On Thu, Jan 1, 2015 at 6:59 PM, Shaohua Li <s...@fb.com> wrote: > On Fri, Dec 19, 2014 at 06:03:34PM +0100, Peter Zijlstra 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. > > Hi Peter & Andy, > I'm wondering how we could use the perf to implament a clock_gettime. > reading the perf fd or using ioctl is slow so reading the mmap > ringbuffer is the only option. But as far as I know the ringbuffer has > data only when an event is generated. Between two events, there is > nothing we can read from the ringbuffer. Then how can application get > time info in the interval?
Don't use the ringbuffer. Instead, use a counting event, mmap it, and look at struct perf_event_mmap_page's comments to see how to read the time stamps. There's some code here that does this: https://github.com/andikleen/pmu-tools but you won't actually need the rdpmc part, since you just want overall times instead of hardware event counts. --Andy > > Thanks, > Shaohua -- 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/
