[PATCH tip/core/rcu 15/18] rcu: Add online/offline info to expedited stall warning message

[Paul E. McKenney]

This commit makes the RCU CPU stall warning message print online/offline
indications immediately after the CPU number.  A "O" indicates global
offline, a "." global online, and a "o" indicates RCU believes that the
CPU is offline for the current grace period and "." otherwise, and an
"N" indicates that RCU believes that the CPU will be offline for the
next grace period, and "." otherwise, all right after the CPU number.
So for CPU 10, you would normally see "10-...:" indicating that everything
believes that the CPU is online.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
 kernel/rcu/tree.c        |  9 ++++++++-
 kernel/rcu/tree.h        |  1 +
 kernel/rcu/tree_plugin.h | 31 +++++++++++++++++++++++++++++++
 3 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index ff68087d93a3..b246aa0470dc 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3737,11 +3737,18 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 		pr_err("INFO: %s detected expedited stalls on CPUs: {",
 		       rsp->name);
 		rcu_for_each_leaf_node(rsp, rnp) {
+			(void)rcu_print_task_exp_stall(rnp);
 			mask = 1;
 			for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
+				struct rcu_data *rdp;
+
 				if (!(rnp->expmask & mask))
 					continue;
-				pr_cont(" %d", cpu);
+				rdp = per_cpu_ptr(rsp->rda, cpu);
+				pr_cont(" %d-%c%c%c", cpu,
+					"O."[cpu_online(cpu)],
+					"o."[!!(rdp->grpmask & rnp->expmaskinit)],
+					"N."[!!(rdp->grpmask & rnp->expmaskinitnext)]);
 			}
 			mask <<= 1;
 		}
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 6f3b63b68886..191aa3678575 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -589,6 +589,7 @@ static bool rcu_preempt_has_tasks(struct rcu_node *rnp);
 #endif /* #ifdef CONFIG_HOTPLUG_CPU */
 static void rcu_print_detail_task_stall(struct rcu_state *rsp);
 static int rcu_print_task_stall(struct rcu_node *rnp);
+static int rcu_print_task_exp_stall(struct rcu_node *rnp);
 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp);
 static void rcu_preempt_check_callbacks(void);
 void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu));
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 6cbfbfc58656..7b61cece80c0 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -586,6 +586,27 @@ static int rcu_print_task_stall(struct rcu_node *rnp)
 }
 
 /*
+ * Scan the current list of tasks blocked within RCU read-side critical
+ * sections, printing out the tid of each that is blocking the current
+ * expedited grace period.
+ */
+static int rcu_print_task_exp_stall(struct rcu_node *rnp)
+{
+	struct task_struct *t;
+	int ndetected = 0;
+
+	if (!rnp->exp_tasks)
+		return 0;
+	t = list_entry(rnp->exp_tasks->prev,
+		       struct task_struct, rcu_node_entry);
+	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
+		pr_cont(" P%d", t->pid);
+		ndetected++;
+	}
+	return ndetected;
+}
+
+/*
  * Check that the list of blocked tasks for the newly completed grace
  * period is in fact empty.  It is a serious bug to complete a grace
  * period that still has RCU readers blocked!  This function must be
@@ -846,6 +867,16 @@ static int rcu_print_task_stall(struct rcu_node *rnp)
 }
 
 /*
+ * Because preemptible RCU does not exist, we never have to check for
+ * tasks blocked within RCU read-side critical sections that are
+ * blocking the current expedited grace period.
+ */
+static int rcu_print_task_exp_stall(struct rcu_node *rnp)
+{
+	return 0;
+}
+
+/*
  * Because there is no preemptible RCU, there can be no readers blocked,
  * so there is no need to check for blocked tasks.  So check only for
  * bogus qsmask values.
-- 
2.5.2

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