Re: [PATCH net-next 2/8] perf, bpf: allow bpf programs attach to tracepoints

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Apr 05, 2016 at 11:09:30AM -0700, Alexei Starovoitov wrote:
> >>@@ -67,6 +69,14 @@ perf_trace_##call(void *__data, proto)					\
> >>  									\
> >>  	{ assign; }							\
> >>  									\
> >>+	if (prog) {							\
> >>+		*(struct pt_regs **)entry = __regs;			\
> >>+		if (!trace_call_bpf(prog, entry) || hlist_empty(head)) { \
> >>+			perf_swevent_put_recursion_context(rctx);	\
> >>+			return;						\
> >>+		}							\
> >>+		memset(&entry->ent, 0, sizeof(entry->ent));		\
> >
> >But if not, you destroy it and then feed it to perf?
> 
> yes. If bpf prog returns 1 the buffer goes into normal ring-buffer
> with all perf_event attributes and so on.
> So far there wasn't a single real use case where we went this path.
> Programs always do aggregation inside and pass stuff to user space
> either via bpf maps or via bpf_perf_event_output() helper.
> I wanted to keep perf_trace_xx() calls to be minimal in .text size
> so memset above is one x86 instruction, but I don't mind
> replacing this memset with a call to a helper function that will do:
>    local_save_flags(flags);
>    tracing_generic_entry_update(entry, flags, preempt_count());
>    entry->type = type;
> Then whether bpf attached or not the ring buffer will see the same
> raw tracepoint entry. You think it's cleaner?

Yeah, otherwise you get very weird and surprising behaviour.

Also, one possible use-case is dynamic filters where the BPF program is
basically used to filter events, although I suppose we already have a
hook for that elsewhere.