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Message-ID: <48514BE3.3000506@redhat.com>
Date: Thu, 12 Jun 2008 12:16:35 -0400
From: Masami Hiramatsu <mhiramat@...hat.com>
To: "Frank Ch. Eigler" <fche@...hat.com>
CC: Peter Zijlstra <peterz@...radead.org>,
Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
Hideo AOKI <haoki@...hat.com>, mingo@...e.hu,
linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Kernel marker has no performance impact on ia64.
Hi Frank,
Frank Ch. Eigler wrote:
> Hi -
>
> On Thu, Jun 12, 2008 at 04:27:03PM +0200, Peter Zijlstra wrote:
>> [...]
>>> Well, the string contains each field name and type. Therefore, SystemTAP
>>> can hook on a marker and parse the string looking for some elements by
>>> passing a NULL format string upon probe registration. Alternatively, it
>>> can provide the exact format string expected when it registers its probe
>>> to the marker and a check will be done to verify that the format string
>>> passed along with the registered probe matches the marker format string.
>> Yes, I get that, its one of the ugliest things I've met in this whole
>> marker story. Why can't stap not insert a normal trace handler that
>> extracts the information from prev/next it wants? [...]
>
> Think this through. How should systemtap (or another user-space
> separate-compiled tool like lttng) do this exactly?
>
> (a) rely on debugging information? Even assuming we could get proper
> anchors (PC addresses or unambiguous type names) to start
> searching dwarf data, we lose a key attractions of markers: that
> it can robustly transfer data *without* dwarf data kept around.
>
> (b) rely on hand-written C code (prototypes, pointer derefrencing
> wrappers) distributed with systemtap? Not only would this be a
> brittle maintenance pain in the form of cude duplication, but then
> errors in it couldn't even be detected until the final C
> compilation stage. That would make a lousy user experience.
>
> (c) have systemtap try to parse the mhiramat-proposed "(struct
> task_struct * next, struct task_struct * prev)" format strings?
> Then we're asking systemtap to parse potentially general C type
> expressions, find the kernel headers that declare the types?
> Parse available subfields? That seems too much to ask for.
>
> (d) or another way?
use a lookup table. we can expect that the marking points which
regularly inserted in the upstream kernel are stable(not so
frequently change). In that case, we can easily maintain
a lookup table which has pairs of format strings like as
"sched_switch(struct task_struct * next, struct task_struct * prev)":"next %p prev %p"
out of tree. Thus, you can use the printf-style format parser.
This actually is a kind of duplication, but in this way,
I think we can detect errors before generating C code, and
easily add lookup pairs of format strings.
(additionally, we can choose %s or %p for "char *" ;-))
>
>
> - FChE
Thank you,
--
Masami Hiramatsu
Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division
e-mail: mhiramat@...hat.com
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