Messages in this thread | | | Date | Sun, 04 Nov 2012 18:38:29 +0800 | From | Shan Wei <> | Subject | Re: [PATCH v2 6/9] rcu: use __this_cpu_read helper instead of per_cpu_ptr(p, raw_smp_processor_id()) |
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Paul E. McKenney said, at 2012/11/3 17:19: > OK, I do understand why it happens to work. My question is instead why > it is considered a good idea.
Maybe objdump gives the answer. __this_cpu_read which read member pointer of per-cpu variable can reduce two instructions on x86-64 arch.
*test code:* struct eater_state { u32 state; struct eater __percpu *eater_info; };
struct eater { char name[4]; u32 age; };
static u32 test_func(struct eater_state *tstas) { struct eater *aeater;
//aeater = __this_cpu_ptr(tstas->eater_info); <-----------------1 //return aeater->age; return __this_cpu_read(tstas->eater_info->age); <-----------------2 }
static int __init demo_init(void) { int ret = 0 ; int age; struct eater_state as; struct eater david;
as.state = 1; as.eater_info = &david;
age = test_func(&as);
return ret; }
__this_cpu_ptr <-----------------1 0000000000000000 <init_module>: 0: 55 push %rbp 1: 48 89 e5 mov %rsp,%rbp 4: 48 83 ec 10 sub $0x10,%rsp 8: 48 8d 45 f0 lea -0x10(%rbp),%rax c: 65 48 03 04 25 00 00 00 00 add %gs:0x0,%rax 15: 31 c0 xor %eax,%eax 17: c9 leaveq 18: c3 retq
__this_cpu_read<-----------------2 0000000000000000 <init_module>: 0: 55 push %rbp 1: 31 c0 xor %eax,%eax 3: 48 89 e5 mov %rsp,%rbp 6: 48 83 ec 10 sub $0x10,%rsp a: c9 leaveq b: c3 retq
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