Re: plumbers session on profiling?

Peter Zijlstra
2022-07-04T14:01:52Z

On Fri, Jul 01, 2022 at 11:57:25AM -0700, Bill Wendling wrote:
> On Fri, Jul 1, 2022 at 4:49 AM Peter Zijlstra <peterz@infradead.org> wrote:

> > IIRC Google has LBR sample driven PGO somewhere as well. ISTR that being
> > the whole motivation for that gruesome Zen3 BRS hack.
> >
> > Google got me this: https://research.google.com/pubs/archive/45290.pdf
> >
> Right. However, there's a chicken-and-egg issue with AutoFDO for the
> production kernel. We can't release a kernel that hasn't been compiled
> with PGO/FDO. We could only release it in a test environment, in which
> case we could use AutoFDO. However, the document says that AutoFDO
> only reaches ~90% of FDO. They list some reasons for this, but
> nonetheless I suspect that the delta would be too severe for us to
> release the kernel.

The pertinent question seems to be what's missing? Where does that 10%
go.

> As for LBR, that will work with Intel/AMD, but I thought that LBR
> doesn't exist for Arm processors (my knowledge could be out of date on
> this).

Not totally up to date on the ARM thing either; but I believe you're
right in that they don't yet have such a feature.

> What would make PGO (sample-based or instrumented) easy enough for you
> to use? What're the key elements missing?

The key piece missing is how to feed a perf.data file back into the
compile cycle, something like:

$ make O=build/ PERF=perf.data -j$lots

would be useful I suppose.