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Peter Anvin" , Linus Torvalds , Peter Zijlstra , Thomas Gleixner , Josh Poimboeuf , Nadav Amit Subject: [PATCH -tip v3] x86/percpu: Introduce const-qualified const_pcpu_hot Date: Mon, 23 Oct 2023 16:30:43 +0200 Message-ID: <20231023143100.4014-1-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 --- v2: Export const_pcpu_hot symbol. --- v3: Rename const_pcpu_hot to const__pcpu_hot. Return const__pcpu_hot.top_of_stack from current_top_of_stack(). --- 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 a1168e7b69e5..6d8384f6d62d 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..dd10f35f2af6 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 const__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 b14fc8c1c953..d24e92ad3283 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -2049,6 +2049,7 @@ 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 54a5596adaa6..4afbadbf0ab4 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