From nobody Thu Jan 1 11:02:55 2026 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 02F5CC001E0 for ; Mon, 23 Oct 2023 10:39:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232568AbjJWKji (ORCPT ); Mon, 23 Oct 2023 06:39:38 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59824 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229572AbjJWKjg (ORCPT ); Mon, 23 Oct 2023 06:39:36 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8DD3CDA; Mon, 23 Oct 2023 03:39:33 -0700 (PDT) Date: Mon, 23 Oct 2023 10:39:31 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1698057572; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=gpcW9/05c8vPddJwR7qGwxD6YERGkg5tE+9qWuZlV0I=; b=c9nJ+HzosX3V9UAxcuypDMOfc4SRUWpqytu+xsJMhLII2BZDcYEsS6kmYhnpZN+rFjM4R7 DlnUh25VIMhIwXFF2R6qedQ/9JA0KrVw7AHGF/wYgrrifRQQEH7Z43hVIVWydXvNi8Sn8g XoSGLccjPv1kzf3WK9JU/f7C/b/IeDVyE4Jjf2HV3rsP6GHN3vnB4JN+GYzCo0/9FYaqaa 25KUFEGA7GsNSaPd3nyInfPhseJ3O7aDKQRg/68xwGQ1akn9rM8DVm8uzC59aexjaAMdy9 GSNahgLNR4EKgshfJnnBgTOrZBcDLrDHWwBq9Mt7S5J5rzz89/ZQaFB6ebjJlA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1698057572; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=gpcW9/05c8vPddJwR7qGwxD6YERGkg5tE+9qWuZlV0I=; b=63iiffRXMZf/n9aDfjtAXm42cWzQNh/PieZH/54tqbZoqJ1ijkgXAVyyayO5dwsAbXgSRM 4jhBn6i8L4biJPCg== From: "tip-bot2 for Uros Bizjak" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: x86/percpu] x86/percpu: Introduce const-qualified const_pcpu_hot to micro-optimize code generation Cc: Nadav Amit , Uros Bizjak , Ingo Molnar , x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20231020162004.135244-1-ubizjak@gmail.com> References: <20231020162004.135244-1-ubizjak@gmail.com> MIME-Version: 1.0 Message-ID: <169805757123.3135.742796790384706189.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The following commit has been merged into the x86/percpu branch of tip: Commit-ID: ed2f752e0e0a21d941ca0ee539ef3d4cd576bc5e Gitweb: https://git.kernel.org/tip/ed2f752e0e0a21d941ca0ee539ef3d4cd= 576bc5e Author: Uros Bizjak AuthorDate: Fri, 20 Oct 2023 18:19:20 +02:00 Committer: Ingo Molnar CommitterDate: Mon, 23 Oct 2023 11:27:35 +02:00 x86/percpu: Introduce const-qualified const_pcpu_hot to micro-optimize code= generation 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 25,477,353 4389456 808452 30675261 1d4113d vmlinux-old.o 25,476,074 4389440 808452 30673966 1d40c2e vmlinux-new.o representing a code size reduction of -1279 bytes. [ mingo: Updated the changelog, EXPORT(const_pcpu_hot). ] Co-developed-by: Nadav Amit Signed-off-by: Nadav Amit Signed-off-by: Uros Bizjak Signed-off-by: Ingo Molnar Link: https://lore.kernel.org/r/20231020162004.135244-1-ubizjak@gmail.com --- arch/x86/include/asm/current.h | 7 +++++++ arch/x86/include/asm/percpu.h | 6 +++--- arch/x86/include/asm/processor.h | 3 +++ arch/x86/kernel/cpu/common.c | 1 + arch/x86/kernel/vmlinux.lds.S | 1 + include/linux/compiler.h | 2 +- 6 files changed, 16 insertions(+), 4 deletions(-) diff --git a/arch/x86/include/asm/current.h b/arch/x86/include/asm/current.h index a1168e7..0538d24 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 bbcc1ca..b86b27d 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 0086920..b47a997 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -518,6 +518,9 @@ 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/cpu/common.c b/arch/x86/kernel/cpu/common.c index 382d4e6..4cc0ab0 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -2051,6 +2051,7 @@ DEFINE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot) =3D= { .top_of_stack =3D TOP_OF_INIT_STACK, }; EXPORT_PER_CPU_SYMBOL(pcpu_hot); +EXPORT_PER_CPU_SYMBOL(const_pcpu_hot); =20 #ifdef CONFIG_X86_64 DEFINE_PER_CPU_FIRST(struct fixed_percpu_data, diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S index 83d41c2..e701f2d 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 d7779a1..bf9815e 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"))