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Subject[PATCH 0/9 v4] use efficient this_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. 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 V4:
1. [read|write]ing fields of struct rds_ib_cache_head using __this_cpu_* operation for rds subsystem.
see patch2
2. fix bug in xfrm to read pointer. see patch3.
3. avoid type cast in patch7.

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 d4185bbf62a5d8d777ee445db1581beb17882a07
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.h | 2 +-
net/rds/ib_recv.c | 24 +++++++++++++-----------
net/xfrm/xfrm_ipcomp.c | 8 +++-----
12 files changed, 29 insertions(+), 38 deletions(-)
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