Messages in this thread | | | Subject | Re: [PATCH] Linux Kernel Markers 0.11 for 2.6.17 | From | (Frank Ch. Eigler) | Date | 27 Sep 2006 12:12:09 -0400 |
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Hi -
Mathieu Desnoyers <compudj@krystal.dyndns.org> writes:
> [...] > > That involves new conventions beyond printf. Why not "%p %p %u %u" > > for two blobs ... or why implicitly dereference the given pointers. A > > probe handler unaware of a specific marker's semantics would not know > > whether or not this is implied. > > [...] > So yes, there is a semantic to create, but I don't see the problem with that.
That's a part of my point. The marker data types marked up with printf directives do not fully describe the data - in this case whether it is a raw pointer or a data blob that is being marked.
> And why would the probe actually know what to do with a pointer ? If > it only wants to record the pointer's address or if it wants to > access data inside this pointer, it's up to the probe (or automatic > probe generator, hum ?) to do it.
Of course, but that precludes a general client tool, such as (say) a trace-only handler.
> > > I think that duplicating the number of marker macros could easily make > > > them unflexible and ugly. [...] > > > > Inflexible and ugly in what way? [...] > > I don't expect the kernel programmer community to accept that their code will > call an automatically generated macro. It removes all the idea of "I can see > what code is actually generated by my function", which I believe is necessary.
Not at all - the generated macros can sit in-tree and are easily inspected. Check out gen-stapmark.h and stapmark.h at <http://sources.redhat.com/cgi-bin/cvsweb.cgi/src/?cvsroot=systemtap>.
> Also, people are used to the simplicity and flexibility of printf > style format strings.
True, but is this in context of the existing tracing/probing facilities? Unless I'm mistaken, ltt functions doesn't use them; nor does blktrace. Other than printk, are there any?
> Do you really expect people to start using various macros like > MARK_u_p_llu
I don't know. Using gcc extensions such as __builtin_typeof() could automate the typing aspect, leaving only the arity as a programmer-visible text.
> and start defining their own marker macro each time they want to add > a specific type ?
Well, adding a new type would be at last as hard in the printf case.
> [...] However, if you want to create probes that are type-safe, you > can then create a script that will extract all the format strings in > the markers section of the object and automatically generate all the > probes with their respective va_args setup at the beginning of the > probe. [...]
This could work. OTOH this relies on an as-yet-unwritten script, and additional run-time costs (the parameter-by-parameter va_arg copying).
I wonder if writing a functional back-end for these markers should be considered a corequisite for this work; or in the alternative, whether it's good enough to start putting markers into the code, and revamp their implementation later if necessary.
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