linux-kernel - Re: [PATCH -tip 10/10] perf probe: Accessing members in data structures

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Date:	Tue, 23 Mar 2010 16:55:13 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	Mark Wielaard <mjw@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH -tip 10/10] perf probe: Accessing members in data
 structures

On Thu, 2010-03-18 at 04:28 +0100, Frederic Weisbecker wrote:
> On Wed, Mar 17, 2010 at 03:14:43PM -0400, Masami Hiramatsu wrote:
> > Mark Wielaard wrote:
> > > On Tue, 2010-03-16 at 18:06 -0400, Masami Hiramatsu wrote:
> > >> Support accessing members in the data structures. With this,
> > >> perf-probe accepts data-structure members(IOW, it now accepts
> > >> dot '.' and arrow '->' operators) as probe arguemnts.
> > >>
> > >> e.g.
> > >>
> > >>  ./perf probe --add 'schedule:44 rq->curr'
> > >>
> > >>  ./perf probe --add 'vfs_read file->f_op->read file->f_path.dentry'
> > >>
> > >> Note that '>' can be interpreted as redirection in command-line.
> > > 
> > > If you find that a problem then you can do like SystemTap does and allow
> > > '.' in place of '->'. In the code you already use the
> > > perf_probe_arg_field ref flag only to check that the DIE gives you the
> > > same information. So you could just drop that and use any separator.
> > > Then you decide based on whether you see a DW_TAG_pointer_type. This
> > > gives the user some extra flexibility by letting them not having to care
> > > about specifying extra type information already available elsewhere.
> > 
> > Thanks, when designing this feature, I considered it too.
> > 
> > Since perf probe already support displaying source code by --line option,
> > users will read the probed code itself and try to probe it. In that case,
> > I think they naturally use '.' and '->' as they read (they might try to
> > copy & paste it).
> > 
> > So, I think that it would be good to support both of '.' and '->' as
> > they are used in the code, because it will not confuse users.
> > 
> > Thank you,
> 
> 
> Agreed.
> 
> And lets people use what is common for them: expressions that follow
> C rules in the context.
> 
> And those who will be more familiar with perf probe will know they can
> use the simplified "." based scheme.

I'd expect a syntax error when I mix up '.' and '->'.
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