On Thu, Sep 1, 2011 at 3:06 PM, Ryan Johnson <ryan.john...@cs.utoronto.ca> wrote: > 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? > Yes, it does. I am not sure they're updated correctly, though. I have not tried that in a very long time.
> 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 > ------------------------------------------------------------------------------ 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
