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Re: [PATCH tracing/kprobes v2 1/5] tracing/kprobes: Rename special variables syntax
- From: Frederic Weisbecker <fweisbec at gmail dot com>
- To: Masami Hiramatsu <mhiramat at redhat dot com>
- Cc: Steven Rostedt <rostedt at goodmis dot org>, Ingo Molnar <mingo at elte dot hu>, lkml <linux-kernel at vger dot kernel dot org>, systemtap <systemtap at sources dot redhat dot com>, DLE <dle-develop at lists dot sourceforge dot net>, 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>
- Date: Mon, 5 Oct 2009 21:18:31 +0200
- Subject: Re: [PATCH tracing/kprobes v2 1/5] tracing/kprobes: Rename special variables syntax
- References: <20091002214834.30906.86502.stgit@dhcp-100-2-132.bos.redhat.com> <20091002214842.30906.49220.stgit@dhcp-100-2-132.bos.redhat.com> <20091003015444.GE4828@nowhere> <4AC830F0.2010003@redhat.com>
On Sun, Oct 04, 2009 at 01:21:52AM -0400, Masami Hiramatsu wrote:
> Hmm, # is widely used for comment, including some kernel pseudo
> file interfaces, kprobe_events too. Comments are useful if a
> probe list is restored from a file.
>
Right, let's think about something else.
> For accessing local variables, kprobe-tracer needs to support *at least*
> below variables:
> - Registers
> - Stack address (if a register points stack address, this isn't needed)
Ok.
Well, thinking more about the % sign, we shouldn't worry about
format confusion, since it's a commonly used character for registers,
we can take it for them: %rax, %rbx, etc. (is that what you did
in this patch? I don't remember exactly...)
And for addresses: @addr
> Below special vars are complementary aliases.
> - Function arguments
For the function arguments, I guess we don't need to worry
anymore about r0, r1, etc... but we can deal with the true var
name, without any kind of prefixes.
> - Return value
What about %return ?
As return values are usually stored in a register (at least in Arm
and x86, I don't know about the others), the % prefix fits well for
that.
> - Return address
What about @return :-) ?
That said we shouldn't worry about that in perf, since we can
take snapshots of the backtraces, like in some other perf events.
> and I'd like perf-probe to have a transparent syntax with kprobe-tracer.
That's feasible. But think about the fact that perf probe benefits
from a higher level of code view. Now that we have global and local
variables resolution, we can't anymore expect using r1, r2, rax,
a1, rv, ra without risking to collide with variable names.
But this tracer hasn't been merged yet, so it's still time
to update its interface.
> This means, if we can remove special vars except registers, or rename it
> non-conflictable name with registers, we just need to separate name spaces
> of
> - Regsiters
> - Local variables
Yeah.
> Here, local variables will support fields of data structs, and it will
> use '->' expression. Since '>' means redirection in bash, local variables
> need to be *escaped* in this case. Thus, I think we can use '$' prefix
> for it. (I'm OK, because this is similar syntax to systemtap:-).
>
> So, if you don't like %regs, $svars and locals, we can use regs and $locals :-)
'>' means redirection, but at least inside quotes it's not interpreted.
I'm fine with %regs actually. But I really don't like the "$" because
I really worry about shell substitutions.
Most people delimit their strings with double quotes.
What if we take the following:
[Ftrace and perf probe side]
%reg = registers, we can also play with deref and offsets like (%esp), 8(%esp), etc.
%return = return value
@return = return addr
arg(n) = arg number, where n is the number
[Perf probe only]
var = variable name, global or local, we can deal with shadow issues later
through variable scope: func_name:var, filename:var, whatever for now
it's not a problem. Local also includes argument names.
Hmm?