[PATCH PREEMPT_RT 4.19 STABLE] rt: PREEMPT_RT safety net for backported patches

[Clark Williams <clark.williams@gmail.com>]

While doing some 4.19-rt cleanup work, I stumbled across the fact that parts of
two backported patches were dependent on CONFIG_PREEMPT_RT, rather than
the CONFIG_PREEMPT_RT_FULL used in 4.19 and earlier RT series.  The commits
in the linux-stable-rt v4.19-rt branch are:

dad4c6a33bf4e mm: slub: Don't resize the location tracking cache on PREEMPT_RT
e626b6f873f2c net: Treat __napi_schedule_irqoff() as __napi_schedule() on PREEMPT_RT

Discussing this at the Stable RT maintainers meeting, Steven Rostedt suggested that
we automagically select CONFIG_PREEMPT_RT if CONFIG_PREEMPT_RT_FULL is on, giving
us a safety net for any subsequently backported patches. Here's my first cut at
that patch.

I suspect we'll need a similar patch for stable RT kernels < 4.19.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Clark Williams <williams@redhat.com>
---
 kernel/Kconfig.preempt | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index 306567f72a3e..951f58cbd652 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -6,6 +6,10 @@ config PREEMPT_RT_BASE
 	bool
 	select PREEMPT
 
+config PREEMPT_RT
+       bool
+       default false
+
 config HAVE_PREEMPT_LAZY
 	bool
 
@@ -80,6 +84,7 @@ config PREEMPT_RT_FULL
 	depends on IRQ_FORCED_THREADING
 	select PREEMPT_RT_BASE
 	select PREEMPT_RCU
+	select PREEMPT_RT
 	help
 	  All and everything
 
-- 
2.34.1