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

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Message-ID: <YMcKgyOz331qvO/R@hirez.programming.kicks-ass.net>
Date:   Mon, 14 Jun 2021 09:51:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bill Wendling <morbo@...gle.com>
Cc:     Kees Cook <keescook@...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>
Subject: Re: [PATCH v9] pgo: add clang's Profile Guided Optimization
 infrastructure

On Sat, Jun 12, 2021 at 01:56:41PM -0700, Bill Wendling wrote:
> For example, Fangrui gave you numbers, and you rejected them out of
> hand. I've explained to you why instrumentation is better than
> sampling (at least for clang). Fangrui gave you numbers. Let's move on
> to something else.

I did not dismiss them; I asked for clarification. I would like to
understand what exactly is missed by sampling based PGO data that makes
such a difference.