On Fri, Jan 18, 2019 at 09:09:04AM -0500, Vince Weaver wrote: > On Fri, 18 Jan 2019, Peter Zijlstra wrote: > > > On Fri, Jan 11, 2019 at 04:52:22PM -0500, Vince Weaver wrote: > > > On Thu, 10 Jan 2019, Vince Weaver wrote: > > > > > > > On Thu, 10 Jan 2019, Vince Weaver wrote: > > > > > > > > > On Thu, 10 Jan 2019, Vince Weaver wrote: > > > > > > > > > > > However if you create an all-process attached to CPU event: > > > > > > perf_event_open(attr, -1, X, -1, 0); > > > > > > the mmap event index is set as if this were a valid event and so > > > > > > the rdpmc > > > > > > succeeds even though it shouldn't (we're trying to read an event > > > > > > value > > > > > > on a remote cpu with a local rdpmc). > > > > > > so on further looking at the code, it doesn't appear that rdpmc events > > > are > > > explicitly marked as unavailable in the attach-cpu or attach-pid case, > > > it's just by luck the check for PERF_EVENT_STATE_ACTIVE catches most of > > > the cases? > > > > > > should an explicit check be added to zero out userpg->index in cases > > > where > > > the event being measured is running on a different core? > > > > And how would we konw? We don't know what CPU will be observing the > > mmap(). > > > > RDPMC fundamentally only makes sense on 'self' (either task or CPU). > > so is this a "don't do that then" thing and I should have PAPI > userspace avoid using rdpmc() whenever a proc/cpu was attached to?
You can actually use rdpmc when you attach to a CPU, but you have to ensure that the userspace component is guaranteed to run on that very CPU (sched_setaffinity(2) comes to mind). > Or is there a way to have the kernel indicate this? Does the kernel track > current CPU and original CPU of the mmap and could zero out the index > field in this case? Or would that add too much overhead? Impossible I'm afraid. Memory is not associated with a CPU; it's contents is the same whatever CPU reads from it. The best we could possibly do is put the (target, not current) cpu number in the mmap page; but userspace should already know this, for it created the event and therefore knows this already.
