linux-kernel - Re: plumbers session on profiling?

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Mon, 4 Jul 2022 16:01:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bill Wendling <morbo@...gle.com>
Cc:     "Jose E. Marchesi" <jemarch@....org>,
        Ruud van der Pas <ruud.vanderpas@...cle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Vladimir Mezentsev <vladimir.mezentsev@...cle.com>,
        clang-built-linux <llvm@...ts.linux.dev>,
        LKML <linux-kernel@...r.kernel.org>, Yonghong Song <yhs@...com>,
        Wenlei He <wenlei@...com>, Hongtao Yu <hoy@...com>,
        Ingo Molnar <mingo@...nel.org>,
        linux-toolchains <linux-toolchains@...r.kernel.org>,
        elena.zannoni@...cle.com
Subject: Re: plumbers session on profiling?

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@...radead.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 doesn't exist for Arm processors (my knowledge could be out of date on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.