On Fri, Oct 24, 2014 at 5:41 AM, Vince Weaver <vi...@deater.net> wrote: > 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.
Have you checked recently? IIRC Michael Lespinasse put some effort into improving MAP_POPULATE recently. > > 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. I'm sympathetic enough to this use case that I don't really mind adding a bit of code to support rdpmc=2 meaning that rdpmc is always allowed. Switching in and out of rdpmc=2 mode will be expensive (static key and IPI to all CPUs). PeterZ, is that OK with you? --Andy > > Vince -- 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/
