Re: [PATCH v3 linux-trace 1/8] tracing: attach eBPF programs to tracepoints and syscalls

From: Alexei Starovoitov
Date: Sat Feb 14 2015 - 17:54:49 EST

On Wed, Feb 11, 2015 at 7:51 AM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> On Tue, 10 Feb 2015 22:33:05 -0800
> Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote:
>> fair enough.
>> Something like TRACE_MARKER(arg1, arg2) that prints
>> it was hit without accessing the args would be enough.
>> Without any args it is indeed a 'fast kprobe' only.
>> Debug info would still be needed to access
>> function arguments.
>> On x64 function entry point and x64 abi make it easy
>> to access args, but i386 or kprobe in the middle
>> lose visibility when debug info is not available.
>> TRACE_MARKER (with few key args that function
>> is operating on) is enough to achieve roughly the same
>> as kprobe without debug info.
> Actually, what about a TRACE_EVENT_DEBUG(), that has a few args and
> possibly a full trace event layout.
> The difference would be that the trace events do not show up unless you
> have "trace_debug" on the command line. This should prevent
> applications from depending on them.
> I could even do the nasty dmesg output like I do with trace_printk()s,
> that would definitely keep a production kernel from adding it by
> default.
> When trace_debug is not there, the trace points could still be accessed
> but perhaps only via bpf, or act like a simple trace marker.

I think that is a great idea!
Makes it clear that all prints are for debugging and
no abi guarantees.

> Note, if you need ids, I rather have them in another directory than
> tracefs. Make a eventfs perhaps that holds these. I rather keep tracefs
> simple.

indeed. makes sense. no reason to burn fs memory just
to get an id from name. may be perf_event api can be
extended to lookup id from name. I think perf will benefit as well.
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