Subject: Re: [PATCH v5 3/9] net: xfrm: use __this_cpu_read per-cpu helper

From: Shan Wei <[email-masked]>
Date: Tue, 13 Nov 2012 20:36:00 +0800

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 <[email-masked]>
---
 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