Currently preferred node is set to dst_nid which is the last node in the
iteration whose group weight or task weight is greater than the current
node. However it doesn't guarantee that dst_nid has the numa capacity
to move. It also doesn't guarantee that dst_nid has the best_cpu which
is the CPU/node ideal for node migration.

Lets consider faults on a 4 node system with group weight numbers
in different nodes being in 0 < 1 < 2 < 3 proportion. Consider the task
is running on 3 and 0 is its preferred node but its capacity is full.
Consider nodes 1, 2 and 3 have capacity. Then the task should be
migrated to node 1. Currently the task gets moved to node 2. env.dst_nid
points to the last node whose faults were greater than current node.

Modify to set the preferred node based of best_cpu. Earlier setting
preferred node was skipped if nr_active_nodes is 1. This could result in
the task being moved out of the preferred node to a random node during
regular load balancing.

Also while modifying task_numa_migrate(), use sched_setnuma to set
preferred node. This ensures out numa accounting is correct.

Running SPECjbb2005 on a 4 node machine and comparing bops/JVM
JVMS  LAST_PATCH  WITH_PATCH  %CHANGE
16    25122.9     25549.6     1.698
1     73850       73190       -0.89

Running SPECjbb2005 on a 16 node machine and comparing bops/JVM
JVMS  LAST_PATCH  WITH_PATCH  %CHANGE
8     105930      113437      7.08676
1     178624      196130      9.80047

(numbers from v1 based on v4.17-rc5)
Testcase       Time:         Min         Max         Avg      StdDev
numa01.sh      Real:      435.78      653.81      534.58       83.20
numa01.sh       Sys:      121.93      187.18      145.90       23.47
numa01.sh      User:    37082.81    51402.80    43647.60     5409.75
numa02.sh      Real:       60.64       61.63       61.19        0.40
numa02.sh       Sys:       14.72       25.68       19.06        4.03
numa02.sh      User:     5210.95     5266.69     5233.30       20.82
numa03.sh      Real:      746.51      808.24      780.36       23.88
numa03.sh       Sys:       97.26      108.48      105.07        4.28
numa03.sh      User:    58956.30    61397.05    60162.95     1050.82
numa04.sh      Real:      465.97      519.27      484.81       19.62
numa04.sh       Sys:      304.43      359.08      334.68       20.64
numa04.sh      User:    37544.16    41186.15    39262.44     1314.91
numa05.sh      Real:      411.57      457.20      433.29       16.58
numa05.sh       Sys:      230.05      435.48      339.95       67.58
numa05.sh      User:    33325.54    36896.31    35637.84     1222.64

Testcase       Time:         Min         Max         Avg      StdDev 	 %Change
numa01.sh      Real:      506.35      794.46      599.06      104.26 	 -10.76%
numa01.sh       Sys:      150.37      223.56      195.99       24.94 	 -25.55%
numa01.sh      User:    43450.69    61752.04    49281.50     6635.33 	 -11.43%
numa02.sh      Real:       60.33       62.40       61.31        0.90 	 -0.195%
numa02.sh       Sys:       18.12       31.66       24.28        5.89 	 -21.49%
numa02.sh      User:     5203.91     5325.32     5260.29       49.98 	 -0.513%
numa03.sh      Real:      696.47      853.62      745.80       57.28 	 4.6339%
numa03.sh       Sys:       85.68      123.71       97.89       13.48 	 7.3347%
numa03.sh      User:    55978.45    66418.63    59254.94     3737.97 	 1.5323%
numa04.sh      Real:      444.05      514.83      497.06       26.85 	 -2.464%
numa04.sh       Sys:      230.39      375.79      316.23       48.58 	 5.8343%
numa04.sh      User:    35403.12    41004.10    39720.80     2163.08 	 -1.153%
numa05.sh      Real:      423.09      460.41      439.57       13.92 	 -1.428%
numa05.sh       Sys:      287.38      480.15      369.37       68.52 	 -7.964%
numa05.sh      User:    34732.12    38016.80    36255.85     1070.51 	 -1.704%
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1529514181-9842-5-git-send-email-srikar@linux.vnet.ibm.comSigned-off-by: Ingo Molnar <mingo@kernel.org>