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

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Date:	Wed, 11 Feb 2015 07:51:42 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH v3 linux-trace 1/8] tracing: attach eBPF programs to
 tracepoints and syscalls

On Tue, 10 Feb 2015 22:33:05 -0800
Alexei Starovoitov <ast@...mgrid.com> 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.

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.

This is something that needs to probably be discussed at bit.

-- Steve
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