[rcu] c0f4dfd4f9: -53% perf-stat.cpu-migrations

[Fengguang Wu <fengguang.wu@intel.com>]

Hi Paul,

Just FYI, we noticed -53% perf-stat.cpu-migrations in dd write tests
on btrfs, which looks good. First good commit is

commit c0f4dfd4f90f1667d234d21f15153ea09a2eaa66
Author:     Paul E. McKenney <paul.mckenney@linaro.org>
AuthorDate: Fri Dec 28 11:30:36 2012 -0800
Commit:     Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CommitDate: Tue Mar 26 08:04:51 2013 -0700

    rcu: Make RCU_FAST_NO_HZ take advantage of numbered callbacks
    
    Because RCU callbacks are now associated with the number of the grace
    period that they must wait for, CPUs can now take advance callbacks
    corresponding to grace periods that ended while a given CPU was in
    dyntick-idle mode.  This eliminates the need to try forcing the RCU
    state machine while entering idle, thus reducing the CPU intensiveness
    of RCU_FAST_NO_HZ, which should increase its energy efficiency.
    
    Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

 Documentation/kernel-parameters.txt |  28 ++-
 include/linux/rcupdate.h            |   1 +
 init/Kconfig                        |  17 +-
 kernel/rcutree.c                    |  28 +--
 kernel/rcutree.h                    |  12 +-
 kernel/rcutree_plugin.h             | 374 ++++++++++--------------------------
 kernel/rcutree_trace.c              |   2 -
 7 files changed, 149 insertions(+), 313 deletions(-)

b11cc5760a9c48c  c0f4dfd4f90f1667d234d21f1  
---------------  -------------------------  
     86878 ~138%     -90.3%       8397 ~152%   cpuidle.POLL.time
       154 ~16%     -87.3%         19 ~55%   cpuidle.POLL.usage
  12177976 ~ 4%     -85.6%    1748244 ~20%   cpuidle.C1-NHM.time
    381439 ~ 3%     -68.4%     120538 ~ 2%   softirqs.RCU
      0.53 ~87%    +161.8%       1.40 ~16%   perf-profile.cpu-cycles.copy_user_generic_string.__btrfs_buffered_write.btrfs_file_aio_write.do_sync_write.vfs_write
   5227241 ~ 4%     -58.3%    2180928 ~ 7%   cpuidle.C1E-NHM.time
      0.67 ~88%     +88.6%       1.26 ~21%   perf-profile.cpu-cycles.calc_csum_metadata_size.btrfs_delalloc_release_metadata.btrfs_clear_bit_hook.clear_state_bit.clear_extent_bit
    231531 ~ 2%     -48.3%     119653 ~ 2%   interrupts.LOC
     91019 ~ 2%     -40.2%      54404 ~ 2%   cpuidle.C3-NHM.usage
 1.991e+08 ~ 3%     -36.7%   1.26e+08 ~ 7%   cpuidle.C3-NHM.time
      7.07 ~ 4%     -32.7%       4.76 ~ 8%   turbostat.%c3
     23380 ~33%     +41.2%      33024 ~ 6%   proc-vmstat.kswapd_low_wmark_hit_quickly
     62805 ~ 3%     -28.4%      44960 ~ 2%   softirqs.SCHED
     64678 ~ 1%     -30.1%      45195 ~ 1%   softirqs.TIMER
     55051 ~ 3%     -22.2%      42823 ~ 2%   interrupts.0:IO-APIC-edge.timer
       920 ~ 4%     +19.2%       1097 ~ 5%   slabinfo.kmalloc-512.active_objs
       920 ~ 4%     +19.2%       1097 ~ 5%   slabinfo.kmalloc-512.num_objs
    361987 ~ 2%      +9.9%     397730 ~ 0%   cpuidle.C6-NHM.usage
      5.30 ~ 1%      -9.7%       4.78 ~ 1%   turbostat.%c1
    178105 ~ 3%     -53.5%      82837 ~ 1%   perf-stat.cpu-migrations
      5763 ~ 8%     -44.7%       3186 ~22%   vmstat.system.cs
   3566268 ~ 8%     -44.8%    1968744 ~21%   perf-stat.context-switches
       658 ~ 2%     -30.4%        458 ~ 0%   vmstat.system.in
  53376814 ~12%     -24.2%   40482438 ~22%   perf-stat.node-load-misses
 2.996e+10 ~ 3%     -10.8%  2.672e+10 ~ 3%   perf-stat.L1-icache-load-misses
 1.998e+09 ~ 4%     -11.6%  1.766e+09 ~ 2%   perf-stat.branch-misses
 1.005e+12 ~ 5%     -11.9%  8.852e+11 ~ 6%   perf-stat.stalled-cycles-frontend
 6.344e+08 ~ 2%      -6.8%  5.915e+08 ~ 2%   perf-stat.LLC-store-misses
 2.892e+10 ~ 2%      +5.4%  3.047e+10 ~ 3%   perf-stat.bus-cycles


                              perf-stat.cpu-migrations

   90000 ++*----*----------*-*---*------*-----------------------------------+
         *   **   *.*.**.*     *  *.*.*   **.*.*.*.*   .*.                  |
   80000 ++                                         *.*   *                 |
         |                                                                  |
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   60000 ++                                                                 |
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   50000 ++                                                                 |
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   40000 ++                                                                 |
         | O O                                                              |
   30000 O+     O O O OO O O O O OO O O O OO O O O OO O O O O OO O O O OO O O
         |    O                                                             |
   20000 ++-----------------------------------------------------------------+