linux-kernel - Re: [PATCH v9] pgo: add clang's Profile Guided Optimization infrastructure

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Date:   Mon, 14 Jun 2021 17:46:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Marco Elver <elver@...gle.com>, Bill Wendling <morbo@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Fangrui Song <maskray@...gle.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, johannes.berg@...el.com,
        oberpar@...ux.vnet.ibm.com, linux-toolchains@...r.kernel.org
Subject: Re: [PATCH v9] pgo: add clang's Profile Guided Optimization
 infrastructure

On Mon, Jun 14, 2021 at 08:26:01AM -0700, Kees Cook wrote:
> > 2. Like (1) but also keep GCOV, given proper support for attribute
> > no_instrument_function would probably fix it (?).
> > 
> > 3. Keep GCOV (and KCOV of course). Somehow extract PGO profiles from KCOV.
> > 
> > 4. Somehow extract PGO profiles from GCOV, or modify kernel/gcov to do so.
> 
> If there *is* a way to "combine" these, I don't think it makes sense
> to do it now. PGO has users (and is expanding[1]), and trying to
> optimize the design before even landing the first version seems like a
> needless obstruction, and to likely not address currently undiscovered
> requirements.

Even if that were so (and I'm not yet convinced), the current proposal
is wedded to llvm-pgo, there is no way gcc-pgo could reuse any of this
code afaict, which then means they have to create yet another variant.

Sorting this *before* the first version is exactly the right time.

Since when are we merging code when the requirements are not clear?

Just to clarify:

Nacked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

For all this PGO crud.