From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2C47AC433E9 for ; Tue, 12 Jan 2021 00:45:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 032662253A for ; Tue, 12 Jan 2021 00:45:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730837AbhALApd (ORCPT ); Mon, 11 Jan 2021 19:45:33 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57510 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728218AbhALAp1 (ORCPT ); Mon, 11 Jan 2021 19:45:27 -0500 Received: from mail-pg1-x532.google.com (mail-pg1-x532.google.com [IPv6:2607:f8b0:4864:20::532]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id AB6D9C06179F for ; Mon, 11 Jan 2021 16:44:46 -0800 (PST) Received: by mail-pg1-x532.google.com with SMTP id g15so273265pgu.9 for ; Mon, 11 Jan 2021 16:44:46 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=dPK8pNH4QCaFIUUZrNoOKMv2joVCl+mHPXFylSDuRyY=; b=YYZM3nC4hqQi8K3u2YEghUOFWSp0FudnKJQjHHjiXbYDWf5IBbeiuTdLklLvfbn/1J I/7XeAxo9NGSa2DgDNEJEess0Hv/TD7PjwXe6TVYuO4D2z/QbcC7iIBGWsJRlNwnrdFW wIeOx31f/jqTl1IUALwJLpeqpkBRMQbt7zODBGO0KwTZi7FCQxXRx2dZ2l3MqwHNX6Ab Mqed2aDgr2OddzqXikVgh0iy6Ci1vHR+bOcnnUhsCpj5nbt8gP3y7Kp+E1n24vSxhjOo +8PMmsMQYd+sUx+SCoqjtEmdW68go5RWEpCftxCH7j13OQXSFBpiKuT5l6pQlpRrDuP/ Y3mw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=dPK8pNH4QCaFIUUZrNoOKMv2joVCl+mHPXFylSDuRyY=; b=noXKLOM6O4HD33jfZ60HN45vgbtK2wfXPRSkjROzkKotBbtMa6NgGKNzOxFoSlTeyC CCWjmTGReDBwaEuZ/ojFimGttG+Fcuies0SGJPcd4k8TcvcwtHeucaAGz+CU8n5Dc6KS MnxHv0vgU3dtBl1UUWcPpZja5x2HzVFRGZ2FQ3QSQ+ypSwPc510jUoqcNA98eFdqT2Zr zvp8Fxizt1I6CZbHmfxBlJPxCrSLPYkjmhj1x0PxfMKdvY2sWA19VyyUw5h7oOW9+BjB 3JPfvcc5cQA8OCo5GVvtsDNvY8JbZNZEBw3Dsqv7yk2uvHenoMQEueVvEPkSkF2vmt0F KRSQ== X-Gm-Message-State: AOAM533uBG1gNMKUp/pw7OLfHcsYnV92cewsxpIDsMlYmgZoszRNutX2 4/plHyB378jOXX8WkgBcDi1IWbJFvFnCXwXPOWou/Q== X-Google-Smtp-Source: ABdhPJyUgt2aK/qEK75FZJTwrK4geThG9TgZIvV8yVMdaHJ9s1TRxET7l03tQOHfyl5cXnSua/lKZ6tmizdHX6/XmRs= X-Received: by 2002:a63:5a08:: with SMTP id o8mr2065138pgb.118.1610412285888; Mon, 11 Jan 2021 16:44:45 -0800 (PST) MIME-Version: 1.0 References: <20210111081821.3041587-1-morbo@google.com> <20210111201224.l5r2zxuyd7ayszke@google.com> <20210111203120.daeef4yuwgmk5em4@google.com> In-Reply-To: From: =?UTF-8?B?RsSBbmctcnXDrCBTw7JuZw==?= Date: Mon, 11 Jan 2021 16:44:34 -0800 Message-ID: Subject: Re: [PATCH] pgo: add clang's Profile Guided Optimization infrastructure To: Bill Wendling Cc: Jonathan Corbet , Masahiro Yamada , Linux Doc Mailing List , LKML , Linux Kbuild mailing list , clang-built-linux , Andrew Morton , Nathan Chancellor , Nick Desaulniers , Sami Tolvanen Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 11, 2021 at 4:38 PM Bill Wendling wrote: > > On Mon, Jan 11, 2021 at 12:31 PM Fangrui Song wrote: > > On 2021-01-11, Bill Wendling wrote: > > >On Mon, Jan 11, 2021 at 12:12 PM Fangrui Song wrote: > > >> > > >> On 2021-01-11, 'Bill Wendling' via Clang Built Linux wrote: > > >> >From: Sami Tolvanen > > >> > > > >> >Enable the use of clang's Profile-Guided Optimization[1]. To generate a > > >> >profile, the kernel is instrumented with PGO counters, a representative > > >> >workload is run, and the raw profile data is collected from > > >> >/sys/kernel/debug/pgo/profraw. > > >> > > > >> >The raw profile data must be processed by clang's "llvm-profdata" tool before > > >> >it can be used during recompilation: > > >> > > > >> > $ cp /sys/kernel/debug/pgo/profraw vmlinux.profraw > > >> > $ llvm-profdata merge --output=vmlinux.profdata vmlinux.profraw > > >> > > > >> >Multiple raw profiles may be merged during this step. > > >> > > > >> >The data can be used either by the compiler if LTO isn't enabled: > > >> > > > >> > ... -fprofile-use=vmlinux.profdata ... > > >> > > > >> >or by LLD if LTO is enabled: > > >> > > > >> > ... -lto-cs-profile-file=vmlinux.profdata ... > > >> > > >> This LLD option does not exist. > > >> LLD does have some `--lto-*` options but the `-lto-*` form is not supported > > >> (it clashes with -l) https://reviews.llvm.org/D79371 > > >> > > >That's strange. I've been using that option for years now. :-) Is this > > >a recent change? > > > > The more frequently used options (specifyed by the clang driver) are > > -plugin-opt=... (options implemented by LLVMgold.so). > > `-lto-*` is rare. > > > > >> (There is an earlier -fprofile-instr-generate which does > > >> instrumentation in Clang, but the option does not have broad usage. > > >> It is used more for code coverage, not for optimization. > > >> Noticeably, it does not even implement the Kirchhoff's current law > > >> optimization) > > >> > > >Right. I've been told outside of this email that -fprofile-generate is > > >the prefered flag to use. > > > > > >> -fprofile-use= is used by both regular PGO and context-sensitive PGO (CSPGO). > > >> > > >> clang -flto=thin -fprofile-use= passes -plugin-opt=cs-profile-path= to the linker. > > >> For regular PGO, this option is effectively a no-op (confirmed with CSPGO main developer). > > >> > > >> So I think the "or by LLD if LTO is enabled:" part should be removed. > > > > > >But what if you specify the linking step explicitly? Linux doesn't > > >call "clang" when linking, but "ld.lld". > > > > Regular PGO+LTO does not need -plugin-opt=cs-profile-path= > > CSPGO+LTO needs it. > > Because -fprofile-use= may be used by both, Clang driver adds it. > > CSPGO is relevant in this this patch, so the linker option does not need to be mentioned. > > I'm still a bit confused. Are you saying that when clang uses > `-flto=thin -fprofile-use=foo` that the profile file "foo" is embedded > into the bitcode file so that when the linker's run it'll be used? > > This is the workflow: > > clang ... -fprofile-use=vmlinux.profdata ... -c -o foo.o foo.c > clang ... -fprofile-use=vmlinux.profdata ... -c -o bar.o bar.c > ld.lld ... foo.o bar.o > > Are you saying that we don't need to have > "-plugin-opt=cs-profile-path=vmlinux.profdata" on the "ld.lld ..." > line? > > -bw The backend compile step -flto=thin -fprofile-use=foo has all the information. -plugin-opt=cs-profile-path=vmlinux.profdata is not needed for regular PGO.