[PATCH v3] rcu: Add per-CPU rcuc task info to RCU CPU stall warnings

[Zqiang]

When the 'use_softirq' be set zero, all RCU_SOFTIRQ processing
be moved to per-CPU rcuc kthreads, if the rcuc kthreads is
being starved, quiescent state can not report in time. the
RCU stall may be triggered. this commit adds a stack trace of
this CPU and dump rcuc kthreads stack to help analyze what
prevents rcuc kthreads from running.

Suggested-by: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx>
Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
---
 v1->v2:
 Avoid print anything when CPU is offline or idle.
 v2->v3:
 Make code more compact.

 kernel/rcu/tree.c        |  3 +++
 kernel/rcu/tree.h        |  1 +
 kernel/rcu/tree_plugin.h |  3 +++
 kernel/rcu/tree_stall.h  | 35 +++++++++++++++++++++++++++++++++++
 4 files changed, 42 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a4c25a6283b0..e3fc31a0f546 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2850,10 +2850,12 @@ static void rcu_cpu_kthread(unsigned int cpu)
 {
 	unsigned int *statusp = this_cpu_ptr(&rcu_data.rcu_cpu_kthread_status);
 	char work, *workp = this_cpu_ptr(&rcu_data.rcu_cpu_has_work);
+	unsigned long *j = this_cpu_ptr(&rcu_data.rcuc_activity);
 	int spincnt;
 
 	trace_rcu_utilization(TPS("Start CPU kthread@rcu_run"));
 	for (spincnt = 0; spincnt < 10; spincnt++) {
+		WRITE_ONCE(*j, jiffies);
 		local_bh_disable();
 		*statusp = RCU_KTHREAD_RUNNING;
 		local_irq_disable();
@@ -2874,6 +2876,7 @@ static void rcu_cpu_kthread(unsigned int cpu)
 	schedule_timeout_idle(2);
 	trace_rcu_utilization(TPS("End CPU kthread@rcu_yield"));
 	*statusp = RCU_KTHREAD_WAITING;
+	WRITE_ONCE(*j, jiffies);
 }
 
 static struct smp_hotplug_thread rcu_cpu_thread_spec = {
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 486fc901bd08..4e0fdebb62e8 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -237,6 +237,7 @@ struct rcu_data {
 					/* rcuc per-CPU kthread or NULL. */
 	unsigned int rcu_cpu_kthread_status;
 	char rcu_cpu_has_work;
+	unsigned long rcuc_activity;
 
 	/* 7) Diagnostic data, including RCU CPU stall warnings. */
 	unsigned int softirq_snap;	/* Snapshot of softirq activity. */
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index c5b45c2f68a1..327bbfd79cc6 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -996,12 +996,15 @@ dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
  */
 static void rcu_cpu_kthread_setup(unsigned int cpu)
 {
+	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 #ifdef CONFIG_RCU_BOOST
 	struct sched_param sp;
 
 	sp.sched_priority = kthread_prio;
 	sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
 #endif /* #ifdef CONFIG_RCU_BOOST */
+
+	WRITE_ONCE(rdp->rcuc_activity, jiffies);
 }
 
 #ifdef CONFIG_RCU_BOOST
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 21bebf7c9030..0c5d8516516a 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -379,6 +379,15 @@ static bool rcu_is_gp_kthread_starving(unsigned long *jp)
 	return j > 2 * HZ;
 }
 
+static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp)
+{
+	unsigned long j = jiffies - READ_ONCE(rdp->rcuc_activity);
+
+	if (jp)
+		*jp = j;
+	return j > 2 * HZ;
+}
+
 /*
  * Print out diagnostic information for the specified stalled CPU.
  *
@@ -430,6 +439,29 @@ static void print_cpu_stall_info(int cpu)
 	       falsepositive ? " (false positive?)" : "");
 }
 
+static void rcuc_kthread_dump(struct rcu_data *rdp)
+{
+	int cpu;
+	unsigned long j;
+	struct task_struct *rcuc;
+
+	rcuc = rdp->rcu_cpu_kthread_task;
+	if (!rcuc)
+		return;
+
+	cpu = task_cpu(rcuc);
+	if (cpu_is_offline(cpu) || idle_cpu(cpu))
+		return;
+
+	if (!rcu_is_rcuc_kthread_starving(rdp, &j))
+		return;
+
+	pr_err("%s kthread starved for %ld jiffies\n", rcuc->comm, j);
+	sched_show_task(rcuc);
+	if (!trigger_single_cpu_backtrace(cpu))
+		dump_cpu_task(cpu);
+}
+
 /* Complain about starvation of grace-period kthread.  */
 static void rcu_check_gp_kthread_starvation(void)
 {
@@ -601,6 +633,9 @@ static void print_cpu_stall(unsigned long gps)
 	rcu_check_gp_kthread_expired_fqs_timer();
 	rcu_check_gp_kthread_starvation();
 
+	if (!use_softirq)
+		rcuc_kthread_dump(rdp);
+
 	rcu_dump_cpu_stacks();
 
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
-- 
2.25.1