[PATCH] rcu-tasks: Warn on too-short RCU Tasks Rude grace period

[Zqiang]

The RCU Tasks Rude grace period is implemented by invoking the
schedule_on_each_cpu() with an empty work function, which forces
a context switch on every online CPU and guarantees that all
rude-tasks read-side critical sections have completed. However,
schedule_on_each_cpu() can fail (via alloc_percpu() returning
-ENOMEM) and returns immediately, in which case those context
switches never happen, too-short grace period happen.

This commit therefore wraps the schedule_on_each_cpu() call in
WARN_ONCE() so that this failure is reported as a "Too-short
grace period" warning.

Signed-off-by: Zqiang <qiang.zhang@xxxxxxxxx>
---
 kernel/rcu/tasks.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index fcac7361ec51..91c25717402e 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1361,7 +1361,8 @@ static void rcu_tasks_be_rude(struct work_struct *work)
 static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
 {
 	rtp->n_ipis += cpumask_weight(cpu_online_mask);
-	schedule_on_each_cpu(rcu_tasks_be_rude);
+	WARN_ONCE(schedule_on_each_cpu(rcu_tasks_be_rude),
+		  "%s: Too-short grace period, OOM is now expected behavior!!!\n", rtp->name);
 }
 
 static void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
-- 
2.17.1