[PATCH 0/9 v4] use efficient this_cpu_* helper

* [PATCH 0/9 v4] use efficient this_cpu_* helper
@ 2012-11-13  1:51 Shan Wei
  2012-11-15 14:19 ` Christoph Lameter
  0 siblings, 1 reply; 4+ messages in thread
From: Shan Wei @ 2012-11-13  1:51 UTC (permalink / raw)
  To: cl, David Miller, NetDev, Kernel-Maillist, Shan Wei

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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