Re: user-space probes -- plan B from outer space

["Frank Ch. Eigler" <fche@redhat.com>]

Hi -

varap wrote:
> [...]
> >A fixed pool of predefined handlers seem like an antithesis of
> >systemtap.  Did you have some user interface in mind for these?
>
> Like i mentioned in response to Prasanna's user space probes approach  
> mail there will be two types of handlers for user space probes [...]

I understand that this is what you're proposing.

> SystemTap will use the second approach.

OK, so no user interface required for the first.  But of course if
you're building the first approach also, you must have some tool in
mind to at least demonstrate it.


> [...] you could use the preexisting handlers and don't need to write
> kernel module and possibly don't even need root permission to trace.
> [...]

With some cleverness, we can keep separated the issues of these
predefined handlers (and the hypothetical non-systemtap tool that
might use them) and unprivileged probing.


> >>[...] I am not sure i see the value of process("process name")
> >>syntax if our focus is process specific tracing.
>
> >It would be one way of identifying present or future processes to
> >probe.  For processes that do not yet exist, what other scheme do you
> >have in mind?
>
> The scheme i am thinking is you could start a new process [...]

But you were concerned about the process("name") *syntax*.  Sure,
implementing any of these various probes may involve forked observer
processes.  But if you don't have that *syntax*, how else do you want
a script to target a particular process (other than by pid)?


> >>I am not sure i see lot of value of this solution compared to a gdb
> >>batch job, but for bit better performance than the heavy weight gdb.
> >>[...]
> >
> >How would this gdb batch job alternative work?  Are you intending to
> >compare the expressity of systemtap script with gdb macros?
> 
> I am not saying gdb macros are as powerful as systemtap scripts. All i 
> meant is one can use gdb batch scripts to print the variables you need 
> and use all kinds of post processing using your favorite scripting 
> language. As the handlers are run in the userspace i am not seeing much 
> advantage of using systemtap language to do filtering in the userspace 
> vs post processing using your favorite scripting language.

It has the same kind of advantage in user space as it does in kernel
space: the option of safely and compactly
filtering/analyzing/acting-on data in situ rather than archiving
masses of trace data and passively analyzing it after the fact.

- FChE