rcu: RCU-based detection of stalled CPUs for Classic RCU

This patch adds stalled-CPU detection to Classic RCU.  This capability
is enabled by a new config variable CONFIG_RCU_CPU_STALL_DETECTOR, which
defaults disabled.

This is a debugging feature to detect infinite loops in kernel code, not
something that non-kernel-hackers would be expected to care about.

This feature can detect looping CPUs in !PREEMPT builds and looping CPUs
with preemption disabled in PREEMPT builds.  This is essentially a port of
this functionality from the treercu patch, replacing the stall debug patch
that is already in tip/core/rcu (commit 67182ae1c4).

The changes from the patch in tip/core/rcu include making the config
variable name match that in treercu, changing from seconds to jiffies to
avoid spurious warnings, and printing a boot message when this feature
is enabled.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

3 files changed, 96 insertions, 84 deletions

diff --git a/include/linux/rcuclassic.h b/include/linux/rcuclassic.h
index 29bf528c7dcc..5f89b62e6983 100644
--- a/include/linux/rcuclassic.h
+++ b/include/linux/rcuclassic.h
@@ -40,15 +40,21 @@
 #include <linux/cpumask.h>
 #include <linux/seqlock.h>
 
+#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
+#define RCU_SECONDS_TILL_STALL_CHECK    ( 3 * HZ) /* for rcp->jiffies_stall */
+#define RCU_SECONDS_TILL_STALL_RECHECK  (30 * HZ) /* for rcp->jiffies_stall */
+#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
 
 /* Global control variables for rcupdate callback mechanism. */
 struct rcu_ctrlblk {
         long    cur;            /* Current batch number.                      */
         long    completed;      /* Number of the last completed batch         */
         long    pending;        /* Number of the last pending batch           */
-#ifdef CONFIG_DEBUG_RCU_STALL
-        unsigned long gp_check; /* Time grace period should end, in seconds.  */
-#endif /* #ifdef CONFIG_DEBUG_RCU_STALL */
+#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
+        unsigned long gp_start; /* Time at which GP started in jiffies. */
+        unsigned long jiffies_stall;
+                                /* Time at which to check for CPU stalls. */
+#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
 
         int     signaled;
 
diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c
index ed15128ca2c9..0d07e6e51578 100644
--- a/kernel/rcuclassic.c
+++ b/kernel/rcuclassic.c
@@ -164,6 +164,87 @@ static void __call_rcu(struct rcu_head *head, struct rcu_ctrlblk *rcp,
         }
 }
 
+#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
+
+static void record_gp_stall_check_time(struct rcu_ctrlblk *rcp)
+{
+        rcp->gp_start = jiffies;
+        rcp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_CHECK;
+}
+
+static void print_other_cpu_stall(struct rcu_ctrlblk *rcp)
+{
+        int cpu;
+        long delta;
+        unsigned long flags;
+
+        /* Only let one CPU complain about others per time interval. */
+
+        spin_lock_irqsave(&rcp->lock, flags);
+        delta = jiffies - rcp->jiffies_stall;
+        if (delta < 2 || rcp->cur != rcp->completed) {
+                spin_unlock_irqrestore(&rcp->lock, flags);
+                return;
+        }
+        rcp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
+        spin_unlock_irqrestore(&rcp->lock, flags);
+
+        /* OK, time to rat on our buddy... */
+
+        printk(KERN_ERR "RCU detected CPU stalls:");
+        for_each_possible_cpu(cpu) {
+                if (cpu_isset(cpu, rcp->cpumask))
+                        printk(" %d", cpu);
+        }
+        printk(" (detected by %d, t=%ld jiffies)\n",
+               smp_processor_id(), (long)(jiffies - rcp->gp_start));
+}
+
+static void print_cpu_stall(struct rcu_ctrlblk *rcp)
+{
+        unsigned long flags;
+
+        printk(KERN_ERR "RCU detected CPU %d stall (t=%lu/%lu jiffies)\n",
+                        smp_processor_id(), jiffies,
+                        jiffies - rcp->gp_start);
+        dump_stack();
+        spin_lock_irqsave(&rcp->lock, flags);
+        if ((long)(jiffies - rcp->jiffies_stall) >= 0)
+                rcp->jiffies_stall =
+                        jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
+        spin_unlock_irqrestore(&rcp->lock, flags);
+        set_need_resched();  /* kick ourselves to get things going. */
+}
+
+static void check_cpu_stall(struct rcu_ctrlblk *rcp)
+{
+        long delta;
+
+        delta = jiffies - rcp->jiffies_stall;
+        if (cpu_isset(smp_processor_id(), rcp->cpumask) && delta >= 0) {
+
+                /* We haven't checked in, so go dump stack. */
+                print_cpu_stall(rcp);
+
+        } else if (rcp->cur != rcp->completed && delta >= 2) {
+
+                /* They had two seconds to dump stack, so complain. */
+                print_other_cpu_stall(rcp);
+        }
+}
+
+#else /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
+
+static void record_gp_stall_check_time(struct rcu_ctrlblk *rcp)
+{
+}
+
+static void check_cpu_stall(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
+{
+}
+
+#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
+
 /**
  * call_rcu - Queue an RCU callback for invocation after a grace period.
  * @head: structure to be used for queueing the RCU updates.
@@ -293,84 +374,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
  *   period (if necessary).
  */
 
-#ifdef CONFIG_DEBUG_RCU_STALL
-
-static inline void record_gp_check_time(struct rcu_ctrlblk *rcp)
-{
-        rcp->gp_check = get_seconds() + 3;
-}
-
-static void print_other_cpu_stall(struct rcu_ctrlblk *rcp)
-{
-        int cpu;
-        long delta;
-        unsigned long flags;
-
-        /* Only let one CPU complain about others per time interval. */
-
-        spin_lock_irqsave(&rcp->lock, flags);
-        delta = get_seconds() - rcp->gp_check;
-        if (delta < 2L || cpus_empty(rcp->cpumask)) {
-                spin_unlock(&rcp->lock);
-                return;
-        }
-        rcp->gp_check = get_seconds() + 30;
-        spin_unlock_irqrestore(&rcp->lock, flags);
-
-        /* OK, time to rat on our buddy... */
-
-        printk(KERN_ERR "RCU detected CPU stalls:");
-        for_each_cpu_mask(cpu, rcp->cpumask)
-                printk(" %d", cpu);
-        printk(" (detected by %d, t=%lu/%lu)\n",
-               smp_processor_id(), get_seconds(), rcp->gp_check);
-}
-
-static void print_cpu_stall(struct rcu_ctrlblk *rcp)
-{
-        unsigned long flags;
-
-        printk(KERN_ERR "RCU detected CPU %d stall (t=%lu/%lu)\n",
-                        smp_processor_id(), get_seconds(), rcp->gp_check);
-        dump_stack();
-        spin_lock_irqsave(&rcp->lock, flags);
-        if ((long)(get_seconds() - rcp->gp_check) >= 0L)
-                rcp->gp_check = get_seconds() + 30;
-        spin_unlock_irqrestore(&rcp->lock, flags);
-}
-
-static void check_cpu_stall(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
-{
-        long delta;
-
-        delta = get_seconds() - rcp->gp_check;
-        if (cpu_isset(smp_processor_id(), rcp->cpumask) && delta >= 0L) {
-
-                /* We haven't checked in, so go dump stack. */
-
-                print_cpu_stall(rcp);
-
-        } else {
-                if (!cpus_empty(rcp->cpumask) && delta >= 2L) {
-                        /* They had two seconds to dump stack, so complain. */
-                        print_other_cpu_stall(rcp);
-                }
-        }
-}
-
-#else /* #ifdef CONFIG_DEBUG_RCU_STALL */
-
-static inline void record_gp_check_time(struct rcu_ctrlblk *rcp)
-{
-}
-
-static inline void
-check_cpu_stall(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
-{
-}
-
-#endif /* #else #ifdef CONFIG_DEBUG_RCU_STALL */
-
 /*
  * Register a new batch of callbacks, and start it up if there is currently no
  * active batch and the batch to be registered has not already occurred.
@@ -381,7 +384,7 @@ static void rcu_start_batch(struct rcu_ctrlblk *rcp)
         if (rcp->cur != rcp->pending &&
                         rcp->completed == rcp->cur) {
                 rcp->cur++;
-                record_gp_check_time(rcp);
+                record_gp_stall_check_time(rcp);
 
                 /*
                  * Accessing nohz_cpu_mask before incrementing rcp->cur needs a
@@ -603,7 +606,7 @@ static void rcu_process_callbacks(struct softirq_action *unused)
 static int __rcu_pending(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
 {
         /* Check for CPU stalls, if enabled. */
-        check_cpu_stall(rcp, rdp);
+        check_cpu_stall(rcp);
 
         if (rdp->nxtlist) {
                 long completed_snap = ACCESS_ONCE(rcp->completed);
@@ -769,6 +772,9 @@ static struct notifier_block __cpuinitdata rcu_nb = {
  */
 void __init __rcu_init(void)
 {
+#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
+        printk(KERN_INFO "RCU-based detection of stalled CPUs is enabled.\n");
+#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
         rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE,
                         (void *)(long)smp_processor_id());
         /* Register notifier for non-boot CPUs */
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index ccede1aeab38..9fee969dd60e 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -597,7 +597,7 @@ config RCU_TORTURE_TEST_RUNNABLE
           Say N here if you want the RCU torture tests to start only
           after being manually enabled via /proc.
 
-config RCU_CPU_STALL
+config RCU_CPU_STALL_DETECTOR
         bool "Check for stalled CPUs delaying RCU grace periods"
         depends on CLASSIC_RCU
         default n