sched: zap the migration init / cache-hot balancing code

the SMP load-balancer uses the boot-time migration-cost estimation
code to attempt to improve the quality of balancing. The reason for
this code is that the discrete priority queues do not preserve
the order of scheduling accurately, so the load-balancer skips
tasks that were running on a CPU 'recently'.

this code is fundamental fragile: the boot-time migration cost detector
doesnt really work on systems that had large L3 caches, it caused boot
delays on large systems and the whole cache-hot concept made the
balancing code pretty undeterministic as well.

(and hey, i wrote most of it, so i can say it out loud that it sucks ;-)

under CFS the same purpose of cache affinity can be achieved without
any special cache-hot special-case: tasks are sorted in the 'timeline'
tree and the SMP balancer picks tasks from the left side of the
tree, thus the most cache-cold task is balanced automatically.

Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 0 insertions, 43 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index af50f9bbe68e..4d880b3d1f35 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1014,49 +1014,6 @@ and is between 256 and 4096 characters. It is defined in the file
 
         mga=            [HW,DRM]
 
-        migration_cost=
-                        [KNL,SMP] debug: override scheduler migration costs
-                        Format: <level-1-usecs>,<level-2-usecs>,...
-                        This debugging option can be used to override the
-                        default scheduler migration cost matrix. The numbers
-                        are indexed by 'CPU domain distance'.
-                        E.g. migration_cost=1000,2000,3000 on an SMT NUMA
-                        box will set up an intra-core migration cost of
-                        1 msec, an inter-core migration cost of 2 msecs,
-                        and an inter-node migration cost of 3 msecs.
-
-                        WARNING: using the wrong values here can break
-                        scheduler performance, so it's only for scheduler
-                        development purposes, not production environments.
-
-        migration_debug=
-                        [KNL,SMP] migration cost auto-detect verbosity
-                        Format=<0|1|2>
-                        If a system's migration matrix reported at bootup
-                        seems erroneous then this option can be used to
-                        increase verbosity of the detection process.
-                        We default to 0 (no extra messages), 1 will print
-                        some more information, and 2 will be really
-                        verbose (probably only useful if you also have a
-                        serial console attached to the system).
-
-        migration_factor=
-                        [KNL,SMP] multiply/divide migration costs by a factor
-                        Format=<percent>
-                        This debug option can be used to proportionally
-                        increase or decrease the auto-detected migration
-                        costs for all entries of the migration matrix.
-                        E.g. migration_factor=150 will increase migration
-                        costs by 50%. (and thus the scheduler will be less
-                        eager migrating cache-hot tasks)
-                        migration_factor=80 will decrease migration costs
-                        by 20%. (thus the scheduler will be more eager to
-                        migrate tasks)
-
-                        WARNING: using the wrong values here can break
-                        scheduler performance, so it's only for scheduler
-                        development purposes, not production environments.
-
         mousedev.tap_time=
                         [MOUSE] Maximum time between finger touching and
                         leaving touchpad surface for touch to be considered