rcu: Print scheduling-clock information on RCU CPU stall-warning messages

There have been situations where RCU CPU stall warnings were caused by
issues in scheduling-clock timer initialization.  To make it easier to
track these down, this commit causes the RCU CPU stall-warning messages
to print out the number of scheduling-clock interrupts taken in the
current grace period for each stalled CPU.

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

4 files changed, 194 insertions, 14 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index cfdab9898e33..dccd2f78db4e 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -689,12 +689,6 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
                 return;
         }
         rsp->jiffies_stall = jiffies + 3 * jiffies_till_stall_check() + 3;
-
-        /*
-         * Now rat on any tasks that got kicked up to the root rcu_node
-         * due to CPU offlining.
-         */
-        ndetected = rcu_print_task_stall(rnp);
         raw_spin_unlock_irqrestore(&rnp->lock, flags);
 
         /*
@@ -702,8 +696,9 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
          * See Documentation/RCU/stallwarn.txt for info on how to debug
          * RCU CPU stall warnings.
          */
-        printk(KERN_ERR "INFO: %s detected stalls on CPUs/tasks: {",
+        printk(KERN_ERR "INFO: %s detected stalls on CPUs/tasks:",
                rsp->name);
+        print_cpu_stall_info_begin();
         rcu_for_each_leaf_node(rsp, rnp) {
                 raw_spin_lock_irqsave(&rnp->lock, flags);
                 ndetected += rcu_print_task_stall(rnp);
@@ -712,11 +707,22 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
                         continue;
                 for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
                         if (rnp->qsmask & (1UL << cpu)) {
-                                printk(" %d", rnp->grplo + cpu);
+                                print_cpu_stall_info(rsp, rnp->grplo + cpu);
                                 ndetected++;
                         }
         }
-        printk("} (detected by %d, t=%ld jiffies)\n",
+
+        /*
+         * Now rat on any tasks that got kicked up to the root rcu_node
+         * due to CPU offlining.
+         */
+        rnp = rcu_get_root(rsp);
+        raw_spin_lock_irqsave(&rnp->lock, flags);
+        ndetected = rcu_print_task_stall(rnp);
+        raw_spin_unlock_irqrestore(&rnp->lock, flags);
+
+        print_cpu_stall_info_end();
+        printk(KERN_CONT "(detected by %d, t=%ld jiffies)\n",
                smp_processor_id(), (long)(jiffies - rsp->gp_start));
         if (ndetected == 0)
                 printk(KERN_ERR "INFO: Stall ended before state dump start\n");
@@ -740,8 +746,11 @@ static void print_cpu_stall(struct rcu_state *rsp)
          * See Documentation/RCU/stallwarn.txt for info on how to debug
          * RCU CPU stall warnings.
          */
-        printk(KERN_ERR "INFO: %s detected stall on CPU %d (t=%lu jiffies)\n",
-               rsp->name, smp_processor_id(), jiffies - rsp->gp_start);
+        printk(KERN_ERR "INFO: %s self-detected stall on CPU", rsp->name);
+        print_cpu_stall_info_begin();
+        print_cpu_stall_info(rsp, smp_processor_id());
+        print_cpu_stall_info_end();
+        printk(KERN_CONT " (t=%lu jiffies)\n", jiffies - rsp->gp_start);
         if (!trigger_all_cpu_backtrace())
                 dump_stack();
 
@@ -831,6 +840,7 @@ static void __note_new_gpnum(struct rcu_state *rsp, struct rcu_node *rnp, struct
                         rdp->passed_quiesce = 0;
                 } else
                         rdp->qs_pending = 0;
+                zero_cpu_stall_ticks(rdp);
         }
 }
 
@@ -1496,6 +1506,7 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
 void rcu_check_callbacks(int cpu, int user)
 {
         trace_rcu_utilization("Start scheduler-tick");
+        increment_cpu_stall_ticks();
         if (user || rcu_is_cpu_rrupt_from_idle()) {
 
                 /*
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 0328a537846a..e2ac8ee415bb 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -239,6 +239,12 @@ struct rcu_data {
         bool            preemptible;    /* Preemptible RCU? */
         struct rcu_node *mynode;        /* This CPU's leaf of hierarchy */
         unsigned long grpmask;          /* Mask to apply to leaf qsmask. */
+#ifdef CONFIG_RCU_CPU_STALL_INFO
+        unsigned long   ticks_this_gp;  /* The number of scheduling-clock */
+                                        /*  ticks this CPU has handled */
+                                        /*  during and after the last grace */
+                                        /* period it is aware of. */
+#endif /* #ifdef CONFIG_RCU_CPU_STALL_INFO */
 
         /* 2) batch handling */
         /*
@@ -466,5 +472,10 @@ static void __cpuinit rcu_prepare_kthreads(int cpu);
 static void rcu_prepare_for_idle_init(int cpu);
 static void rcu_cleanup_after_idle(int cpu);
 static void rcu_prepare_for_idle(int cpu);
+static void print_cpu_stall_info_begin(void);
+static void print_cpu_stall_info(struct rcu_state *rsp, int cpu);
+static void print_cpu_stall_info_end(void);
+static void zero_cpu_stall_ticks(struct rcu_data *rdp);
+static void increment_cpu_stall_ticks(void);
 
 #endif /* #ifndef RCU_TREE_NONCORE */
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 0e74e1c6333f..aa93b074bb2f 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -63,7 +63,10 @@ static void __init rcu_bootup_announce_oddness(void)
         printk(KERN_INFO "\tRCU torture testing starts during boot.\n");
 #endif
 #if defined(CONFIG_TREE_PREEMPT_RCU) && !defined(CONFIG_RCU_CPU_STALL_VERBOSE)
-        printk(KERN_INFO "\tVerbose stalled-CPUs detection is disabled.\n");
+        printk(KERN_INFO "\tDump stacks of tasks blocking RCU-preempt GP.\n");
+#endif
+#if defined(CONFIG_RCU_CPU_STALL_INFO)
+        printk(KERN_INFO "\tAdditional per-CPU info printed with stalls.\n");
 #endif
 #if NUM_RCU_LVL_4 != 0
         printk(KERN_INFO "\tExperimental four-level hierarchy is enabled.\n");
@@ -490,6 +493,31 @@ static void rcu_print_detail_task_stall(struct rcu_state *rsp)
 
 #endif /* #else #ifdef CONFIG_RCU_CPU_STALL_VERBOSE */
 
+#ifdef CONFIG_RCU_CPU_STALL_INFO
+
+static void rcu_print_task_stall_begin(struct rcu_node *rnp)
+{
+        printk(KERN_ERR "\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
+               rnp->level, rnp->grplo, rnp->grphi);
+}
+
+static void rcu_print_task_stall_end(void)
+{
+        printk(KERN_CONT "\n");
+}
+
+#else /* #ifdef CONFIG_RCU_CPU_STALL_INFO */
+
+static void rcu_print_task_stall_begin(struct rcu_node *rnp)
+{
+}
+
+static void rcu_print_task_stall_end(void)
+{
+}
+
+#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_INFO */
+
 /*
  * Scan the current list of tasks blocked within RCU read-side critical
  * sections, printing out the tid of each.
@@ -501,12 +529,14 @@ static int rcu_print_task_stall(struct rcu_node *rnp)
 
         if (!rcu_preempt_blocked_readers_cgp(rnp))
                 return 0;
+        rcu_print_task_stall_begin(rnp);
         t = list_entry(rnp->gp_tasks,
                        struct task_struct, rcu_node_entry);
         list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
-                printk(" P%d", t->pid);
+                printk(KERN_CONT " P%d", t->pid);
                 ndetected++;
         }
+        rcu_print_task_stall_end();
         return ndetected;
 }
 
@@ -2004,7 +2034,7 @@ static void rcu_cleanup_after_idle(int cpu)
 }
 
 /*
- * Do the idle-entry grace-period work, which, because CONFIG_RCU_FAST_NO_HZ=y,
+ * Do the idle-entry grace-period work, which, because CONFIG_RCU_FAST_NO_HZ=n,
  * is nothing.
  */
 static void rcu_prepare_for_idle(int cpu)
@@ -2273,3 +2303,117 @@ static void rcu_prepare_for_idle(int cpu)
 }
 
 #endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
+
+#ifdef CONFIG_RCU_CPU_STALL_INFO
+
+#ifdef CONFIG_RCU_FAST_NO_HZ
+
+static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
+{
+        struct hrtimer *hrtp = &per_cpu(rcu_idle_gp_timer, cpu);
+
+        sprintf(cp, "drain=%d %c timer=%lld",
+                per_cpu(rcu_dyntick_drain, cpu),
+                per_cpu(rcu_dyntick_holdoff, cpu) == jiffies ? 'H' : '.',
+                hrtimer_active(hrtp)
+                        ? ktime_to_us(hrtimer_get_remaining(hrtp))
+                        : -1);
+}
+
+#else /* #ifdef CONFIG_RCU_FAST_NO_HZ */
+
+static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
+{
+}
+
+#endif /* #else #ifdef CONFIG_RCU_FAST_NO_HZ */
+
+/* Initiate the stall-info list. */
+static void print_cpu_stall_info_begin(void)
+{
+        printk(KERN_CONT "\n");
+}
+
+/*
+ * Print out diagnostic information for the specified stalled CPU.
+ *
+ * If the specified CPU is aware of the current RCU grace period
+ * (flavor specified by rsp), then print the number of scheduling
+ * clock interrupts the CPU has taken during the time that it has
+ * been aware.  Otherwise, print the number of RCU grace periods
+ * that this CPU is ignorant of, for example, "1" if the CPU was
+ * aware of the previous grace period.
+ *
+ * Also print out idle and (if CONFIG_RCU_FAST_NO_HZ) idle-entry info.
+ */
+static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
+{
+        char fast_no_hz[72];
+        struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
+        struct rcu_dynticks *rdtp = rdp->dynticks;
+        char *ticks_title;
+        unsigned long ticks_value;
+
+        if (rsp->gpnum == rdp->gpnum) {
+                ticks_title = "ticks this GP";
+                ticks_value = rdp->ticks_this_gp;
+        } else {
+                ticks_title = "GPs behind";
+                ticks_value = rsp->gpnum - rdp->gpnum;
+        }
+        print_cpu_stall_fast_no_hz(fast_no_hz, cpu);
+        printk(KERN_ERR "\t%d: (%lu %s) idle=%03x/%llx/%d %s\n",
+               cpu, ticks_value, ticks_title,
+               atomic_read(&rdtp->dynticks) & 0xfff,
+               rdtp->dynticks_nesting, rdtp->dynticks_nmi_nesting,
+               fast_no_hz);
+}
+
+/* Terminate the stall-info list. */
+static void print_cpu_stall_info_end(void)
+{
+        printk(KERN_ERR "\t");
+}
+
+/* Zero ->ticks_this_gp for all flavors of RCU. */
+static void zero_cpu_stall_ticks(struct rcu_data *rdp)
+{
+        rdp->ticks_this_gp = 0;
+}
+
+/* Increment ->ticks_this_gp for all flavors of RCU. */
+static void increment_cpu_stall_ticks(void)
+{
+        __get_cpu_var(rcu_sched_data).ticks_this_gp++;
+        __get_cpu_var(rcu_bh_data).ticks_this_gp++;
+#ifdef CONFIG_TREE_PREEMPT_RCU
+        __get_cpu_var(rcu_preempt_data).ticks_this_gp++;
+#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
+}
+
+#else /* #ifdef CONFIG_RCU_CPU_STALL_INFO */
+
+static void print_cpu_stall_info_begin(void)
+{
+        printk(KERN_CONT " {");
+}
+
+static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
+{
+        printk(KERN_CONT " %d", cpu);
+}
+
+static void print_cpu_stall_info_end(void)
+{
+        printk(KERN_CONT "} ");
+}
+
+static void zero_cpu_stall_ticks(struct rcu_data *rdp)
+{
+}
+
+static void increment_cpu_stall_ticks(void)
+{
+}
+
+#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_INFO */
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 3cc419d05f2f..d27a2aa3e815 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -927,6 +927,20 @@ config RCU_CPU_STALL_VERBOSE
 
           Say Y if you want to enable such checks.
 
+config RCU_CPU_STALL_INFO
+        bool "Print additional diagnostics on RCU CPU stall"
+        depends on (TREE_RCU || TREE_PREEMPT_RCU) && DEBUG_KERNEL
+        default n
+        help
+          For each stalled CPU that is aware of the current RCU grace
+          period, print out additional per-CPU diagnostic information
+          regarding scheduling-clock ticks, idle state, and,
+          for RCU_FAST_NO_HZ kernels, idle-entry state.
+
+          Say N if you are unsure.
+
+          Say Y if you want to enable such diagnostics.
+
 config RCU_TRACE
         bool "Enable tracing for RCU"
         depends on DEBUG_KERNEL