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

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Message-ID: <20210612203138.GH68208@worktop.programming.kicks-ass.net>
Date:   Sat, 12 Jun 2021 22:31:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Bill Wendling <morbo@...gle.com>, 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>,
        "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 01:20:15PM -0700, Fangrui Song wrote:

> For applications, I think instrumentation based PGO can be 1%~4% faster
> than sample-based PGO (e.g. AutoFDO) on x86.

Why? What specifically is missed by sample-based? I thought that LBR
augmented samples were very useful for exactly this.

> Sample-based PGO has CPU requirement (e.g. Performance Monitoring Unit).
> (my gut feeling is that there may be larger gap between instrumentation
> based PGO and sample-based PGO for aarch64/ppc64, even though they can
> use sample-based PGO.)
> Instrumentation based PGO can be ported to more architectures.

Every architecture that cares about performance had better have a
hardware PMU. Both argh64 and ppc64 have one.

> In addition, having an infrastructure for instrumentation based PGO
> makes it easy to deploy newer techniques like context-sensitive PGO
> (just changed compile options; it doesn't need new source level
> annotation).

What's this context sensitive stuff you speak of? The link provided
earlier is devoid of useful information.