[RFC PATCH 47/86] rcu: select PREEMPT_RCU if PREEMPT

[Ankur Arora]

With PREEMPTION being always-on, some configurations might prefer
the stronger forward-progress guarantees provided by PREEMPT_RCU=n
as compared to PREEMPT_RCU=y.

So, select PREEMPT_RCU=n for PREEMPT_VOLUNTARY and PREEMPT_NONE and
enabling PREEMPT_RCU=y for PREEMPT or PREEMPT_RT.

Note that the preemption model can be changed at runtime (modulo
configurations with ARCH_NO_PREEMPT), but the RCU configuration
is statically compiled.

Cc: Simon Horman <horms@xxxxxxxxxxxx>
Cc: Julian Anastasov <ja@xxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>

---
CC-note: Paul had flagged some code that might be impacted
with the proposed RCU changes:

1. My guess is that the IPVS_EST_TICK_CHAINS heuristic remains
   unchanged, but I must defer to the include/net/ip_vs.h people.

2. I need to check with the BPF folks on the BPF verifier's
   definition of BTF_ID(func, rcu_read_unlock_strict).

3. I must defer to others on the mm/pgtable-generic.c file's
   #ifdef that depends on CONFIG_PREEMPT_RCU.

Detailed here:
 https://lore.kernel.org/lkml/a375674b-de27-4965-a4bf-e0679229e28e@paulmck-laptop/

---
 include/linux/rcutree.h | 2 +-
 kernel/rcu/Kconfig      | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 126f6b418f6a..75aaa6294421 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -104,7 +104,7 @@ extern int rcu_scheduler_active;
 void rcu_end_inkernel_boot(void);
 bool rcu_inkernel_boot_has_ended(void);
 bool rcu_is_watching(void);
-#ifndef CONFIG_PREEMPTION
+#ifndef CONFIG_PREEMPT
 void rcu_all_qs(void);
 #endif
 
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index bdd7eadb33d8..a808cb29ab7c 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -18,7 +18,7 @@ config TREE_RCU
 
 config PREEMPT_RCU
 	bool
-	default y if PREEMPTION
+	default y if PREEMPT || PREEMPT_RT
 	select TREE_RCU
 	help
 	  This option selects the RCU implementation that is
@@ -31,7 +31,7 @@ config PREEMPT_RCU
 
 config TINY_RCU
 	bool
-	default y if !PREEMPTION && !SMP
+	default y if !PREEMPT && !SMP
 	help
 	  This option selects the RCU implementation that is
 	  designed for UP systems from which real-time response
-- 
2.31.1