On Fri, Sep 28, 2012 at 7:49 AM, Namhyung Kim <namhy...@kernel.org> wrote: > Hi Frederic, > > On Fri, 28 Sep 2012 01:01:48 +0200, Frederic Weisbecker wrote: >> When Arun was working on this, I asked him to explore if it could make sense >> to reuse >> the "-b, --branch-stack" perf report option. Because after all, this >> feature is doing >> about the same than "-b" except it's using callchains instead of full branch >> tracing. >> But callchains are branches. Just a limited subset of all branches taken on >> excecution. >> So you can probably reuse some interface and even ground code there. >> >> What do you think? > > Umm.. first of all, I'm not familiar with the branch stack thing. It's > intel-specific, right? > The kernel API is NOT specific to Intel. It is abstracted to be portable across architecture. The implementation only exists on certain Intel X86 processors.
> Also I don't understand what exactly you want here. What kind of > interface did you say? Can you elaborate it bit more? > Not clear to me either. > And AFAIK branch stack can collect much more branch information than > just callstacks. Can we differentiate which is which easily? Is there > any limitation on using it? What if callstacks are not sync'ed with > branch stacks - is it possible though? > First of all branch stack is not a branch tracing mechanism. This is a branch sampling mechanism. Not all branches are captured. Only the last N consecutive branches leading to a PMU interrupt are captured in each sample. Yes, the branch stack mechanism as it exists on Intel processors can capture more then call branches. It is HW based and provides a branch type filter. Filtering capability is exposed at the API level in a generic fashion. The hw filter is based on opcodes. Call branches all cover call, syscall instructions. As such, the branch stack mechanism cannot be used to capture callstacks to shared libraries, simply because there a a non call instruction in the trampoline. To obtain a better quality callstack you have instead to sample return branches. So yes, callstacks are not sync'ed with branch stack even if limited to call branches. > But I think it'd be good if the branch stack can be changed to call > stack in general. Did you mean this? > That's not going to happen. The mechanism is much more generic than that. Quite frankly, I don't understand Frederic's motivation here. The mechanism are not quite the same. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
