This commit prints out the non-zero per-CPU callback counts when a
forware-progress error (OOM event) occurs.

Signed-off-by: Paul E. McKenney <paul...@linux.ibm.com>
[ paulmck: Fix a pair of uninitialized locals spotted by kbuild test robot. ]
---
 kernel/rcu/tree.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 67f2c7a055b6..cef7d9867508 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2672,6 +2672,10 @@ rcu_check_gp_start_stall(struct rcu_node *rnp, struct 
rcu_data *rdp)
  */
 void rcu_fwd_progress_check(unsigned long j)
 {
+       unsigned long cbs;
+       int cpu;
+       unsigned long max_cbs = 0;
+       int max_cpu = -1;
        struct rcu_data *rdp;
 
        if (rcu_gp_in_progress()) {
@@ -2682,6 +2686,20 @@ void rcu_fwd_progress_check(unsigned long j)
                rcu_check_gp_start_stall(rdp->mynode, rdp, j);
                preempt_enable();
        }
+       for_each_possible_cpu(cpu) {
+               cbs = rcu_get_n_cbs_cpu(cpu);
+               if (!cbs)
+                       continue;
+               if (max_cpu < 0)
+                       pr_info("%s: callbacks", __func__);
+               pr_cont(" %d: %lu", cpu, cbs);
+               if (cbs <= max_cbs)
+                       continue;
+               max_cbs = cbs;
+               max_cpu = cpu;
+       }
+       if (max_cpu >= 0)
+               pr_cont("\n");
 }
 EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
 
-- 
2.17.1

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