Hi Paul,

Just FYI, we noticed the following changes (which looks good) on old commit
c0f4dfd4f9 ("rcu: Make RCU_FAST_NO_HZ take advantage of numbered callbacks") 
in test case dd-write/4HDD-JBOD-cfq-btrfs-1dd:

b11cc5 (parent)  c0f4dfd4f90f1667d234d21f1  
---------------  -------------------------  
    213757 ~ 4%     -65.4%      73929 ~ 3%  softirqs.RCU
     21193 ~ 5%     -36.5%      13451 ~ 4%  softirqs.SCHED
      2036 ~ 4%     -59.4%        825 ~ 3%  vmstat.system.cs
   1304520 ~ 4%     -59.2%     532451 ~ 3%  perf-stat.context-switches
     95685 ~ 4%     -44.0%      53598 ~ 2%  perf-stat.cpu-migrations

The "~ N%" numbers are stddev percent.

                               perf-stat.cpu-migrations

   120000 ++----------------------------------------------------------------+
          |                                                                 |
   110000 ++          *     *    *.       *   **.                           |
   100000 *+ .*    *  :+    ::  +  *.*   : : :   *    .*                    |
          | *  *. + +:  *  : : *      :  : : :    + .*  :                   |
    90000 ++     *   *   + :  *       : *   *      *    *.*                 |
          |               *            *                                    |
    80000 ++                                                                |
          |                                                                 |
    70000 ++                                                                |
    60000 ++                                                                |
          | O        O  O O O  O O O        O  O   O   O    O   O    O      O
    50000 O+  OO O O  O       O      O OO O               O   O  O O   OO O |
          |                                   O  O   O  O                   |
    40000 ++----------------------------------------------------------------+


                              perf-stat.context-switches

   1.5e+06 ++---------------------------------------------------------------+
   1.4e+06 ++       *  *     *    *.       *  *.*.*    *                    |
           *. .**  : + :+   + *. +  **    + :+     :  + +                   |
   1.3e+06 ++*   + :  *  *.*    *     + .*  *      *.*   **                 |
   1.2e+06 ++     *                    *                                    |
   1.1e+06 ++                                                               |
     1e+06 ++                                                               |
           |                                                                |
    900000 ++                                                               |
    800000 ++                                                               |
    700000 ++                                                               |
    600000 ++O               O  O                                           |
           O   OO O   OO O O  O   O OO O O OO O O OO O O OO O O O  O O    O O
    500000 ++       O                                            O     OO   |
    400000 ++---------------------------------------------------------------+


                                  vmstat.system.cs

   2400 ++------------------------------------------------------------------+
        |            *    *.    *.       *   *. *     *                     |
   2200 *+ .*.*  *. + +   : *. +  *.*   + + +  * +   + +                    |
   2000 ++*    :+  *   *.*    *      *.*   *      *.*   *.*                 |
        |      *                                                            |
   1800 ++                                                                  |
   1600 ++                                                                  |
        |                                                                   |
   1400 ++                                                                  |
   1200 ++                                                                  |
        |                                                                   |
   1000 ++O              OO   O                                             |
    800 O+  O OO   O O O    O   O O  O O O O O OO O O O O OO O O   O O  O O O
        |        O                  O                            O    O     |
    600 ++------------------------------------------------------------------+

Thanks,
Fengguang