Messages in this thread | | | Date | Wed, 04 Jun 2008 11:18:19 -0700 | From | Mike Travis <> | Subject | Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations |
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> cpu_local_inc() does all this: it takes the name of a local_t var, and is > expected to increment this cpu's version of that. You ripped this out and > called it CPU_INC().
Hi,
I'm attempting to test both approaches to compare the object generated in order to understand the issues involved here. Here's my code:
void test_cpu_inc(int *s) { __CPU_INC(s); }
void test_local_inc(local_t *t) { __local_inc(THIS_CPU(t)); }
void test_cpu_local_inc(local_t *t) { __cpu_local_inc(t); }
But I don't know how I can use cpu_local_inc because the pointer to the object is not &__get_cpu_var(l):
#define __cpu_local_inc(l) cpu_local_inc((l)) #define cpu_local_inc(l) cpu_local_wrap(local_inc(&__get_cpu_var((l))))
At the minimum, we would need a new local_t op to get the correct CPU_ALLOC'd pointer value for the increment. These new local_t ops for CPU_ALLOC'd variables could use CPU_XXX primitives to implement them, or just a base val_to_ptr primitive to replace __get_cpu_var().
I did notice this in local.h:
* X86_64: This could be done better if we moved the per cpu data directly * after GS.
... which it now is, so true per_cpu variables could be optimized better as well.
Also, the above cpu_local_wrap(...) adds:
#define cpu_local_wrap(l) \ ({ \ preempt_disable(); \ (l); \ preempt_enable(); \ }) \
... and there isn't a non-preemption version that I can find.
Here are the objects.
0000000000000000 <test_cpu_inc>: 0: 55 push %rbp 1: 48 89 e5 mov %rsp,%rbp 4: 48 83 ec 08 sub $0x8,%rsp 8: 48 89 7d f8 mov %rdi,0xfffffffffffffff8(%rbp) c: 65 48 ff 45 f8 incq %gs:0xfffffffffffffff8(%rbp) 11: c9 leaveq 12: c3 retq
0000000000000013 <test_local_inc>: 13: 55 push %rbp 14: 65 48 8b 05 00 00 00 mov %gs:0(%rip),%rax # 1c <test_local_inc+0x9> 1b: 00 1c: 48 89 e5 mov %rsp,%rbp 1f: 48 ff 04 07 incq (%rdi,%rax,1) 23: c9 leaveq 24: c3 retq
With a new local_t op then test_local_inc probably could be optimized to be the same instructions as test_cpu_inc.
One other distinction is CPU_INC increments an arbitrary sized variable while local_inc requires a local_t variable. This may not make it usable in all cases.
Thanks, Mike
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