[rcu] 10a94227ba2: -2.0% will-it-scale.per_process_ops

[Fengguang Wu]

Paul,

FYI, we noticed the below changes on

git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git next.2014.04.16b
commit 10a94227ba229f1b05672754dc318a8fe7982c95 ("rcu: Update cpu_needs_another_gp() for futures from non-NOCB CPUs")

test case: nhm4/micro/will-it-scale/lseek1

11ba5ab363b9359  10a94227ba229f1b05672754d  
---------------  -------------------------  
  11210675 ~ 0%      -2.0%   10985451 ~ 0%  TOTAL will-it-scale.per_process_ops
      1.24 ~ 5%     -33.4%       0.83 ~ 5%  TOTAL perf-profile.cpu-cycles.trace_hardirqs_off_caller.lseek64
      3.88 ~ 2%     +49.0%       5.79 ~ 0%  TOTAL perf-profile.cpu-cycles.trace_hardirqs_on_thunk.lseek64
       295 ~16%     +27.0%        375 ~ 8%  TOTAL cpuidle.C1E-NHM.usage
     45061 ~ 2%     +16.7%      52590 ~ 2%  TOTAL cpuidle.C6-NHM.usage
      1.21 ~ 4%      +5.8%       1.28 ~ 4%  TOTAL perf-profile.cpu-cycles.shmem_file_llseek.sys_lseek.system_call_fastpath.lseek64
      4206 ~ 1%     -78.6%        900 ~ 8%  TOTAL interrupts.IWI
     14303 ~ 1%     +26.7%      18120 ~ 1%  TOTAL interrupts.0:IO-APIC-edge.timer
      3228 ~ 4%     -17.2%       2672 ~ 6%  TOTAL interrupts.RES
       182 ~ 2%      -8.1%        167 ~ 3%  TOTAL time.user_time
       235 ~ 2%      +6.2%        250 ~ 2%  TOTAL time.system_time
    379471 ~ 0%      +1.2%     384127 ~ 0%  TOTAL interrupts.LOC

Legend:
	~XX%    - stddev percent
	[+-]XX% - change percent

It does effectively eliminate interrupts.IWI:

                                   interrupts.IWI

   4500 ++-*-----*-------*--------------------------------------------------+
        |..   *.  +  .*.      .*..*.  .*..*..*..          .*..*..*..*.*..*..*
   4000 *+         *.       *.      *.          *.. .*..*.                  |
   3500 ++                                         *                        |
        |                                                                   |
   3000 ++                                                                  |
   2500 ++                                                                  |
        |                                                                   |
   2000 ++                                                                  |
   1500 ++                                                                  |
        |                                                                   |
   1000 ++                                              O  O  O  O    O  O  O
    500 ++                                                          O       |
        |                                                                   |
      0 O+-O--O--O-O--O--O--O--O--O-O--O--O--O--O--O-O----------------------+


	[*] bisect-good sample
	[O] bisect-bad  sample

Thanks,
Fengguang
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