smp/hotplug: Add state diagram

Add a state diagram to clarify when which states are ran where.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: bigeasy@linutronix.de
Cc: efault@gmx.de
Cc: rostedt@goodmis.org
Cc: max.byungchul.park@gmail.com
Link: https://lkml.kernel.org/r/20170920170546.661598270@infradead.org

1 files changed, 18 insertions, 0 deletions

diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index f24bfb2b9a2d..477b2e6f60f7 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -3,6 +3,24 @@
 
 #include <linux/types.h>
 
+/*
+ * CPU-up                       CPU-down
+ *
+ * BP           AP              BP              AP
+ *
+ * OFFLINE                      OFFLINE
+ *   |                            ^
+ *   v                            |
+ * BRINGUP_CPU->AP_OFFLINE      BRINGUP_CPU  <- AP_IDLE_DEAD (idle thread/play_dead)
+ *                |                             AP_OFFLINE
+ *                v (IRQ-off)     ,---------------^
+ *              AP_ONLNE          | (stop_machine)
+ *                |             TEARDOWN_CPU <- AP_ONLINE_IDLE
+ *                |                               ^
+ *                v                               |
+ *              AP_ACTIVE                       AP_ACTIVE
+ */
+
 enum cpuhp_state {
         CPUHP_OFFLINE,
         CPUHP_CREATE_THREADS,