Commit f7c83bcb authored Nov 13, 2012 by Shan Wei Committed by Steffen Klassert Nov 13, 2012

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 <davidshan@tencent.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

parent aafd0d88

net/xfrm/xfrm_ipcomp.c

+3 −5

Original line number	Diff line number	Diff line
		@@ -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;
		}
		}

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