mm: Allow only SLUB on RT

Memory allocation disables interrupts as part of the allocation and freeing
process. For -RT it is important that this section remain short and don't
depend on the size of the request or an internal state of the memory allocator.
At the beginning the SLAB memory allocator was adopted for RT's needs and it
required substantial changes. Later, with the addition of the SLUB memory
allocator we adopted this one as well and the changes were smaller. More
important, due to the design of the SLUB allocator it performs better and its
worst case latency was smaller. In the end only SLUB remained supported.

Disable SLAB and SLOB on -RT. Only SLUB is adopted to -RT needs.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

init/Kconfig

@@ -1896,6 +1896,7 @@ choice
 
 config SLAB
 	bool "SLAB"
+	depends on !PREEMPT_RT
 	select HAVE_HARDENED_USERCOPY_ALLOCATOR
 	help
 	  The regular slab allocator that is established and known to work
@@ -1916,6 +1917,7 @@ config SLUB
 config SLOB
 	depends on EXPERT
 	bool "SLOB (Simple Allocator)"
+	depends on !PREEMPT_RT
 	help
 	   SLOB replaces the stock allocator with a drastically simpler
 	   allocator. SLOB is generally more space efficient but