Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 4 Jul 2022 16:01:15 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Bill Wendling <morbo@google.com>
Cc:     "Jose E. Marchesi" <jemarch@gnu.org>,
        Ruud van der Pas <ruud.vanderpas@oracle.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        Vladimir Mezentsev <vladimir.mezentsev@oracle.com>,
        clang-built-linux <llvm@lists.linux.dev>,
        LKML <linux-kernel@vger.kernel.org>, Yonghong Song <yhs@fb.com>,
        Wenlei He <wenlei@fb.com>, Hongtao Yu <hoy@fb.com>,
        Ingo Molnar <mingo@kernel.org>,
        linux-toolchains <linux-toolchains@vger.kernel.org>,
        elena.zannoni@oracle.com
Subject: Re: plumbers session on profiling?
Message-ID: <YsLyq5FUZj9NLy3V@worktop.programming.kicks-ass.net>
References: <CAKwvOdnsZekEM77axBf67MDqQVP0n6PTKH=njSyPSWTNiWAOiA@mail.gmail.com>
 <87mtf7z0rt.fsf@gnu.org>
 <6F9E9D93-3913-4022-9384-D809C8EF7715@oracle.com>
 <CAKwvOdm=_YqBpuBzouqoWHYNe6MMUE10vqF0PUkU=hcOj+UqrQ@mail.gmail.com>
 <B0A01DE7-1B50-479A-92DF-DAFAB3F06E0F@oracle.com>
 <878rpgpvfj.fsf@gnu.org>
 <Yr638aOIaaEBPICy@worktop.programming.kicks-ass.net>
 <CAGG=3QWbW-Dang49Jx3fyNExWtL8syuMkMJmcPHA7J25cHQ0zw@mail.gmail.com>
 <Yr7fMre18pUMz9rA@worktop.programming.kicks-ass.net>
 <CAGG=3QUA8BnhN9QaJao6nH6EyNks0yWO+cWWyvE0pJbOUm+cMw@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAGG=3QUA8BnhN9QaJao6nH6EyNks0yWO+cWWyvE0pJbOUm+cMw@mail.gmail.com>
Precedence: bulk

On Fri, Jul 01, 2022 at 11:57:25AM -0700, Bill Wendling wrote:
> On Fri, Jul 1, 2022 at 4:49 AM Peter Zijlstra <peterz@infradead.org> wrote:

> > IIRC Google has LBR sample driven PGO somewhere as well. ISTR that being
> > the whole motivation for that gruesome Zen3 BRS hack.
> >
> > Google got me this: https://research.google.com/pubs/archive/45290.pdf
> >
> Right. However, there's a chicken-and-egg issue with AutoFDO for the
> production kernel. We can't release a kernel that hasn't been compiled
> with PGO/FDO. We could only release it in a test environment, in which
> case we could use AutoFDO. However, the document says that AutoFDO
> only reaches ~90% of FDO. They list some reasons for this, but
> nonetheless I suspect that the delta would be too severe for us to
> release the kernel.

The pertinent question seems to be what's missing? Where does that 10%
go.

> As for LBR, that will work with Intel/AMD, but I thought that LBR
> doesn't exist for Arm processors (my knowledge could be out of date on
> this).

Not totally up to date on the ARM thing either; but I believe you're
right in that they don't yet have such a feature.

> What would make PGO (sample-based or instrumented) easy enough for you
> to use? What're the key elements missing?

The key piece missing is how to feed a perf.data file back into the
compile cycle, something like:

  $ make O=build/ PERF=perf.data -j$lots

would be useful I suppose.