On Fri, Jan 18, 2019 at 12:24:20PM -0500, Vince Weaver wrote: > On Fri, 18 Jan 2019, Peter Zijlstra wrote: > > > > 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). > > unfortunately the HPC people using PAPI would probably be annoyed if we > started binding their threads to cores out from under them.
Quite.. :-) > > 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. > > one other thing the kernel would do is just disable rdpmc (setting index > to 0) in the case where the original perf_event_open() cpu paramater!=0 > > though that would stop the case where we were on the same CPU from > working. Indeed. > The issue is currently if you're not careful the rdpmc() interface will > sometimes return plausible (but wrong) results for a cross-CPU rdpmc() > call, even if you are properly falling back to read() on ->index being 0. > It's a bit surprising and it looks like it will take a decent amount of > userspace code to work around the issue, which cuts into the low-overhead > nature of rdpmc. > > If the answer is simply this is the way the kernel is going to do it, > that's fine, I just have to add workarounds to PAPI and then get the > perf_even_open() manpage updated to make sure people are aware of the > issue. I'm not sure there really is anything the kernel can do to help here... One thing you could look at is using rseq together with adding a CPU number to the userspace descriptor, and if rseq gives you a matching CPU number use rdpmc, otherwise, or on rseq abort, use read().
