rcu: add grace-period age and more kthread state to tracing

This commit adds the age in jiffies of the current grace period along
with the duration in jiffies of the longest grace period since boot
to the rcu/rcugp debugfs file.  It also adds an additional "O" state
to kthread tracing to differentiate between the kthread waiting due to
having nothing to do on the one hand and waiting due to being on the
wrong CPU on the other hand.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/RCU/trace.txt

@@ -159,8 +159,8 @@ o	"qs" gives an indication of the state of the callback queue
 	the corresponding character is replaced by ".".
 
 o	"kt" is the per-CPU kernel-thread state.  The digit preceding
-	the slash is zero if there is no work pending and 1 otherwise.
-	The character after the slash is as follows:
+	the first slash is zero if there is no work pending and 1
+	otherwise.  The character between the slashes is as follows:
 
 	"S"	The kernel thread is stopped, in other words, all
 		CPUs corresponding to this rcu_node structure are
@@ -171,10 +171,18 @@ o	"kt" is the per-CPU kernel-thread state.  The digit preceding
 	"W"	The kernel thread is waiting because there is no work
 		for it to do.
 
+	"O"	The kernel thread is waiting because it has been
+		forced off of its designated CPU or because its
+		->cpus_allowed mask permits it to run on other than
+		its designated CPU.
+
 	"Y"	The kernel thread is yielding to avoid hogging CPU.
 
 	"?"	Unknown value, indicates a bug.
 
+	The number after the final slash is the CPU that the kthread
+	is actually running on.
+
 o	"b" is the batch limit for this CPU.  If more than this number
 	of RCU callbacks is ready to invoke, then the remainder will
 	be deferred.

kernel/rcutree.c

@@ -92,6 +92,7 @@ EXPORT_SYMBOL_GPL(rcu_scheduler_active);
  */
 static DEFINE_PER_CPU(struct task_struct *, rcu_cpu_kthread_task);
 DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_status);
+DEFINE_PER_CPU(int, rcu_cpu_kthread_cpu);
 static DEFINE_PER_CPU(wait_queue_head_t, rcu_cpu_wq);
 DEFINE_PER_CPU(char, rcu_cpu_has_work);
 static char rcu_kthreads_spawnable;
@@ -888,13 +889,18 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
 static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
 	__releases(rcu_get_root(rsp)->lock)
 {
+	unsigned long gp_duration;
+
 	WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
 
 	/*
 	 * Ensure that all grace-period and pre-grace-period activity
 	 * is seen before the assignment to rsp->completed.
 	 */
 	smp_mb(); /* See above block comment. */
+	gp_duration = jiffies - rsp->gp_start;
+	if (gp_duration > rsp->gp_max)
+		rsp->gp_max = gp_duration;
 	rsp->completed = rsp->gpnum;
 	rsp->signaled = RCU_GP_IDLE;
 	rcu_start_gp(rsp, flags);  /* releases root node's rnp->lock. */
@@ -1583,12 +1589,15 @@ static int rcu_cpu_kthread_should_stop(int cpu)
 	       smp_processor_id() != cpu) {
 		if (kthread_should_stop())
 			return 1;
+		per_cpu(rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
+		per_cpu(rcu_cpu_kthread_cpu, cpu) = raw_smp_processor_id();
 		local_bh_enable();
 		schedule_timeout_uninterruptible(1);
 		if (!cpumask_equal(&current->cpus_allowed, cpumask_of(cpu)))
 			set_cpus_allowed_ptr(current, cpumask_of(cpu));
 		local_bh_disable();
 	}
+	per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
 	return 0;
 }
 
@@ -1656,6 +1665,7 @@ static int __cpuinit rcu_spawn_one_cpu_kthread(int cpu)
 	if (IS_ERR(t))
 		return PTR_ERR(t);
 	kthread_bind(t, cpu);
+	per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
 	WARN_ON_ONCE(per_cpu(rcu_cpu_kthread_task, cpu) != NULL);
 	per_cpu(rcu_cpu_kthread_task, cpu) = t;
 	wake_up_process(t);

kernel/rcutree.h

@@ -93,8 +93,9 @@ struct rcu_dynticks {
 #define RCU_KTHREAD_STOPPED  0
 #define RCU_KTHREAD_RUNNING  1
 #define RCU_KTHREAD_WAITING  2
-#define RCU_KTHREAD_YIELDING 3
-#define RCU_KTHREAD_MAX      3
+#define RCU_KTHREAD_OFFCPU   3
+#define RCU_KTHREAD_YIELDING 4
+#define RCU_KTHREAD_MAX      4
 
 /*
  * Definition for node within the RCU grace-period-detection hierarchy.
@@ -383,6 +384,8 @@ struct rcu_state {
 						/*  but in jiffies. */
 	unsigned long jiffies_stall;		/* Time at which to check */
 						/*  for CPU stalls. */
+	unsigned long gp_max;			/* Maximum GP duration in */
+						/*  jiffies. */
 	char *name;				/* Name of structure. */
 };
 

kernel/rcutree_trace.c

@@ -47,13 +47,14 @@
 #include "rcutree.h"
 
 DECLARE_PER_CPU(unsigned int, rcu_cpu_kthread_status);
+DECLARE_PER_CPU(unsigned int, rcu_cpu_kthread_cpu);
 DECLARE_PER_CPU(char, rcu_cpu_has_work);
 
 static char convert_kthread_status(unsigned int kthread_status)
 {
 	if (kthread_status > RCU_KTHREAD_MAX)
 		return '?';
-	return "SRWY"[kthread_status];
+	return "SRWOY"[kthread_status];
 }
 
 static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp)
@@ -74,7 +75,7 @@ static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp)
 		   rdp->dynticks_fqs);
 #endif /* #ifdef CONFIG_NO_HZ */
 	seq_printf(m, " of=%lu ri=%lu", rdp->offline_fqs, rdp->resched_ipi);
-	seq_printf(m, " ql=%ld qs=%c%c%c%c kt=%d/%c b=%ld",
+	seq_printf(m, " ql=%ld qs=%c%c%c%c kt=%d/%c/%d b=%ld",
 		   rdp->qlen,
 		   ".N"[rdp->nxttail[RCU_NEXT_READY_TAIL] !=
 			rdp->nxttail[RCU_NEXT_TAIL]],
@@ -86,6 +87,7 @@ static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp)
 		   per_cpu(rcu_cpu_has_work, rdp->cpu),
 		   convert_kthread_status(per_cpu(rcu_cpu_kthread_status,
 					  rdp->cpu)),
+		   per_cpu(rcu_cpu_kthread_cpu, rdp->cpu),
 		   rdp->blimit);
 	seq_printf(m, " ci=%lu co=%lu ca=%lu\n",
 		   rdp->n_cbs_invoked, rdp->n_cbs_orphaned, rdp->n_cbs_adopted);
@@ -312,16 +314,35 @@ static const struct file_operations rcuhier_fops = {
 	.release = single_release,
 };
 
+static void show_one_rcugp(struct seq_file *m, struct rcu_state *rsp)
+{
+	unsigned long flags;
+	unsigned long completed;
+	unsigned long gpnum;
+	unsigned long gpage;
+	unsigned long gpmax;
+	struct rcu_node *rnp = &rsp->node[0];
+
+	raw_spin_lock_irqsave(&rnp->lock, flags);
+	completed = rsp->completed;
+	gpnum = rsp->gpnum;
+	if (rsp->completed == rsp->gpnum)
+		gpage = 0;
+	else
+		gpage = jiffies - rsp->gp_start;
+	gpmax = rsp->gp_max;
+	raw_spin_unlock_irqrestore(&rnp->lock, flags);
+	seq_printf(m, "%s: completed=%ld  gpnum=%lu  age=%ld  max=%ld\n",
+		   rsp->name, completed, gpnum, gpage, gpmax);
+}
+
 static int show_rcugp(struct seq_file *m, void *unused)
 {
 #ifdef CONFIG_TREE_PREEMPT_RCU
-	seq_printf(m, "rcu_preempt: completed=%ld  gpnum=%lu\n",
-		   rcu_preempt_state.completed, rcu_preempt_state.gpnum);
+	show_one_rcugp(m, &rcu_preempt_state);
 #endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
-	seq_printf(m, "rcu_sched: completed=%ld  gpnum=%lu\n",
-		   rcu_sched_state.completed, rcu_sched_state.gpnum);
-	seq_printf(m, "rcu_bh: completed=%ld  gpnum=%lu\n",
-		   rcu_bh_state.completed, rcu_bh_state.gpnum);
+	show_one_rcugp(m, &rcu_sched_state);
+	show_one_rcugp(m, &rcu_bh_state);
 	return 0;
 }