sched/numa: Limit the conditions where scan period is reset

migrate_task_rq_fair resets the scan rate for NUMA balancing on every
cross-node migration. In the event of excessive load balancing due to
saturation, this may result in the scan rate being pegged at maximum and
further overloading the machine.

This patch only resets the scan if NUMA balancing is active, a preferred
node has been selected and the task is being migrated from the preferred
node as these are the most harmful. For example, a migration to the preferred
node does not justify a faster scan rate. Similarly, a migration between two
nodes that are not preferred is probably bouncing due to over-saturation of
the machine.  In that case, scanning faster and trapping more NUMA faults
will further overload the machine.

specjbb2005 / bops/JVM / higher bops are better
on 2 Socket/2 Node Intel
JVMS  Prev    Current  %Change
4     208862  209029   0.0799571
1     307007  326585   6.37705

on 2 Socket/4 Node Power8 (PowerNV)
JVMS  Prev     Current  %Change
8     89911.4  89627.8  -0.315422
1     216176   221299   2.36983

on 2 Socket/2 Node Power9 (PowerNV)
JVMS  Prev    Current  %Change
4     196078  195444   -0.323341
1     214664  222390   3.59911

on 4 Socket/4 Node Power7
JVMS  Prev     Current  %Change
8     60719.2  60152.4  -0.933477
1     112615   111458   -1.02739

dbench / transactions / higher numbers are better
on 2 Socket/2 Node Intel
count  Min      Max      Avg      Variance  %Change
5      12511.7  12559.4  12539.5  15.5883
5      12904.6  12969    12942.6  23.9053   3.21464

on 2 Socket/4 Node Power8 (PowerNV)
count  Min      Max      Avg      Variance  %Change
5      4709.28  4979.28  4919.32  105.126
5      4984.25  5025.95  5004.5   14.2253   1.73154

on 2 Socket/2 Node Power9 (PowerNV)
count  Min      Max      Avg      Variance  %Change
5      9388.38  9406.29  9395.1   5.98959
5      9277.64  9357.22  9322.07  26.3558   -0.77732

on 4 Socket/4 Node Power7
count  Min      Max      Avg      Variance  %Change
5      157.71   184.929  174.754  10.7275
5      160.632  175.558  168.655  5.26823   -3.49005

Cc: Rik van Riel <riel@surriel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1533276841-16341-7-git-send-email-srikar@linux.vnet.ibm.com