[PATCH] rcu-tasks: Allow RCU-Task trace stall warning dump late IPI CPU stacks

[Zqiang]

The task structure's->trc_ipi_to_cpu and percpu trc_ipi_to_cpu is
used to record whether the IPI is completed, if the percpu trc_ipi_to_cpu
is true and task structure's->trc_ipi_to_cpu is non-negative value,
indicates that IPI is not completed, if the IPI is unresponsive for
along time for some reason, there will be a possibility of causing
RCU-Tasks trace stall. this commit therefore allow dump late IPI CPU
stacks to show the path that current CPU is executing.

Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
---
 In the real world, the IPI delay will be very small, so the
 probability of triggering dump_cpu_stack() may be very low,
 so if I makes noise, please ignore it.

 kernel/rcu/tasks.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index baf7ec178155..85237fc1d0f0 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1658,8 +1658,13 @@ static void show_stalled_ipi_trace(void)
 	int cpu;
 
 	for_each_possible_cpu(cpu)
-		if (per_cpu(trc_ipi_to_cpu, cpu))
+		if (per_cpu(trc_ipi_to_cpu, cpu)) {
 			pr_alert("\tIPI outstanding to CPU %d\n", cpu);
+			if (cpu_is_offline(cpu))
+				pr_alert("offline CPU %d blocking gp\n", cpu);
+			else
+				dump_cpu_task(cpu);
+		}
 }
 
 /* Do one scan of the holdout list. */
-- 
2.25.1