Merge "sched: Ensure watchdog is enabled before disabling"

include/linux/sched.h

@@ -446,6 +446,7 @@ extern unsigned int  hardlockup_panic;
 void lockup_detector_init(void);
 extern void watchdog_enable(unsigned int cpu);
 extern void watchdog_disable(unsigned int cpu);
+extern bool watchdog_configured(unsigned int cpu);
 #else
 static inline void touch_softlockup_watchdog_sched(void)
 {
@@ -468,6 +469,14 @@ static inline void watchdog_enable(unsigned int cpu)
 static inline void watchdog_disable(unsigned int cpu)
 {
 }
+static inline bool watchdog_configured(unsigned int cpu)
+{
+	/*
+	 * Predend the watchdog is always configured.
+	 * We will be waiting for the watchdog to be enabled in core isolation
+	 */
+	return true;
+}
 #endif
 
 #ifdef CONFIG_DETECT_HUNG_TASK

kernel/sched/core.c

@@ -5696,6 +5696,22 @@ int sched_isolate_cpu(int cpu)
 	if (++cpu_isolation_vote[cpu] > 1)
 		goto out;
 
+	/*
+	 * There is a race between watchdog being enabled by hotplug and
+	 * core isolation disabling the watchdog. When a CPU is hotplugged in
+	 * and the hotplug lock has been released the watchdog thread might
+	 * not have run yet to enable the watchdog.
+	 * We have to wait for the watchdog to be enabled before proceeding.
+	 */
+	if (!watchdog_configured(cpu)) {
+		msleep(20);
+		if (!watchdog_configured(cpu)) {
+			--cpu_isolation_vote[cpu];
+			ret_code = -EBUSY;
+			goto out;
+		}
+	}
+
 	set_cpu_isolated(cpu, true);
 	cpumask_clear_cpu(cpu, &avail_cpus);
 

kernel/watchdog.c

@@ -588,17 +588,13 @@ static void watchdog_set_prio(unsigned int policy, unsigned int prio)
 	sched_setscheduler(current, policy, &param);
 }
 
-/* Must be called with hotplug lock (lock_device_hotplug()) held. */
 void watchdog_enable(unsigned int cpu)
 {
 	struct hrtimer *hrtimer = raw_cpu_ptr(&watchdog_hrtimer);
 	unsigned int *enabled = raw_cpu_ptr(&watchdog_en);
 
-	lock_device_hotplug_assert();
-
 	if (*enabled)
 		return;
-	*enabled = 1;
 
 	/* kick off the timer for the hardlockup detector */
 	hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
@@ -614,24 +610,40 @@ void watchdog_enable(unsigned int cpu)
 	/* initialize timestamp */
 	watchdog_set_prio(SCHED_FIFO, MAX_RT_PRIO - 1);
 	__touch_watchdog();
+
+	/*
+	 * Need to ensure above operations are observed by other CPUs before
+	 * indicating that timer is enabled. This is to synchronize core
+	 * isolation and hotplug. Core isolation will wait for this flag to be
+	 * set.
+	 */
+	mb();
+	*enabled = 1;
 }
 
-/* Must be called with hotplug lock (lock_device_hotplug()) held. */
 void watchdog_disable(unsigned int cpu)
 {
 	struct hrtimer *hrtimer = raw_cpu_ptr(&watchdog_hrtimer);
 	unsigned int *enabled = raw_cpu_ptr(&watchdog_en);
 
-	lock_device_hotplug_assert();
-
 	if (!*enabled)
 		return;
-	*enabled = 0;
 
 	watchdog_set_prio(SCHED_NORMAL, 0);
 	hrtimer_cancel(hrtimer);
 	/* disable the perf event */
 	watchdog_nmi_disable(cpu);
+
+	/*
+	 * No need for barrier here since disabling the watchdog is
+	 * synchronized with hotplug lock
+	 */
+	*enabled = 0;
+}
+
+bool watchdog_configured(unsigned int cpu)
+{
+	return *per_cpu_ptr(&watchdog_en, cpu);
 }
 
 static void watchdog_cleanup(unsigned int cpu, bool online)