[PATCH] Introduce CONFIG_DEFAULT_MIGRATION_COST

Heiko Carstens <[email protected]> wrote:

  The boot sequence on s390 sometimes takes ages and we spend a very long
  time (up to one or two minutes) in calibrate_migration_costs.  The time
  spent there differs from boot to boot.  Also the calculated costs differ
  a lot.  I've seen differences by up to a factor of 15 (yes, factor not
  percent).  Also I doubt that making these measurements make much sense on
  a completely virtualized architecture where you cannot tell how much cpu
  time you will get anyway.

So introduce the CONFIG_DEFAULT_MIGRATION_COST method for an architecture
to set the scheduler migration costs.  This turns off automatic detection
of migration costs.  Makes sense on virtual platforms, where migration
costs are hard to measure accurately.

Signed-off-by: Ingo Molnar <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

arch/s390/Kconfig

@@ -80,6 +80,10 @@ config HOTPLUG_CPU
 	  can be controlled through /sys/devices/system/cpu/cpu#.
 	  Say N if you want to disable CPU hotplug.
 
+config DEFAULT_MIGRATION_COST
+	int
+	default "1000000"
+
 config MATHEMU
 	bool "IEEE FPU emulation"
 	depends on MARCH_G5

kernel/sched.c

@@ -5058,7 +5058,18 @@ static void init_sched_build_groups(struct sched_group groups[], cpumask_t span,
 #define MAX_DOMAIN_DISTANCE 32
 
 static unsigned long long migration_cost[MAX_DOMAIN_DISTANCE] =
-		{ [ 0 ... MAX_DOMAIN_DISTANCE-1 ] = -1LL };
+		{ [ 0 ... MAX_DOMAIN_DISTANCE-1 ] =
+/*
+ * Architectures may override the migration cost and thus avoid
+ * boot-time calibration. Unit is nanoseconds. Mostly useful for
+ * virtualized hardware:
+ */
+#ifdef CONFIG_DEFAULT_MIGRATION_COST
+			CONFIG_DEFAULT_MIGRATION_COST
+#else
+			-1LL
+#endif
+};
 
 /*
  * Allow override of migration cost - in units of microseconds.