[PATCH tip/core/rcu 09/20] rcu: Detect stalls caused by failure to propagate up rcu_node tree

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

If all CPUs have passed through quiescent states, then stalls might be
due to starvation of the grace-period kthread or to failure to propagate
the quiescent states up the rcu_node combining tree.  The current stall
warning messages do not differentiate, so this commit adds a printout
of the root rcu_node structure's ->qsmask field.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 98da632d1d49..3b7e4133ca99 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1196,9 +1196,10 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 		} else {
 			j = jiffies;
 			gpa = ACCESS_ONCE(rsp->gp_activity);
-			pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld\n",
+			pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
 			       rsp->name, j - gpa, j, gpa,
-			       jiffies_till_next_fqs);
+			       jiffies_till_next_fqs,
+			       rcu_get_root(rsp)->qsmask);
 			/* In this case, the current CPU might be at fault. */
 			sched_show_task(current);
 		}
-- 
1.8.1.5