powerpc: Always enable queued spinlocks for 64s, disable for others

Queued spinlocks have shown to have good performance and fairness
properties even on smaller (2 socket) POWER systems. This selects them
automatically for 64s. For other platforms they are de-selected, the
standard spinlock is far simpler and smaller code, and single chips
with a handful of cores is unlikely to show any improvement.

CONFIG_EXPERT still allows this to be changed, e.g., to help debug
performance or correctness issues.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210118123451.1452206-1-npiggin@gmail.com

Author: npiggin

arch/powerpc/Kconfig

@@ -505,18 +505,14 @@ config HOTPLUG_CPU
 	  Say N if you are unsure.
 
 config PPC_QUEUED_SPINLOCKS
-	bool "Queued spinlocks"
+	bool "Queued spinlocks" if EXPERT
 	depends on SMP
+	default PPC_BOOK3S_64
 	help
 	  Say Y here to use queued spinlocks which give better scalability and
 	  fairness on large SMP and NUMA systems without harming single threaded
 	  performance.
 
-	  This option is currently experimental, the code is more complex and
-	  less tested so it defaults to "N" for the moment.
-
-	  If unsure, say "N".
-
 config ARCH_CPU_PROBE_RELEASE
 	def_bool y
 	depends on HOTPLUG_CPU