rcu: Add BOOTPARAM_RCU_STALL_PANIC Kconfig option

Add a Kconfig option to set the default value of the
kernel.panic_on_rcu_stall sysctl, allowing the kernel to be built
with panic-on-RCU-stall enabled by default.

This is useful for high-availability systems that require automatic
recovery (via panic_timeout) when a CPU stall is detected, without
needing userspace to configure the sysctl at boot.

This follows the pattern established by BOOTPARAM_SOFTLOCKUP_PANIC
and BOOTPARAM_HUNG_TASK_PANIC.  The runtime sysctl can still override
the Kconfig default.

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>

Author: gustavold

kernel/rcu/Kconfig.debug

@@ -175,6 +175,30 @@ config RCU_CPU_STALL_NOTIFIER
 	  Say Y here if you want RCU CPU stall notifiers (you don't want them)
 	  Say N if you are unsure.
 
+config BOOTPARAM_RCU_STALL_PANIC
+	bool "Panic (reboot) on RCU CPU stall"
+	depends on RCU_STALL_COMMON
+	default n
+	help
+	  Say Y here to enable the kernel to panic when an RCU CPU stall
+	  is detected.
+
+	  The panic can be used in combination with panic_timeout,
+	  to cause the system to reboot automatically after an
+	  RCU CPU stall has been detected. This feature is useful for
+	  high-availability systems that have uptime guarantees and
+	  where a CPU stall must be resolved ASAP.
+
+	  The kernel.max_rcu_stall_to_panic sysctl can be used to set
+	  a minimum number of stalls before panicking, allowing the
+	  system to tolerate a given number of RCU CPU stalls before
+	  triggering a panic.
+
+	  This setting can be overridden at runtime via the
+	  kernel.panic_on_rcu_stall sysctl parameter.
+
+	  Say N if unsure.
+
 config RCU_TRACE
 	bool "Enable tracing for RCU"
 	depends on DEBUG_KERNEL

kernel/rcu/tree_stall.h

@@ -17,7 +17,7 @@
 // Controlling CPU stall warnings, including delay calculation.
 
 /* panic() on RCU Stall sysctl. */
-static int sysctl_panic_on_rcu_stall __read_mostly;
+static int sysctl_panic_on_rcu_stall __read_mostly = IS_ENABLED(CONFIG_BOOTPARAM_RCU_STALL_PANIC);
 static int sysctl_max_rcu_stall_to_panic __read_mostly;
 
 static const struct ctl_table rcu_stall_sysctl_table[] = {