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Re: [PATCH -tip tracing/kprobes 0/9] tracing/kprobes, perf: perf probe and kprobe-tracer bugfixes
- From: Masami Hiramatsu <mhiramat at redhat dot com>
- To: Ingo Molnar <mingo at elte dot hu>
- Cc: Frederic Weisbecker <fweisbec at gmail dot com>, Steven Rostedt <rostedt at goodmis dot org>, lkml <linux-kernel at vger dot kernel dot org>, Thomas Gleixner <tglx at linutronix dot de>, Arnaldo Carvalho de Melo <acme at redhat dot com>, Mike Galbraith <efault at gmx dot de>, Paul Mackerras <paulus at samba dot org>, Peter Zijlstra <a dot p dot zijlstra at chello dot nl>, Christoph Hellwig <hch at infradead dot org>, Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>, Jim Keniston <jkenisto at us dot ibm dot com>, "Frank Ch. Eigler" <fche at redhat dot com>, "H. Peter Anvin" <hpa at zytor dot com>, systemtap <systemtap at sources dot redhat dot com>, DLE <dle-develop at lists dot sourceforge dot net>
- Date: Sun, 18 Oct 2009 02:01:52 -0400
- Subject: Re: [PATCH -tip tracing/kprobes 0/9] tracing/kprobes, perf: perf probe and kprobe-tracer bugfixes
- References: <20091017000711.16556.69935.stgit@dhcp-100-2-132.bos.redhat.com> <20091017080203.GA4155@elte.hu> <20091017103427.GA31238@elte.hu>
Ingo Molnar wrote:
>
> I took a good look at the current bits, and there are a few more things
> that need to be fixed before we can consider 'perf probe' for upstream.
>
> Firstly, this decoder bug is still not fixed:
>
> CHK include/linux/compile.h
> TEST posttest
> Error: ffffffff81068fe0: 66 0f 73 fd 04 pslldq $0x4,%xmm5
> Error: objdump says 5 bytes, but insn_get_length() says 4 (attr:8000)
> make[1]: *** [posttest] Error 2
>
> 64-bit allyesconfig will trigger this for example, but the attached
> smaller config too. This needs to be fixed before we can apply any
> new commits.
Absolutely, yes. Thank you for reporting. I'm checking it again.
> Secondly, the probe syntax is quite non-obvious currently. All the 'p'
> and -P gymnastics is just a barrier to the first-time user getting his
> first probe inserted successfully.
Hmm...
> The user need not worry about whether it's a 'kprobe' or a 'kretprobe'.
> The user should _specify_ what it wants to probe, and _that_ will lead
> to 'perf probe' picking the most suitable facility to achieve that kind
> of probing.
>
> It might be a kprobe, a kretprobe, or an mcount driven function probe,
> an existing tracepoint if it happens to be present in that place already
> - or some other future mechanism. The driving force must be a robust
> specification of 'what', not the mechanics of 'how'.
Agreed.
> Considering that, the current 'perf probe' syntax does not achieve that
> goal yet.
>
> Here are a few syntax suggestions
>
> The simpest probe syntax should be to add a probe to a single function
> name:
>
> perf probe +schedule
>
> _nothing else_.
>
> To remove it, the user should just do something like:
>
> perf probe -schedule
>
> (to be symmetric 'perf probe +schedule' should work as well)
I think '-<symbol>' syntax doesn't work good with other command-line
options and multiple definitions.
(However, it will be good for input-from-file syntax. :-))
So, what would you think about using -D (def) and -U (undef) ?
> perf probe will make up a synthetic probe name for that - probe-1 for
> example. It will also pick the suitable probe mechanism (kprobes).
How about [perfprobe:symbol_offs] ?
> All the other extensions and possibilities - arguments, variables,
> source code lines, etc. should be natural and intuitive extensions of
> this basic, minimal syntax.
Don't you like current space(' ') separated arguments? :-)
I mean, what is 'natural' syntax in your opinion?
>
> To insert a simple probe no -P should be needed, 'p', no ':' - no probe
> name even.
Yeah, return-probe and event-name should be optional.
> Furthermore, there should be a way to list existing probes (and only
> probes), probably via 'perf list --probes' or 'perf probe --list'.
OK. I think perf probe --list will list up all probes including
user-defined ftrace-event via debugfs (not perf-probe), since
perf can use and delete it.
> Plus, 'perf probe --help' should list a few simple examples, beyond the
> syntax.
>
> Ok?
Sure.
Thank you!
--
Masami Hiramatsu
Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division
e-mail: mhiramat@redhat.com