XDP to bpf_perf_event_output() will fail with -EOPNOTSUPP. I tracked this down to this if statement in __bpf_perf_event_output() in bpf_trace.c: > if (unlikely(event->oncpu != cpu)) > return -EOPNOTSUPP; I *think* that the way to fix this is for the userspace program to open a perf file descriptor for each CPU in the system and poll all of them, in which case the XDP program can pass the BPF_F_CURRENT_CPU flag to access the right one. I would love for someone more knowledgeable about perf internals to confirm this, though. And, well, the polling function in trace_helpers.c doesn't support currently this, and I didn't have the time to fix that while writing this example :) -Toke
