Messages in this thread | | | Date | Fri, 11 Aug 2017 16:53:30 +0200 | From | Peter Zijlstra <> | Subject | Re: perf: multiple mmap of fd behavior on x86/ARM |
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On Fri, Aug 11, 2017 at 12:06:39PM +0100, Mark Rutland wrote: > On Fri, Aug 11, 2017 at 12:52:52PM +0200, Peter Zijlstra wrote: > > On Fri, Aug 11, 2017 at 11:01:27AM +0100, Mark Rutland wrote: > > > On Thu, Aug 10, 2017 at 02:48:52PM -0400, Vince Weaver wrote: > > > > > > > > So I was working on my perf_event_tests on ARM/ARM64 (the end goal was to > > > > get ARM64 rdpmc support working, but apparently those patches never made > > > > it upstream?) > > > > > > IIUC by 'rdpmc' you mean direct userspace counter access? > > > > > > Patches for that never made it upstream. Last I saw, there were no > > > patches in a suitable state for review. > > > > > > There are also difficulties (e.g. big.LITTLE systems where the number of > > > counters can differ across CPUs) which have yet to be solved. > > > > How would that be a problem? The API gives an explicit index to use with > > the 'rdpmc' instruction. > > It's a problem because access to unimplemented counters trap. So if a > task gets migrated from a CPU with N counters to one with N-1, accessing > counter N would be problematic. > > So we'd need to account for that somehow, in addition to the usual > sequence counter fun to verify the index was valid when the access was > performed.
Aah, you need restartable-sequences :-)
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