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.
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. > - Corey > > ------------------------------------------------------------------------------ > 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
