* Frédéric Weisbecker <fweisbec@xxxxxxxxx> wrote: > 2008/12/16 Ingo Molnar <mingo@xxxxxxx>: > > > > * Frédéric Weisbecker <fweisbec@xxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
