kernel/rcu/tree_stall: add /sys/kernel/rcu_stall_count

Expose a simple counter to userspace for monitoring tools.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Max Kellermann <[email protected]>
Cc: Core Minyard <[email protected]>
Cc: Doug Anderson <[email protected]>
Cc: Joel Granados <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Song Liu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: MaxKellermann

Documentation/ABI/testing/sysfs-kernel-rcu_stall_count

@@ -0,0 +1,6 @@
+What:		/sys/kernel/rcu_stall_count
+Date:		May 2025
+KernelVersion:	6.16
+Contact:	Linux kernel mailing list <[email protected]>
+Description:
+		Shows how many times the system has detected an RCU stall since last boot.