On 01/09/2011 1:55 AM, stephane eranian wrote: > On Thu, Sep 1, 2011 at 1:07 AM, Corey Ashford > <cjash...@linux.vnet.ibm.com> wrote: >> On 08/25/2011 07:19 AM, stephane eranian wrote: >>> Hi, >>> >>> Sorry for late reply. >>> >>> The current support for mmaped count is broken on perf_event x86. >>> It simply does not work. I think it only works on PPC at this point. >> Just as an aside, you can access the counter registers from user space >> on Power (aka PPC) machines, but because the kernel is free to schedule >> the events onto whatever counters that meet the resource constraints, >> it's not at all clear which hardware counter to read from user space, >> and in fact, with event rotation, the counter being used can change from >> one system tick till the next. >> >> If you program a single event, you can be guaranteed that it won't move >> around, but you still will have to guess or somehow determine which >> hardware counter is being used by the kernel. >> > Yes, and that's why they have this 'lock' field in there.It's not really a > lock > but rather a generation counter. You need to read it before you attempt to > read and you need to check it when you're done reading. If the two values > don't match then the counter changed and you need to retry. And changes > means it may have moved to a different counter. This protocol is actually documented pretty well in <linux/perf_event.h>, too. Read the lock, read the index, read hw counter[index-1], read lock again to verify.
> But the key problem here is the time scaling. In case you are multiplex > you need to be able to retrieve time_enabled and time_running to scale > the count. But that's not exposed, thus it does not work as soon as you > have multiplexing. Well, unless you only care about deltas and not the > absolute values. Doesn't perf_event_mmap_page expose both those, also protected by the generation counter? Or are you saying the kernel doesn't actually update those fields right now? Ryan ------------------------------------------------------------------------------ Special Offer -- Download ArcSight Logger for FREE! Finally, a world-class log management solution at an even better price-free! And you'll get a free "Love Thy Logs" t-shirt when you download Logger. Secure your free ArcSight Logger TODAY! http://p.sf.net/sfu/arcsisghtdev2dev _______________________________________________ perfmon2-devel mailing list perfmon2-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/perfmon2-devel
