Re: [PATCH] samples/bpf: Add xdp_sample_pkts example

["Toke Høiland-Jørgensen" <toke@toke.dk>]

Song Liu <liu.song.a23@gmail.com> writes:

> On Wed, May 30, 2018 at 9:45 AM, Toke Høiland-Jørgensen <toke@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