On Thu, 23 Oct 2014, Peter Zijlstra wrote: > On Tue, Oct 21, 2014 at 01:05:49PM -0400, Vince Weaver wrote:
> > There are various reasons why you might want to start events at times > > other than the beginning of the program. Some people don't like kernel > > multiplexing so they start/stop manually if they want to switch eventsets. > > I suppose you could pre-create all events and use ioctl()s to start/stop > them where/when desired, this should be faster I think. But yes, this is > not a use-case I've though much about. The scheduling step is most of what makes the perf_event start call have high overhead. The other annoyance is the fact that due to the NMI watchdog your can successfully perf_event_open() an event group but still have it fail at start time, so a lot of code has to be done that does extraneous open/start/close calls to make sure the events really fit. > MAP_POPULATE is your friend there, but yes manually prefaulting is > perfectly fine too, and the HPC people are quite familiar with the > concept, they do it for a lot of things. MAP_POPULATE actually has noticably more overhead than manually prefaulting. It's on my todo list to drop ftrace on there and find out why, but I've been stuck chasing kernel-crashing fuzzer bugs instead in my spare time. perfctr and possibly perfmon2 would automatically pre-fault the mmap page for you in the kernel, so there was no need for the user to do it. In any case I wasn't really trying to make trouble here, it's just I came across the people using rdpmc w/o perf_event just the other day (on USENET of all places). They were so happy it worked w/o patches now, that I felt bad breaking it to them that there were patches floating around that were going to make their usecase not work anymore. I guess like all things though, you can't have anything fun and useful in the kernel without the security people taking it away. Vince -- 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/
