[PATCH v3 1/2] Track hard and soft "short lockups" or "stalls."

[Alex Neronskiy]

Enable the lockup watchdog to not only respond to lockups beyond a
certain threshold, but also keep track of the worst sub-threshold
lockup (called a "stall") seen so far. The stalls are communicated
to userspace via /proc/sys/kernel/softstall_worst
and hardstall_worst, as well as through the kernel log, where a
stack trace is also included. Hard lockups are checked for more
frequently to collect additional data. Soft stalls are described
by the length of time between successive instances of scheduling
of the watchdog thread, and hard stalls are described by the
number of times a hard lockup check found that no interrupts had
been recorded. Some thresholds on these are set, to attempt to
prevent floods of useless data.

Signed-off-by: Alex Neronskiy <zakmagnus@xxxxxxxxxx>
---
The motivation here is to be able to gather more data about just how
"extreme" latency conditions get inside the kernel, without
necessarily triggering a threshold and potentially setting off a
panic.
The way the hard lockup watchdog works has been changed to make it
more compatible with this purpose. Previously it was hard to
collect any data from it other than "lockup" or "not lockup."
Now it has to fail 5 consecutive checks to be considered a lockup,
somewhat similarly to how soft lockups work.

 include/linux/nmi.h |    6 ++-
 kernel/sysctl.c     |   18 +++++++-
 kernel/watchdog.c   |  124 ++++++++++++++++++++++++++++++++++++++++++++-------
 3 files changed, 129 insertions(+), 19 deletions(-)

diff --git a/include/linux/nmi.h b/include/linux/nmi.h
index 2d304ef..57cda21 100644
--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h
@@ -48,9 +48,13 @@ int hw_nmi_is_cpu_stuck(struct pt_regs *);
 u64 hw_nmi_get_sample_period(int watchdog_thresh);
 extern int watchdog_enabled;
 extern int watchdog_thresh;
+extern unsigned long worst_softstall;
+#ifdef CONFIG_HARDLOCKUP_DETECTOR
+extern unsigned long worst_hardstall;
+#endif /* CONFIG_HARDLOCKUP_DETECTOR */
 struct ctl_table;
 extern int proc_dowatchdog(struct ctl_table *, int ,
 			   void __user *, size_t *, loff_t *);
-#endif
+#endif /* CONFIG_LOCKUP_DETECTOR */
 
 #endif
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index f175d98..47decb8 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -767,7 +767,23 @@ static struct ctl_table kern_table[] = {
 		.extra1		= &zero,
 		.extra2		= &one,
 	},
-#endif
+	{
+		.procname       = "softstall_worst",
+		.data           = &worst_softstall,
+		.maxlen         = sizeof(unsigned long),
+		.mode           = 0644,
+		.proc_handler   = proc_doulongvec_minmax,
+	},
+#ifdef CONFIG_HARDLOCKUP_DETECTOR
+	{
+		.procname       = "hardstall_worst",
+		.data           = &worst_hardstall,
+		.maxlen         = sizeof(unsigned long),
+		.mode           = 0644,
+		.proc_handler   = proc_doulongvec_minmax,
+	},
+#endif /* CONFIG_HARDLOCKUP_DETECTOR */
+#endif /* CONFIG_LOCKUP_DETECTOR */
 #if defined(CONFIG_X86_LOCAL_APIC) && defined(CONFIG_X86)
 	{
 		.procname       = "unknown_nmi_panic",
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 3d0c56a..bf26810 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -29,6 +29,8 @@
 
 int watchdog_enabled = 1;
 int __read_mostly watchdog_thresh = 10;
+unsigned long worst_softstall;
+static DEFINE_SPINLOCK(softstall_lock);
 
 static DEFINE_PER_CPU(unsigned long, watchdog_touch_ts);
 static DEFINE_PER_CPU(struct task_struct *, softlockup_watchdog);
@@ -40,8 +42,29 @@ static DEFINE_PER_CPU(bool, hard_watchdog_warn);
 static DEFINE_PER_CPU(bool, watchdog_nmi_touch);
 static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts);
 static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts_saved);
+static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts_missed);
 static DEFINE_PER_CPU(struct perf_event *, watchdog_ev);
-#endif
+
+unsigned long worst_hardstall;
+static DEFINE_SPINLOCK(hardstall_lock);
+
+/* The number of consecutive hard check failures before it's a lockup */
+#define hardlockup_thresh (5)
+
+/*
+ * The minimum expected number of missed interrupts in a normal hard
+ * lockup check.
+ */
+#define hardstall_thresh (2)
+
+#endif /* CONFIG_HARDLOCKUP_DETECTOR */
+
+/*
+ * The number of milliseconds by which a stall must be worse than the
+ * current worst in order to be recorded. This prevents bursts of near-
+ * identical stalls.
+ */
+#define softstall_diff_thresh (100)
 
 /* boot commands */
 /*
@@ -98,19 +121,28 @@ __setup("nosoftlockup", nosoftlockup_setup);
  * the thresholds with a factor: we make the soft threshold twice the amount of
  * time the hard threshold is.
  */
-static int get_softlockup_thresh(void)
+static inline int get_softlockup_thresh(void)
 {
 	return watchdog_thresh * 2;
 }
 
 /*
- * Returns seconds, approximately.  We don't need nanosecond
+ * This is just the period of the timer function. A "stall" shorter than
+ * this is the expected behavior and is therefore not noteworthy.
+ */
+static inline int get_softstall_thresh(void)
+{
+	return 1000 / 5 * get_softlockup_thresh();
+}
+
+/*
+ * Returns milliseconds, approximately.  We don't need nanosecond
  * resolution, and we don't need to waste time with a big divide when
- * 2^30ns == 1.074s.
+ * 2^20ns == 1.049ms.
  */
 static unsigned long get_timestamp(int this_cpu)
 {
-	return cpu_clock(this_cpu) >> 30LL;  /* 2^30 ~= 10^9 */
+	return cpu_clock(this_cpu) >> 20LL;  /* 2^20 ~= 10^6 */
 }
 
 static unsigned long get_sample_period(void)
@@ -176,25 +208,79 @@ void touch_softlockup_watchdog_sync(void)
 
 #ifdef CONFIG_HARDLOCKUP_DETECTOR
 /* watchdog detector functions */
-static int is_hardlockup(void)
+static void update_hardstall(unsigned long stall, int this_cpu)
+{
+	int update_stall = 0;
+
+	if (stall > hardstall_thresh && stall > worst_hardstall) {
+		unsigned long flags;
+		spin_lock_irqsave(&hardstall_lock, flags);
+		update_stall = stall > worst_hardstall;
+		if (update_stall)
+			worst_hardstall = stall;
+		spin_unlock_irqrestore(&hardstall_lock, flags);
+	}
+
+	if (update_stall) {
+		printk(KERN_WARNING "LOCKUP may be in progress!"
+			"Worst hard stall seen on CPU#%d: %lu interrupts missed\n",
+			this_cpu, stall);
+		dump_stack();
+	}
+}
+
+static int is_hardlockup(int this_cpu)
 {
 	unsigned long hrint = __this_cpu_read(hrtimer_interrupts);
+	unsigned long hrint_saved = __this_cpu_read(hrtimer_interrupts_saved);
+	unsigned long ints_missed = 0;
+
+	__this_cpu_write(hrtimer_interrupts_saved, hrint);
 
-	if (__this_cpu_read(hrtimer_interrupts_saved) == hrint)
+	if (hrint_saved == hrint)
+		ints_missed = per_cpu(hrtimer_interrupts_missed, this_cpu)++;
+	else
+		__this_cpu_write(hrtimer_interrupts_missed, 0);
+
+	if (ints_missed >= hardlockup_thresh)
 		return 1;
 
-	__this_cpu_write(hrtimer_interrupts_saved, hrint);
+	update_hardstall(ints_missed, this_cpu);
 	return 0;
 }
 #endif
 
-static int is_softlockup(unsigned long touch_ts)
+static void update_softstall(unsigned long stall, int this_cpu)
 {
-	unsigned long now = get_timestamp(smp_processor_id());
+	int update_stall = 0;
+	if (stall > get_softstall_thresh() &&
+			stall > worst_softstall + softstall_diff_thresh) {
+		unsigned long flags;
+		spin_lock_irqsave(&softstall_lock, flags);
+		update_stall = stall > worst_softstall + softstall_diff_thresh;
+		if (update_stall)
+			worst_softstall = stall;
+		spin_unlock_irqrestore(&softstall_lock, flags);
+	}
+
+	if (update_stall) {
+		printk(KERN_WARNING "LOCKUP may be in progress!"
+				"Worst soft stall seen on CPU#%d: %lums\n",
+				this_cpu, stall);
+		dump_stack();
+	}
+}
+
+static int is_softlockup(unsigned long touch_ts, int this_cpu)
+{
+	unsigned long now = get_timestamp(this_cpu);
+	unsigned long stall = now - touch_ts;
 
 	/* Warn about unreasonable delays: */
-	if (time_after(now, touch_ts + get_softlockup_thresh()))
-		return now - touch_ts;
+	if (time_after(now, touch_ts + 1000 * get_softlockup_thresh()))
+		return stall;
+
+	update_softstall(stall, this_cpu);
 
 	return 0;
 }
@@ -213,6 +299,9 @@ static void watchdog_overflow_callback(struct perf_event *event, int nmi,
 		 struct perf_sample_data *data,
 		 struct pt_regs *regs)
 {
+	int lockup;
+	int this_cpu = smp_processor_id();
+
 	/* Ensure the watchdog never gets throttled */
 	event->hw.interrupts = 0;
 
@@ -227,9 +316,8 @@ static void watchdog_overflow_callback(struct perf_event *event, int nmi,
 	 * fired multiple times before we overflow'd.  If it hasn't
 	 * then this is a good indication the cpu is stuck
 	 */
-	if (is_hardlockup()) {
-		int this_cpu = smp_processor_id();
-
+	lockup = is_hardlockup(this_cpu);
+	if (lockup) {
 		/* only print hardlockups once */
 		if (__this_cpu_read(hard_watchdog_warn) == true)
 			return;
@@ -260,6 +348,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 	unsigned long touch_ts = __this_cpu_read(watchdog_touch_ts);
 	struct pt_regs *regs = get_irq_regs();
 	int duration;
+	int this_cpu = smp_processor_id();
 
 	/* kick the hardlockup detector */
 	watchdog_interrupt_count();
@@ -289,14 +378,14 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 	 * indicate it is getting cpu time.  If it hasn't then
 	 * this is a good indication some task is hogging the cpu
 	 */
-	duration = is_softlockup(touch_ts);
+	duration = is_softlockup(touch_ts, this_cpu);
 	if (unlikely(duration)) {
 		/* only warn once */
 		if (__this_cpu_read(soft_watchdog_warn) == true)
 			return HRTIMER_RESTART;
 
 		printk(KERN_ERR "BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
-			smp_processor_id(), duration,
+			this_cpu, duration >> 10L,
 			current->comm, task_pid_nr(current));
 		print_modules();
 		print_irqtrace_events(current);
@@ -371,6 +460,7 @@ static int watchdog_nmi_enable(int cpu)
 	/* Try to register using hardware perf events */
 	wd_attr = &wd_hw_attr;
 	wd_attr->sample_period = hw_nmi_get_sample_period(watchdog_thresh);
+	do_div(wd_attr->sample_period, hardlockup_thresh);
 	event = perf_event_create_kernel_counter(wd_attr, cpu, NULL, watchdog_overflow_callback);
 	if (!IS_ERR(event)) {
 		printk(KERN_INFO "NMI watchdog enabled, takes one hw-pmu counter.\n");
-- 
1.7.3.1

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