Messages in this thread | | | Date | Mon, 14 Aug 2017 11:57:29 +0100 | From | Will Deacon <> | Subject | Re: perf: multiple mmap of fd behavior on x86/ARM |
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On Fri, Aug 11, 2017 at 04:53:30PM +0200, Peter Zijlstra wrote: > 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 :-)
Or, in the absence of those, I wouldn't mind only supporting this for non-big/little platforms initially.
Will
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