Steffen Klassert said, at 2012/11/13 18:48:
>
> Ok, so please add a commit message to describe your changes.
>
> Thanks.
>
[PATCH v5] net: xfrm: use __this_cpu_read per-cpu helper
this_cpu_ptr/this_cpu_read is faster than per_cpu_ptr(p, smp_processor_id())
and can reduce memory accesses.
The latter helper needs to find the offset for current cpu,
and needs more assembler instructions which objdump shows in following.
this_cpu_ptr relocates and address. this_cpu_read() relocates the address
and performs the fetch. this_cpu_read() saves you more instructions
since it can do the relocation and the fetch in one instruction.
per_cpu_ptr(p, smp_processor_id())：
1e: 65 8b 04 25 00 00 00 00 mov %gs:0x0,%eax
26: 48 98 cltq
28: 31 f6 xor %esi,%esi
2a: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
31: 48 8b 04 c5 00 00 00 00 mov 0x0(,%rax,8),%rax
39: c7 44 10 04 14 00 00 00 movl $0x14,0x4(%rax,%rdx,1)
this_cpu_ptr(p)
1e: 65 48 03 14 25 00 00 00 00 add %gs:0x0,%rdx
27: 31 f6 xor %esi,%esi
29: c7 42 04 14 00 00 00 movl $0x14,0x4(%rdx)
30: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
Signed-off-by: Shan Wei <davids...@tencent.com>
---
net/xfrm/xfrm_ipcomp.c | 8 +++-----
1 files changed, 3 insertions(+), 5 deletions(-)
diff --git a/net/xfrm/xfrm_ipcomp.c b/net/xfrm/xfrm_ipcomp.c
index e5246fb..2906d52 100644
--- a/net/xfrm/xfrm_ipcomp.c
+++ b/net/xfrm/xfrm_ipcomp.c
@@ -276,18 +276,16 @@ static struct crypto_comp * __percpu
*ipcomp_alloc_tfms(const char *alg_name)
struct crypto_comp * __percpu *tfms;
int cpu;
- /* This can be any valid CPU ID so we don't need locking. */
- cpu = raw_smp_processor_id();
list_for_each_entry(pos, &ipcomp_tfms_list, list) {
struct crypto_comp *tfm;
- tfms = pos->tfms;
- tfm = *per_cpu_ptr(tfms, cpu);
+ /* This can be any valid CPU ID so we don't need locking. */
+ tfm = __this_cpu_read(*pos->tfms);
if (!strcmp(crypto_comp_name(tfm), alg_name)) {
pos->users++;
- return tfms;
+ return pos->tfms;
}
}
--
1.7.1
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