rcu: Add GP-kthread-starvation checks to CPU stall warnings

This commit adds a message that is printed if the relevant grace-period
kthread has not been able to run for the two seconds preceding the
stall warning.  (The two seconds is double the maximum interval between
successive bouts of quiescent-state forcing.)

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/RCU/stallwarn.txt

@@ -152,6 +152,15 @@ no non-lazy callbacks ("." is printed otherwise, as shown above) and
 "D" indicates that dyntick-idle processing is enabled ("." is printed
 otherwise, for example, if disabled via the "nohz=" kernel boot parameter).
 
+If the relevant grace-period kthread has been unable to run prior to
+the stall warning, the following additional line is printed:
+
+	rcu_preempt kthread starved for 2023 jiffies!
+
+Starving the grace-period kthreads of CPU time can of course result in
+RCU CPU stall warnings even when all CPUs and tasks have passed through
+the required quiescent states.
+
 
 Multiple Warnings From One Stall
 

kernel/rcu/tree.c

@@ -1072,6 +1072,21 @@ static void record_gp_stall_check_time(struct rcu_state *rsp)
 	rsp->n_force_qs_gpstart = ACCESS_ONCE(rsp->n_force_qs);
 }
 
+/*
+ * Complain about starvation of grace-period kthread.
+ */
+static void rcu_check_gp_kthread_starvation(struct rcu_state *rsp)
+{
+	unsigned long gpa;
+	unsigned long j;
+
+	j = jiffies;
+	gpa = ACCESS_ONCE(rsp->gp_activity);
+	if (j - gpa > 2 * HZ)
+		pr_err("%s kthread starved for %ld jiffies!\n",
+		       rsp->name, j - gpa);
+}
+
 /*
  * Dump stacks of all tasks running on stalled CPUs.
  */
@@ -1169,9 +1184,10 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 	}
 
 	/* Complain about tasks blocking the grace period. */
-
 	rcu_print_detail_task_stall(rsp);
 
+	rcu_check_gp_kthread_starvation(rsp);
+
 	force_quiescent_state(rsp);  /* Kick them all. */
 }
 
@@ -1196,6 +1212,9 @@ static void print_cpu_stall(struct rcu_state *rsp)
 	pr_cont(" (t=%lu jiffies g=%ld c=%ld q=%lu)\n",
 		jiffies - rsp->gp_start,
 		(long)rsp->gpnum, (long)rsp->completed, totqlen);
+
+	rcu_check_gp_kthread_starvation(rsp);
+
 	rcu_dump_cpu_stacks(rsp);
 
 	raw_spin_lock_irqsave(&rnp->lock, flags);