diff options
author | Uros Bizjak <ubizjak@gmail.com> | 2023-10-20 18:19:20 +0200 |
---|---|---|
committer | Ingo Molnar <mingo@kernel.org> | 2023-10-23 11:27:35 +0200 |
commit | ed2f752e0e0a21d941ca0ee539ef3d4cd576bc5e (patch) | |
tree | 69748caac82f958df3f9667e70347911f7ebc679 | |
parent | 59bec00ace28d565ae0a68b23063ef3b961d82d5 (diff) |
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 <namit@vmware.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20231020162004.135244-1-ubizjak@gmail.com
-rw-r--r-- | arch/x86/include/asm/current.h | 7 | ||||
-rw-r--r-- | arch/x86/include/asm/percpu.h | 6 | ||||
-rw-r--r-- | arch/x86/include/asm/processor.h | 3 | ||||
-rw-r--r-- | arch/x86/kernel/cpu/common.c | 1 | ||||
-rw-r--r-- | arch/x86/kernel/vmlinux.lds.S | 1 | ||||
-rw-r--r-- | 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..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) == 64); DECLARE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot); +/* 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); } diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h index bbcc1ca737f0..b86b27d15e52 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/processor.h index 0086920cda06..b47a9975233f 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_stack(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); } diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index 382d4e6b848d..4cc0ab0dfbb5 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) = { .top_of_stack = TOP_OF_INIT_STACK, }; EXPORT_PER_CPU_SYMBOL(pcpu_hot); +EXPORT_PER_CPU_SYMBOL(const_pcpu_hot); #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 83d41c2601d7..e701f2dcea29 100644 --- a/arch/x86/kernel/vmlinux.lds.S +++ b/arch/x86/kernel/vmlinux.lds.S @@ -46,6 +46,7 @@ ENTRY(phys_startup_64) #endif jiffies = jiffies_64; +const_pcpu_hot = pcpu_hot; #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)) = (void *)&sym; + __UNIQUE_ID(__PASTE(__addressable_,sym)) = (void *)(uintptr_t)&sym; #define __ADDRESSABLE(sym) \ ___ADDRESSABLE(sym, __section(".discard.addressable")) |