Re: [PATCH] x86, perf: Use INST_RETIRED.PREC_DIST for cycles:pp on Skylake

[Peter Zijlstra]

On Mon, Oct 19, 2015 at 03:58:16PM -0700, Andi Kleen wrote:

> Switch the cycles:pp alias from UOPS_RETITRED to INST_RETIRED.PREC_DIST.
> The basic mechanism of abusing the inverse cmask to get all cycles
> works the same as before.
> 
> PREC_DIST has special support for avoiding shadow effects, which
> can give better results compare to UOPS_RETIRED. The drawback is
> that PREC_DIST can only schedule on counter 1, but that is ok for
> cycle sampling, as there is normally no need to do multiple cycle
> sampling runs in parallel. It is still possible to run perf top
> in parallel, as that doesn't use precise mode. Also of course
> the multiplexing can still allow parallel operation.

So the worry I have with this is that there might indeed be people
wanting to use this in parallel.

Typically on workstations you do not, because there's only a single
user, but on servers it might be more common.  The thing I expect to be
most common is having both a CPU wide and a per task cycle counter
enabled.

This means a fairly visible change in behaviour depending on uarch.

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.

We could maybe abuse .precise_ip = 3 for this?

> On earlier parts there were various hardware bugs in it
> (but no show stopper on IvyBridge and up I believe),
> so it could be enabled there after sufficient testing.

Just enable it for IVB+ then.

> On Sandy Bridge PREC_DIST can only be scheduled as a single
> event on the PMU, which is too limiting. Before Sandy
> Bridge it was not supported.

Right, that was a bit cumbersome :-)
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