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

From: Alexei Starovoitov <[email-masked]>
Date: Tue, 5 Apr 2016 11:21:49 -0700

On 4/5/16 11:16 AM, Peter Zijlstra wrote:
> 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.

ok. will respin.

> 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.

There is no other bpf hook for tracepoints. This patch is it.
And yes, after respin such use case will be possible.
Thanks!