On Fri, Dec 19, 2014 at 9:03 AM, Peter Zijlstra <pet...@infradead.org> wrote: > On Fri, Dec 19, 2014 at 08:48:07AM -0800, Andy Lutomirski wrote: >> On Fri, Dec 19, 2014 at 3:23 AM, Peter Zijlstra <pet...@infradead.org> wrote: >> > On Thu, Dec 18, 2014 at 04:22:59PM -0800, Andy Lutomirski wrote: >> >> Bad news: this patch is incorrect, I think. Take a look at >> >> update_rq_clock -- it does fancy things involving irq time and >> >> paravirt steal time. So this patch could result in extremely >> >> non-monotonic results. >> > >> > Yeah, I'm not sure how (and if) we could make all that work :/ >> >> I obviously can't comment on what Facebook needs, but if I were >> rigging something up to profile my own code*, I'd want a count of >> elapsed time, including user, system, and probably interrupt as well. >> I would probably not want to count time during which I'm not >> scheduled, and I would also probably not want to count steal time. >> The latter makes any implementation kind of nasty. >> >> The API presumably doesn't need to be any particular clock id for >> clock_gettime, and it may not even need to be clock_gettime at all. >> >> Is perf self-monitoring good enough for this? If not, can we make it >> good enough? > > Yeah, I think you should be able to use that. You could count a NOP > event and simply use its activated time. We have PERF_COUNT_SW_DUMMY for > such purposes iirc. > > The advantage of using perf self profiling is that it (obviously) > extends to more than just walltime.
Re-asking my old question: would it make sense to add a vdso helper for the magic self-monitoring interface? Or, at the very least, we could try to tidy up the docs a bit. --Andy -- Andy Lutomirski AMA Capital Management, LLC -- 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/
