Re: [PATCH] pgo: add clang's Profile Guided Optimization infrastructure

[Sedat Dilek]

On Fri, Jan 22, 2021 at 2:34 AM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> On Wed, Jan 20, 2021 at 6:21 PM Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
> >
> > Hi,
> >
> > When I looked through the code I wondered why we do not add a
> > "CONFIG_PGO_CLANG_PROFDATA" which can be helpful when doing the PGO
> > rebuild with a vmlinux.profdata.
> >
> > This introduces a "PGO_PROFDATA" to turn on/off to pass
> > "-fprofile-use=vmlinux.profdata" (see CFLAGS_PGO_CLANG_PROFDATA in
> > top-level Makefile).
> >
> > If we turn off via "PGO_PROFILE := n" in several Makefiles - we should
> > do the same and add "PGO_PROFDATA := n" to the same Makefiles?
> >
> > Please see the attached diff.
>
> This is a good idea; something that I brought up in initial code
> review (on github).  Would it be ok with you to land the core first,
> then follow up with this suggestion?
>
> Also, AutoFDO production builds are so incredibly similar to PGO
> builds that I could see a possible path forward:
> 1. land PGO upstream
> 2. adds docs for AutoFDO
> 3. consider a config for hardcoding the location of the profiling data
> so that we don't need to specify it at the command line invocation of
> make.
>

I made a v3 - with some small nits.
The idea was to do the "PGO-rebuild" handling a bit easier.

But as you say that can wait.

Some personal notes:

I will be very happy when people verify/confirm what's going on with
PGO-rebuild + LLVM_IAS=1.
As said GNU/AS and PGO-rebuild is fine.
( This seems to be independent of clang-12 or clang-11. )
( This seems to be independent of DWARF v4 or v5 enabled. )

The benefit here I saw was a reduction in build-time of 00:30 seen
from a total 04:30 when using a PGO-rebuilt Linux-kernel.
Approx. 10%?

This is not much compared to a ThinLTO + PGO optimized LLVM toolchain
whcih saved here 40% of build-time.

- Sedat -