Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 21 Oct 2015 08:26:08 -0700 | From | Andi Kleen <> | Subject | Re: [PATCH] x86, perf: Use INST_RETIRED.PREC_DIST for cycles:pp on Skylake |
| |
> Right, but suppose one user (root) would be sampling machine wide with > PEBS whilst another user is using it just on his one task.
The root case is likely perf top which does not use PEBS.
> > > And you having killed the flag bits for PEBS events precludes people > > > from using this manually, right? I think we want to exempt .inv=1 > > > .cmask=16 from that general rule on general utility value. > > > > I don't want to do this for every event. It has caused problems > > in the past. There's also no reason to use the other events. > > But it does allow users to play around with alternative encodings for > cycles etc.. We've made use of this in the past when SNB turned out to > have broken the event.
If they want to violate the SDM they can hack the kernel. It's risky to do as you well know.
> > > We could maybe abuse .precise_ip = 3 for this? > > > > I implemented this now, even though it's ugly. > > Agreed, its not ideal.
So it turns out that UOPS_RETIRED.ALL+pebs only works by accident on Skylake. It's not in the specification.
So on Skylake it has to be unconditional for any cycles:p On earlier parts it can be only for three p to handle your unlikely scenario.
-Andi
-- ak@linux.intel.com -- Speaking for myself only
| ![\](/images/icornerr.gif) |