example opensnoop doesn't run and I've fixed it

[lilydjwg]

First try with the exmple program, I get todo: oh no. It seems that bpf_get_next_key for the last CPU will fail so I fixed it.

Second try, I get SIGSEGV at the callback. The pointer passed to bpf_open_perf_buffer refers to an object on stack.

rust-bcc/src/perf.rs

Lines 117 to 122 in f06554a

	let mut callback = PerfCallback { raw_cb };
	let reader = unsafe {
	bpf_open_perf_buffer(
	Some(raw_callback),
	None,
	&mut callback as *mut _ as MutPointer,

I didn't figure out what PerfCallback does so I replaced it altogether with plain function pointers and it seems to work.

I didn't update other examples for now. If the changes seem good, I can update others.

[brayniac]

Hi @lilydjwg - thanks for your PR. I'm still taking a look, but I believe there are a couple issues with the changes in this PR.

[brayniac]

I think this will break the case where lower number cores are offlined.

[brayniac]

@lilydjwg - I've tested the opensnoop example and it works as it is in master. I'd prefer to work this through an issue to figure out what's going on before making code changes to try to address what you're seeing.

I'm not comfortable with the changes in this PR as they don't appear to be addressing a confirmed issue. I'm closing this PR without merging.