Re: documentation for user-space usage?

[Grant Edwards <grant dot b dot edwards at gmail dot com>]

On 2010-11-01, Frank Ch. Eigler <fche@redhat.com> wrote:
>
> grant.b.edwards wrote:
>
>> [...]
>> Assuming utrace/uprobes gets ported to ARM, does such a trace events
>> involve a context switch to kernel-mode and then back to user-mode?
>
> Yes.

That might be a concern if it affects timings too much.  If we roll
our own, the plan would be to keep it purely user-space in order to
avoid the impact of context switching when logging is enabled.

>> [...]  I'm using ARM9.  Utrace support patches have apparently been
>> rejected by the kernel maintainers, so I'd hae to maintain my own
>> fork of the kernel as well as port uprobes.  I think.
>
> How close is your current kernel to fedora's?

I've no idea.  I'm currently using vanilla 2.6.30 sources with some
AT91-specific patches from Atmel.

>>> [...] User-space probing and kernel-space probing are basically
>>> identical from the point of view of the stap user.
>>
>> Is the timing impact of a user-space probe no different than that of
>> a kernel-space probe?
>
> It's very similar.
>
>>> [...]
>>> The theoretical fit is pretty good.  If in practice you are blocked
>>> by some missing unported layer, you could decide between helping
>>> port, prototyping on x86 while someone else ports, and/or
>>> investigating other logging-related tools/libraries.
>>
>> I wouldn't mind working on porting uprobes if I was confident that it
>> would be accepted upstream.  Since utrace support was apparently not
>> accepted, I'm not too optimistic.
>
> Another option is to go ahead and try to port uprobes, leave
> ARM/utrace to us / fedora people.  When/if the newer lkml-track
> uprobes gets merged, the hypothetical ARM port could go into the main
> kernel that way, bypassing the utrace kerfuffle.  IOW, doing an ARM
> port of the current systemtap-resident uprobes would not be a wasted
> effort, if LKML gets its act together and merges the other one.

OK, thanks.  Can anybody provide a guess as to how much porting needs
to done (assuming a competent kernel-mode programmer who knows nothing
about the tracing stuff)?

-- 
Grant Edwards               grant.b.edwards        Yow! Spreading peanut
                                  at               butter reminds me of
                              gmail.com            opera!!  I wonder why?