rcutorture: Export RCU grace-period kthread wait state to rcutorture

This commit allows rcutorture to print additional state for the
RCU grace-period kthreads in cases where RCU seems reluctant to
start a new grace period.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

include/linux/rcutiny.h

@@ -119,6 +119,10 @@ static inline void rcu_sched_force_quiescent_state(void)
 {
 }
 
+static inline void show_rcu_gp_kthreads(void)
+{
+}
+
 static inline void rcu_cpu_stall_reset(void)
 {
 }

include/linux/rcutree.h

@@ -84,6 +84,7 @@ extern unsigned long rcutorture_vernum;
 long rcu_batches_completed(void);
 long rcu_batches_completed_bh(void);
 long rcu_batches_completed_sched(void);
+void show_rcu_gp_kthreads(void);
 
 void rcu_force_quiescent_state(void);
 void rcu_bh_force_quiescent_state(void);

kernel/rcu/rcutorture.c

@@ -1034,6 +1034,7 @@ rcu_torture_printk(char *page)
 				"??? Writer stall state %d g%lu c%lu f%#x\n",
 				rcu_torture_writer_state,
 				gpnum, completed, flags);
+		show_rcu_gp_kthreads();
 		rcutorture_trace_dump();
 	}
 	rtcv_snap = rcu_torture_current_version;

kernel/rcu/tree.c

@@ -279,6 +279,21 @@ void rcu_bh_force_quiescent_state(void)
 }
 EXPORT_SYMBOL_GPL(rcu_bh_force_quiescent_state);
 
+/*
+ * Show the state of the grace-period kthreads.
+ */
+void show_rcu_gp_kthreads(void)
+{
+	struct rcu_state *rsp;
+
+	for_each_rcu_flavor(rsp) {
+		pr_info("%s: wait state: %d ->state: %#lx\n",
+			rsp->name, rsp->gp_state, rsp->gp_kthread->state);
+		/* sched_show_task(rsp->gp_kthread); */
+	}
+}
+EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
+
 /*
  * Record the number of times rcutorture tests have been initiated and
  * terminated.  This information allows the debugfs tracing stats to be
@@ -1626,6 +1641,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
 			trace_rcu_grace_period(rsp->name,
 					       ACCESS_ONCE(rsp->gpnum),
 					       TPS("reqwait"));
+			rsp->gp_state = RCU_GP_WAIT_GPS;
 			wait_event_interruptible(rsp->gp_wq,
 						 ACCESS_ONCE(rsp->gp_flags) &
 						 RCU_GP_FLAG_INIT);
@@ -1653,6 +1669,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
 			trace_rcu_grace_period(rsp->name,
 					       ACCESS_ONCE(rsp->gpnum),
 					       TPS("fqswait"));
+			rsp->gp_state = RCU_GP_WAIT_FQS;
 			ret = wait_event_interruptible_timeout(rsp->gp_wq,
 					((gf = ACCESS_ONCE(rsp->gp_flags)) &
 					 RCU_GP_FLAG_FQS) ||

kernel/rcu/tree.h

@@ -406,7 +406,8 @@ struct rcu_state {
 	unsigned long completed;		/* # of last completed gp. */
 	struct task_struct *gp_kthread;		/* Task for grace periods. */
 	wait_queue_head_t gp_wq;		/* Where GP task waits. */
-	int gp_flags;				/* Commands for GP task. */
+	short gp_flags;				/* Commands for GP task. */
+	short gp_state;				/* GP kthread sleep state. */
 
 	/* End of fields guarded by root rcu_node's lock. */
 
@@ -469,6 +470,11 @@ struct rcu_state {
 #define RCU_GP_FLAG_INIT 0x1	/* Need grace-period initialization. */
 #define RCU_GP_FLAG_FQS  0x2	/* Need grace-period quiescent-state forcing. */
 
+/* Values for rcu_state structure's gp_flags field. */
+#define RCU_GP_WAIT_INIT 0	/* Initial state. */
+#define RCU_GP_WAIT_GPS  1	/* Wait for grace-period start. */
+#define RCU_GP_WAIT_FQS  2	/* Wait for force-quiescent-state time. */
+
 extern struct list_head rcu_struct_flavors;
 
 /* Sequence through rcu_state structures for each RCU flavor. */