From nobody Tue Dec 16 23:41:40 2025 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 33D7AC001DF for ; Fri, 20 Oct 2023 16:20:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1377840AbjJTQUU (ORCPT ); Fri, 20 Oct 2023 12:20:20 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59802 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229554AbjJTQUR (ORCPT ); Fri, 20 Oct 2023 12:20:17 -0400 Received: from mail-ed1-x536.google.com (mail-ed1-x536.google.com [IPv6:2a00:1450:4864:20::536]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 89958A3 for ; Fri, 20 Oct 2023 09:20:15 -0700 (PDT) Received: by mail-ed1-x536.google.com with SMTP id 4fb4d7f45d1cf-53b32dca0bfso2219929a12.0 for ; Fri, 20 Oct 2023 09:20:15 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1697818814; x=1698423614; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:from:to:cc:subject:date:message-id:reply-to; bh=djUEh+V9HtHiOjNuky1TdbS1mFyQzEaDgKE3FsLY6IY=; b=DlZaxkEWg9jvpB2t6HAReKvsB38lpHgXkDvI82tn13Eag8rxurI/nxVV1fIFJK9yDf mV2fvrW2IR9gqaoih9JTnVdpVgdFZsWjNbY0yxK7wd9n6bwYwCpg+bNM0PAGk8l0TFSV Amkgl0A1QvzEHh1WvHEvi4iik+R5Kb1vuAPaI3NAZzZSjEaBYzUlxmfbmsEcSdgmvbsf Si/f3w0cQibSpCSl9eyfBxUh9r+1kz4mWisCSC3W2TpL10ih5iPQps87j6BgVDQuX9Ot PncABcMBtVvFIZ/sfXZfqdBEJirakI25SkZqIG9XjIehAUsFrcleDhmOE496z4PdNVId tVpA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1697818814; x=1698423614; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=djUEh+V9HtHiOjNuky1TdbS1mFyQzEaDgKE3FsLY6IY=; b=itmOkqm7pIqbbF5XvfaHJoVmo/Rwya9p/8D+QpB1lQXWqwFJVVzEF9tt7/IYw+5bqC McvZPwG+DWwJ/DdTwOdi118ohsCIlOUm3K+i/c6thMKPcvOu9DfRBcM/WJa1JznE7POw 1M4UcIGerLrOKnlO648eqp5IPILiGbCNH0Ktk3vq+ki4fi/a9Gz964Ja/JQvYub87WwZ UXq2ZQldlNupMUbI+2fxdPvDL2dpQzS39OZyTxOa8tkvowEGwcund8EkVyhQLAffIltK ItcftLlz1Nf0vQnwI4Iq5aKZe24Sqb7Ia2Z4UpPmOgI747ACUgqDphIuxPBl7cqHjscA kdFQ== X-Gm-Message-State: AOJu0YyXoMQWzbj0OUpNFjOdasC2NGtxXfcsv5kOIFUAWCIbIbutICox kuVzOWvFTpPTnVm1RnmLSEw= X-Google-Smtp-Source: AGHT+IEj3e2bk7Bg/YiorS/mvcBZuzF6TnwBpeYilqbXp9IM33eD702o2G8HA4UQ73AZJgHvDbR/iA== X-Received: by 2002:a05:6402:40c5:b0:52f:86a1:3861 with SMTP id z5-20020a05640240c500b0052f86a13861mr2186184edb.7.1697818813673; Fri, 20 Oct 2023 09:20:13 -0700 (PDT) Received: from localhost.localdomain ([46.248.82.114]) by smtp.gmail.com with ESMTPSA id e26-20020a50d4da000000b0053f29ba5adesm1755874edj.50.2023.10.20.09.20.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 20 Oct 2023 09:20:13 -0700 (PDT) From: Uros Bizjak To: x86@kernel.org, linux-kernel@vger.kernel.org Cc: Uros Bizjak , Andy Lutomirski , Brian Gerst , Denys Vlasenko , Ingo Molnar , "H . Peter Anvin" , Linus Torvalds , Peter Zijlstra , Thomas Gleixner , Josh Poimboeuf , Nadav Amit Subject: [PATCH] x86/percpu: Introduce const-qualified const_pcpu_hot Date: Fri, 20 Oct 2023 18:19:20 +0200 Message-ID: <20231020162004.135244-1-ubizjak@gmail.com> X-Mailer: git-send-email 2.41.0 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" Some variables in pcpu_hot, currently current_task and top_of_stack are actually per-thread variables implemented as per-cpu variables and thus stable for the duration of the respective task. There is already an attempt to eliminate redundant reads from these variables using this_cpu_read_stable() asm macro, which hides the dependency on the read memory address. However, the compiler has limited ability to eliminate asm common subexpressions, so this approach results in a limited success. The solution is to allow more aggressive elimination by aliasing pcpu_hot into a const-qualified const_pcpu_hot, and to read stable per-cpu variables from this constant copy. The current per-cpu infrastructure does not support reads from const-qualified variables. However, when the compiler supports segment qualifiers, it is possible to declare the const-aliased variable in the relevant named address space. The compiler considers access to the variable, declared in this way, as a read from a constant location, and will optimize reads from the variable accordingly. By implementing constant-qualified const_pcpu_hot, the compiler can eliminate redundant reads from the constant variables, reducing the number of loads from current_task from 3766 to 3217 on a test build, a -14.6% reduction. The reduction of loads translates to the following code savings: text data bss dec hex filename 25477353 4389456 808452 30675261 1d4113d vmlinux-old.o 25476074 4389440 808452 30673966 1d40c2e vmlinux-new.o representing a code size reduction of 1279 bytes. Cc: Andy Lutomirski Cc: Brian Gerst Cc: Denys Vlasenko Cc: Ingo Molnar Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Josh Poimboeuf Co-developed-by: Nadav Amit Signed-off-by: Nadav Amit Signed-off-by: Uros Bizjak --- arch/x86/include/asm/current.h | 7 +++++++ arch/x86/include/asm/percpu.h | 6 +++--- arch/x86/include/asm/processor.h | 3 +++ arch/x86/kernel/vmlinux.lds.S | 1 + include/linux/compiler.h | 2 +- 5 files changed, 15 insertions(+), 4 deletions(-) diff --git a/arch/x86/include/asm/current.h b/arch/x86/include/asm/current.h index a1168e7b69e5..0538d2436673 100644 --- a/arch/x86/include/asm/current.h +++ b/arch/x86/include/asm/current.h @@ -36,8 +36,15 @@ static_assert(sizeof(struct pcpu_hot) =3D=3D 64); =20 DECLARE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot); =20 +/* const-qualified alias to pcpu_hot, aliased by linker. */ +DECLARE_PER_CPU_ALIGNED(const struct pcpu_hot __percpu_seg_override, + const_pcpu_hot); + static __always_inline struct task_struct *get_current(void) { + if (IS_ENABLED(CONFIG_USE_X86_SEG_SUPPORT)) + return const_pcpu_hot.current_task; + return this_cpu_read_stable(pcpu_hot.current_task); } =20 diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h index bbcc1ca737f0..630bb912a46b 100644 --- a/arch/x86/include/asm/percpu.h +++ b/arch/x86/include/asm/percpu.h @@ -413,9 +413,9 @@ do { \ * accessed while this_cpu_read_stable() allows the value to be cached. * this_cpu_read_stable() is more efficient and can be used if its value * is guaranteed to be valid across cpus. The current users include - * get_current() and get_thread_info() both of which are actually - * per-thread variables implemented as per-cpu variables and thus - * stable for the duration of the respective task. + * pcpu_hot.current_task and pcpu_hot.top_of_stack, both of which are + * actually per-thread variables implemented as per-cpu variables and + * thus stable for the duration of the respective task. */ #define this_cpu_read_stable_1(pcp) percpu_stable_op(1, "mov", pcp) #define this_cpu_read_stable_2(pcp) percpu_stable_op(2, "mov", pcp) diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/proces= sor.h index ae81a7191c1c..a807025a4dee 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -533,6 +533,9 @@ static __always_inline unsigned long current_top_of_sta= ck(void) * and around vm86 mode and sp0 on x86_64 is special because of the * entry trampoline. */ + if (IS_ENABLED(CONFIG_USE_X86_SEG_SUPPORT)) + return pcpu_hot.top_of_stack; + return this_cpu_read_stable(pcpu_hot.top_of_stack); } =20 diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S index 54a5596adaa6..1239be7cc8d8 100644 --- a/arch/x86/kernel/vmlinux.lds.S +++ b/arch/x86/kernel/vmlinux.lds.S @@ -46,6 +46,7 @@ ENTRY(phys_startup_64) #endif =20 jiffies =3D jiffies_64; +const_pcpu_hot =3D pcpu_hot; =20 #if defined(CONFIG_X86_64) /* diff --git a/include/linux/compiler.h b/include/linux/compiler.h index d7779a18b24f..bf9815eaf4aa 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -212,7 +212,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f,= int val, */ #define ___ADDRESSABLE(sym, __attrs) \ static void * __used __attrs \ - __UNIQUE_ID(__PASTE(__addressable_,sym)) =3D (void *)&sym; + __UNIQUE_ID(__PASTE(__addressable_,sym)) =3D (void *)(uintptr_t)&sym; #define __ADDRESSABLE(sym) \ ___ADDRESSABLE(sym, __section(".discard.addressable")) =20 --=20 2.41.0