Re: [rfd] function-graph augmentation

[Ingo Molnar <mingo@elte.hu>]

* Arnaldo Carvalho de Melo <acme@redhat.com> wrote:

> Em Fri, Feb 20, 2009 at 10:30:11AM -0300, Arnaldo Carvalho de Melo escreveu:
> > Em Fri, Feb 20, 2009 at 09:56:27AM +0100, Ingo Molnar escreveu:
> > > 2)
> > > 
> > > Another, entirely different, and i think complementary approach, 
> > > which exciting new possibilities would be to (also) 
> > > automatically pick up arguments from the stack, on function 
> > > entry.
> > > 
> > > If there's a (read-mostly, lockless-to-read and scalable) 
> > > function attributes hash, then we could encode the parameters 
> > > signature of functions (or at least, of certain functions) in 
> > > the attributes hash. Then the tracer will know how many 
> > > arguments to pick up from the stack.
> > > 
> > > This approach has the advantage that we could reconstruct the 
> > > parameters of _arbitrary_ functions, without having to touch 
> > > those functions. We already enumerate all functions during build 
> > > time, it would take some more dwarf2 magic to recover the 
> > > call/parameter signature. Oh, and at that time we could also 
> > > record the _return type_ - easing the return value.
> > > 
> > > Note that it does not take a full, bloated DEBUG_INFO build - we 
> > > can build a -g object to get the dwarf2 data and then strip out 
> > > the dwarf2 data.
> > > 
> > > Arnaldo, what do you think about this, how feasible would it be 
> > > to put dwarf2 magic into scripts/recordmcount.pl?
> > 
> > /me reading scripts/recordmcount.pl...
> 
> So you want to:
> 
> 1. build object with -g
> 2. just after it is built, get what we want from the DWARF sections,
>    then strip it, stash what we collected
> 3. what we want is:
> 	- parameter names
> 	- _where_ each parameter is (DWARF location expressions)
> 	- type signature (CTF like stuff)
> 4. allow users to ask for parameters (all? just some?) for certain functions
>    to be collected at function entry
> 5. at function entry check if parameters should be collected, go over
>    each parameter DWARF location expression and collect the values,
>    encoding them into the ring buffer
> 6. at cat /d/tracing/trace time pretty print the parameters collected,
>    i.e. name=value-formatting-according-to-its-type

yeah.

> Ok, base types are easy, enums are easy, what about structs? forget
> about them and just print the pointer? i.e.:
> 
> 	_spin_lock(.lock=0xabcdef)

yeah.

> versus:
> 
> 	_spin_lock(.lock={.raw_lock={.slock=0}})
> 
> All members should be collected? Just some? User decides?

I think we should concentrate on the simplest, most obvious use, 
and iterate from there, gradually.

There's a lot of unknowns here - how reliable is the dwarf2 data 
in practice: we _really_ dont want to trust dwarf2 data by 
default becaus it can crash the kernel - so it's best to put it 
in some trusted format controlled by us - just like recordmcount 
works as a post-processor.

So if we have something simple and obvious and robust to start 
from we'll know a lot more once we started using it.

	Ingo