Re: [PATCH][RFC 2/23]: SCST core

Frédéric Weisbecker
2008-12-16T23:47:25Z

2008/12/16 Ingo Molnar <mingo@elte.hu>:
>
> * Frédéric Weisbecker <fweisbec@gmail.com> wrote:
>
>> 2008/12/16 Ingo Molnar <mingo@elte.hu>:
>> >
>> > * Frédéric Weisbecker <fweisbec@gmail.com> wrote:
>> >
>> >> > All the above functionality is almost what we need. The only thing
>> >> > left, which I forgot to mention, is possibility to log also functions
>> >> > return value on exit. This is what TRACE_EXIT_RES() in SCST does. Is
>> >> > it possible to add those?
>> >>
>> >> I want to add that on the function graph tracer. That can be done pretty
>> >> easily. The only problem comes with the type of the return value. Would
>> >> this tracer be supposed to always return a 64 bits value regardless of
>> >> the real typ of the value? There would be some pointless bytes on most
>> >> return values. I don't know how to proceed for this problem.
>> >
>> > Things like mov ...,%eax are zero-extend so they'll zap the high 32 bits.
>>
>>
>> That's right, but the problem occurs under 32 bits. The return values
>> for 64 bits are in eax and edx. And most of the time, the high part
>> (edx) will be junk.
>
> for wider types i'd suggest to just print the low bits. Most of the
> interesting return types fit into machine word.
>
>> > The real problem are byte return values generated via things like:
>> >
>> > movb $1, %al
>> >
>> > those wont zero-extend, so you could get garbage in the output. One
>> > approach would be to try a quick hack just to see how common a problem
>> > this is.
>>
>>
>> Yes, I will try something.
>>
>> > We could extract the return type from the debuginfo, hash it in a
>> > read-mostly table and then look it up, but that seems complex both in
>> > terms of build overhead and in terms of runtime overhead.
>>
>> I thought about it too and as you say it's rather complex. And thinking
>> about non primitive types (like pid_t....) that would require a second
>> pass of analysis to retrieve the corresponding primitive...
>
> structure returns would be rather evil to handle, agreed.
>
> Unrelated:
>
> it would be really nice if we could extend ftrace to trace system calls
> and their parameters and return values. We used to have that in the
> latency-tracer in -rt, and it was rather useful. Controlled by a
> trace_option i guess - and blendable into any of the tracer outputs
> (function tracer most notably).
>
> Explicit calls in the entry.S files would be OK for this - but maybe we
> can get this via the tricky use of a TIF_ flags as well, to force the code
> into the ptrace callbacks and then divert it for ftrace's pleasure?
>
> Ingo
>

I recently thought about making a syscalls tracer and only thought
about the tracepoints. But that could be better to
trace them and all the function they call with the function graph
tracer. I think about the "graph" one because time of execution
measure could be useful for the -rt development.

I think that putting new calls on entryxx.S would be an overload and
would overlap what already does ptrace on syscall entry/exit.
So I like the idea of using the ptrace callbacks by manipulating the TIF_ flags.

Yeah that's interesting. I will do that for 2.6.30 :-)