Date: Wed, 11 Feb 2015 07:51:42 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: Alexei Starovoitov <ast@plumgrid.com>
Cc: Ingo Molnar <mingo@kernel.org>, Namhyung Kim <namhyung@kernel.org>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Jiri Olsa <jolsa@redhat.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Linux API <linux-api@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Subject: Re: [PATCH v3 linux-trace 1/8] tracing: attach eBPF programs to
 tracepoints and syscalls
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On Tue, 10 Feb 2015 22:33:05 -0800
Alexei Starovoitov <ast@plumgrid.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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