Re: Kernel marker has no performance impact on ia64.

[Masami Hiramatsu <mhiramat@redhat.com>]

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@redhat.com