Re: [GIT PULL] Clang feature updates for v5.14-rc1

[Kees Cook]

On Mon, Jun 28, 2021 at 07:49:04PM -0700, Linus Torvalds wrote:
> On Mon, Jun 28, 2021 at 12:32 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >
> > The big addition for this merge window is the core support for Clang's
> > Profile Guided Optimization, which lets Clang build the kernel for
> > improved performance when running specific kernel workloads. This
> > currently covers only vmlinux, but module support is under active
> > development. (Sami Tolvanen, Bill Wendling, Kees Cook, Jarmo Tiitto,
> > Lukas Bulwahn)
> 
> Am I misreading this?
> 
> The PGO data seems to be done by using clang instrumentation, instead
> of done sanely using sample data from a regular "perf" run?

Right, yes. My understanding is that PGO is measurably better than
sample-based profiling. Additionally, it's arch-agnostic (not that that's
meaningful here with only x86 finished), and can gain other analysis
features that aren't possible with perf. I'll let Nick, Fangrui, Bill,
or Sami answer this more directly.

In the meantime I will split the pull request into "PGO" and "everything
else".

> That odd decision seems to not be documented anywhere, and it seems
> odd and counter-productive, and causes all that odd special buffer
> handling and that vmlinux.profraw file etc.
> 
> And it causes the kernel to be bigger and run slower.

Right -- that's expected. It's not designed to be the final kernel
someone uses. :)

-Kees

-- 
Kees Cook