Message-Id: <1310510811-12564-1-git-send-email-zakmagnus@google.com>
Date:	Tue, 12 Jul 2011 15:46:51 -0700
From:	Alex Neronskiy <zakmagnus@...gle.com>
To:	linux-kernel@...r.kernel.org
Cc:	Ingo Molnar <mingo@...e.hu>, Don Zickus <dzickus@...hat.com>,
	Mandeep Singh Baines <msb@...omium.org>,
	Alex Neronskiy <zakmagnus@...gle.com>
Subject: [PATCH] Track hard and soft "short lockups" or "stalls."

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/worst_softstall
and worst_hardstall, as well as through the kernel log, where a
stack trace is also included. 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 interrupts
recorded between successive perf event overflows. Some thresholds
on these are set to attempt to prevent floods of useless data.
Based on 3.0-rc7.

Change-Id: I6db8432fd81ad5a2b40867699709c22c25165ba0
---
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 hardstall_thresh looks magical. Assuming no time is spent halted,
there is time for watchdog_thresh / (2 * watchdog_thresh / 5) = 2.5
interrupts in between the hard lockup checks. So, if only 2 of them
happen, it doesn't indicate that anything is wrong with the system.
I wasn't sure how to express the above programmatically, since in
the code it would be a mixture of timings and cycle counts, plus
various assumptions, so I just hardcoded it...

 include/linux/nmi.h |    6 ++-
 kernel/sysctl.c     |   18 ++++++++-
 kernel/watchdog.c   |  108 +++++++++++++++++++++++++++++++++++++++++++--------
 3 files changed, 114 insertions(+), 18 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..b52a4d5 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       = "worst_softstall",
+		.data           = &worst_softstall,
+		.maxlen         = sizeof(unsigned long),
+		.mode           = 0444,
+		.proc_handler   = proc_doulongvec_minmax,
+	},
+#ifdef CONFIG_HARDLOCKUP_DETECTOR
+	{
+		.procname       = "worst_hardstall",
+		.data           = &worst_hardstall,
+		.maxlen         = sizeof(unsigned long),
+		.mode           = 0444,
+		.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..5f44130 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -29,6 +29,9 @@
 
 int watchdog_enabled = 1;
 int __read_mostly watchdog_thresh = 10;
+unsigned long worst_softstall;
+static int softstall_cpu;
+static DEFINE_SPINLOCK(softstall_lock);
 
 static DEFINE_PER_CPU(unsigned long, watchdog_touch_ts);
 static DEFINE_PER_CPU(struct task_struct *, softlockup_watchdog);
@@ -41,7 +44,24 @@ 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(struct perf_event *, watchdog_ev);
-#endif
+
+unsigned long worst_hardstall;
+static int hardstall_cpu;
+static DEFINE_SPINLOCK(hardstall_lock);
+
+/*
+ * The minimum expected number of saved interrupts in one hard
+ * lockup check period.
+ */
+#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 +118,28 @@ __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)
@@ -176,11 +205,31 @@ void touch_softlockup_watchdog_sync(void)
 
 #ifdef CONFIG_HARDLOCKUP_DETECTOR
 /* watchdog detector functions */
-static int is_hardlockup(void)
+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);
 
-	if (__this_cpu_read(hrtimer_interrupts_saved) == hrint)
+	unsigned long stall = hrint - hrint_saved;
+
+	/*
+	 * Stalls of 0 are actually bites, and will be reported
+	 * anyway. No need to make note of them here.
+	 * worst_hardstall==0 means no stalls recorded thus far.
+	 */
+	if (stall < hardstall_thresh &&
+			((!worst_hardstall) ||
+			 (stall && stall < worst_hardstall))) {
+		unsigned long flags;
+		spin_lock_irqsave(&hardstall_lock, flags);
+		if ((!worst_hardstall) || (stall && stall < worst_hardstall)) {
+			worst_hardstall = stall;
+			hardstall_cpu = this_cpu;
+		}
+		spin_unlock_irqrestore(&hardstall_lock, flags);
+	}
+
+	if (hrint_saved == hrint)
 		return 1;
 
 	__this_cpu_write(hrtimer_interrupts_saved, hrint);
@@ -188,12 +237,24 @@ static int is_hardlockup(void)
 }
 #endif
 
-static int is_softlockup(unsigned long touch_ts)
+static int is_softlockup(unsigned long touch_ts, int this_cpu)
 {
-	unsigned long now = get_timestamp(smp_processor_id());
+	unsigned long now = get_timestamp(this_cpu);
+	unsigned long stall = now - touch_ts;
+
+	if (stall > get_softstall_thresh() &&
+			stall > worst_softstall + softstall_diff_thresh) {
+		unsigned long flags;
+		spin_lock_irqsave(&softstall_lock, flags);
+		if (stall > worst_softstall + softstall_diff_thresh) {
+			worst_softstall = stall;
+			softstall_cpu = this_cpu;
+		}
+		spin_unlock_irqrestore(&softstall_lock, flags);
+	}
 
 	/* Warn about unreasonable delays: */
-	if (time_after(now, touch_ts + get_softlockup_thresh()))
+	if (time_after(now, touch_ts + 1000 * get_softlockup_thresh()))
 		return now - touch_ts;
 
 	return 0;
@@ -213,6 +274,10 @@ static void watchdog_overflow_callback(struct perf_event *event, int nmi,
 		 struct perf_sample_data *data,
 		 struct pt_regs *regs)
 {
+	unsigned long old_stall = worst_hardstall;
+	int lockup;
+	int this_cpu = smp_processor_id();
+
 	/* Ensure the watchdog never gets throttled */
 	event->hw.interrupts = 0;
 
@@ -227,9 +292,13 @@ 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 (this_cpu == hardstall_cpu && worst_hardstall != old_stall) {
+		printk(KERN_WARNING "new worst hard stall seen on CPU#%d: %lu interrupts recorded\n",
+			this_cpu, worst_hardstall);
+		dump_stack();
+	}
+	if (lockup) {
 		/* only print hardlockups once */
 		if (__this_cpu_read(hard_watchdog_warn) == true)
 			return;
@@ -259,7 +328,9 @@ 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();
+	unsigned long old_stall = worst_softstall;
 	int duration;
+	int this_cpu = smp_processor_id();
 
 	/* kick the hardlockup detector */
 	watchdog_interrupt_count();
@@ -289,14 +360,19 @@ 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 (this_cpu == softstall_cpu && old_stall != worst_softstall) {
+		printk(KERN_WARNING "new worst soft stall seen on CPU#%d: %lums\n",
+			this_cpu, worst_softstall);
+		dump_stack();
+	}
 	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,
+		printk(KERN_ERR "BUG: soft lockup - CPU#%d stuck for %ums! [%s:%d]\n",
+			this_cpu, duration,
 			current->comm, task_pid_nr(current));
 		print_modules();
 		print_irqtrace_events(current);
-- 
1.7.3.1

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