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. If you want to operate on rda(defined as per_cpu)
then you can only use this_cpu_ptr. 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
Changelog V3:
1. use this_cpu_read directly read member of per-cpu variable,
so that droping the this_cpu_ptr operation.
2. for preemption off and bottom halves off case,
use __this_cpu_read instead of this_cpu_read.
Changelog V2:
1. Use this_cpu_read directly instead of ref to field of per-cpu variable.
2. Patch5 about ftrace is dropped from this series.
3. Add new patch9 to replace get_cpu;per_cpu_ptr;put_cpu with this_cpu_add opt.
4. For preemption disable case, use __this_cpu_read instead.
$ git diff --stat 7da716aee2532399e213a14f656d304098f67a11..
drivers/clocksource/arm_generic.c | 2 +-
kernel/padata.c | 5 ++---
kernel/rcutree.c | 2 +-
kernel/trace/blktrace.c | 2 +-
kernel/trace/trace.c | 5 +----
net/batman-adv/main.h | 4 +---
net/core/flow.c | 4 +---
net/openvswitch/datapath.c | 4 ++--
net/openvswitch/vport.c | 5 ++---
net/rds/ib_recv.c | 2 +-
net/xfrm/xfrm_ipcomp.c | 7 ++-----
11 files changed, 15 insertions(+), 27 deletions(-)