[PATCH] rcu: detect the preempt_rcu hang for triage jing's board

[he, bo]

Change-Id: I2ffceec2ae4847867753609e45c99afc66956003
Tracked-On:
Signed-off-by: he, bo <bo.he@xxxxxxxxx>
---
 kernel/rcu/tree.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 78c0cf2..d6de363 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2192,8 +2192,13 @@ static int __noreturn rcu_gp_kthread(void *arg)
 	int ret;
 	struct rcu_state *rsp = arg;
 	struct rcu_node *rnp = rcu_get_root(rsp);
+	pid_t rcu_preempt_pid;
 
 	rcu_bind_gp_kthread();
+	if(!strcmp(rsp->name, "rcu_preempt")) {
+		rcu_preempt_pid = rsp->gp_kthread->pid;
+	}
+
 	for (;;) {
 
 		/* Handle grace-period start. */
@@ -2202,8 +2207,19 @@ static int __noreturn rcu_gp_kthread(void *arg)
 					       READ_ONCE(rsp->gp_seq),
 					       TPS("reqwait"));
 			rsp->gp_state = RCU_GP_WAIT_GPS;
-			swait_event_idle_exclusive(rsp->gp_wq, READ_ONCE(rsp->gp_flags) &
-						     RCU_GP_FLAG_INIT);
+			if (current->pid != rcu_preempt_pid) {
+				swait_event_idle_exclusive(rsp->gp_wq, READ_ONCE(rsp->gp_flags) &
+						RCU_GP_FLAG_INIT);
+			} else {
+				ret = swait_event_idle_timeout_exclusive(rsp->gp_wq, READ_ONCE(rsp->gp_flags) &
+						RCU_GP_FLAG_INIT, 2*HZ);
+
+				if(!ret) {

We get here if ret==0.  Therefore, the above "if" statement needs to
instead be "if (ret == 1) {".

In addition, in order to get event traces dumped, we also need:

					rcu_ftrace_dump(DUMP_ALL);

+					show_rcu_gp_kthreads();
+					panic("hung_task: blocked in rcu_gp_kthread init");
+				}
+			}
+
 			rsp->gp_state = RCU_GP_DONE_GPS;
 			/* Locking provides needed memory barrier. */
 			if (rcu_gp_init(rsp))
-- 
2.7.4

------------------------------------------------------------------------

So, again, please change the "if(!ret) {" to "if (ret == 1) {", and
please add "rcu_ftrace_dump(DUMP_ALL);" right after this "if" statement,
as shown above.

With that change, I bet that you will again see failures.

> Compared with the swait_event_idle_timeout_exclusive will do 3 times to check the condition, while swait_event_idle_ exclusive will do 2 times check the condition.
> 
> so today I will do another experiment, only change as below:
> -			swait_event_idle_exclusive(rsp->gp_wq, READ_ONCE(rsp->gp_flags) &
> -						     RCU_GP_FLAG_INIT);
> +			ret = swait_event_idle_timeout_exclusive(rsp->gp_wq, READ_ONCE(rsp->gp_flags) &
> +					RCU_GP_FLAG_INIT, MAX_SCHEDULE_TIMEOUT);
> +
> 
> Can you get some clues from the experiment?

Again, please instead make the changes that I called out above, with
the replacement for your patch 0001 shown below.

							Thanx, Paul

PS.  I have been testing for quite some time, but am still unable
     to reproduce this.  So we must depend on you to reproduce it.

------------------------------------------------------------------------

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0b760c1369f7..86152af1a580 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2153,8 +2153,13 @@ static int __noreturn rcu_gp_kthread(void *arg)
 	int ret;
 	struct rcu_state *rsp = arg;
 	struct rcu_node *rnp = rcu_get_root(rsp);
+	pid_t rcu_preempt_pid;
 
 	rcu_bind_gp_kthread();
+	if(!strcmp(rsp->name, "rcu_preempt")) {
+		rcu_preempt_pid = rsp->gp_kthread->pid;
+	}
+
 	for (;;) {
 
 		/* Handle grace-period start. */
@@ -2163,8 +2168,20 @@ static int __noreturn rcu_gp_kthread(void *arg)
 					       READ_ONCE(rsp->gp_seq),
 					       TPS("reqwait"));
 			rsp->gp_state = RCU_GP_WAIT_GPS;
-			swait_event_idle_exclusive(rsp->gp_wq, READ_ONCE(rsp->gp_flags) &
-						     RCU_GP_FLAG_INIT);
+			if (current->pid != rcu_preempt_pid) {
+				swait_event_idle_exclusive(rsp->gp_wq, READ_ONCE(rsp->gp_flags) &
+						RCU_GP_FLAG_INIT);
+			} else {
+				ret = swait_event_idle_timeout_exclusive(rsp->gp_wq, READ_ONCE(rsp->gp_flags) &
+						RCU_GP_FLAG_INIT, 2*HZ);
+
+				if (ret == 1) {
+					rcu_ftrace_dump(DUMP_ALL);
+					show_rcu_gp_kthreads();
+					panic("hung_task: blocked in rcu_gp_kthread init");
+				}
+			}
+
 			rsp->gp_state = RCU_GP_DONE_GPS;
 			/* Locking provides needed memory barrier. */
 			if (rcu_gp_init(rsp))