watchdog/softlockup: Replace "watchdog/%u" threads with cpu_stop_work

Oleg suggested to replace the "watchdog/%u" threads with
cpu_stop_work. That removes one thread per CPU while at the same time
fixes softlockup vs SCHED_DEADLINE.

But more importantly, it does away with the single
smpboot_update_cpumask_percpu_thread() user, which allows
cleanups/shrinkage of the smpboot interface.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/cpuhotplug.h

@@ -164,6 +164,7 @@ enum cpuhp_state {
 	CPUHP_AP_PERF_POWERPC_NEST_IMC_ONLINE,
 	CPUHP_AP_PERF_POWERPC_CORE_IMC_ONLINE,
 	CPUHP_AP_PERF_POWERPC_THREAD_IMC_ONLINE,
+	CPUHP_AP_WATCHDOG_ONLINE,
 	CPUHP_AP_WORKQUEUE_ONLINE,
 	CPUHP_AP_RCUTREE_ONLINE,
 	CPUHP_AP_ONLINE_DYN,

include/linux/nmi.h

@@ -33,10 +33,15 @@ extern int sysctl_hardlockup_all_cpu_backtrace;
 #define sysctl_hardlockup_all_cpu_backtrace 0
 #endif /* !CONFIG_SMP */
 
+extern int lockup_detector_online_cpu(unsigned int cpu);
+extern int lockup_detector_offline_cpu(unsigned int cpu);
+
 #else /* CONFIG_LOCKUP_DETECTOR */
 static inline void lockup_detector_init(void) { }
 static inline void lockup_detector_soft_poweroff(void) { }
 static inline void lockup_detector_cleanup(void) { }
+#define lockup_detector_online_cpu	NULL
+#define lockup_detector_offline_cpu	NULL
 #endif /* !CONFIG_LOCKUP_DETECTOR */
 
 #ifdef CONFIG_SOFTLOCKUP_DETECTOR

kernel/cpu.c

@@ -1344,6 +1344,11 @@ static struct cpuhp_step cpuhp_hp_states[] = {
 		.startup.single		= perf_event_init_cpu,
 		.teardown.single	= perf_event_exit_cpu,
 	},
+	[CPUHP_AP_WATCHDOG_ONLINE] = {
+		.name			= "lockup_detector:online",
+		.startup.single		= lockup_detector_online_cpu,
+		.teardown.single	= lockup_detector_offline_cpu,
+	},
 	[CPUHP_AP_WORKQUEUE_ONLINE] = {
 		.name			= "workqueue:online",
 		.startup.single		= workqueue_online_cpu,

kernel/watchdog.c

@@ -18,18 +18,14 @@
 #include <linux/init.h>
 #include <linux/module.h>
 #include <linux/sysctl.h>
-#include <linux/smpboot.h>
-#include <linux/sched/rt.h>
-#include <uapi/linux/sched/types.h>
 #include <linux/tick.h>
-#include <linux/workqueue.h>
 #include <linux/sched/clock.h>
 #include <linux/sched/debug.h>
 #include <linux/sched/isolation.h>
+#include <linux/stop_machine.h>
 
 #include <asm/irq_regs.h>
 #include <linux/kvm_para.h>
-#include <linux/kthread.h>
 
 static DEFINE_MUTEX(watchdog_mutex);
 
@@ -169,11 +165,10 @@ static void lockup_detector_update_enable(void)
 unsigned int __read_mostly softlockup_panic =
 			CONFIG_BOOTPARAM_SOFTLOCKUP_PANIC_VALUE;
 
-static bool softlockup_threads_initialized __read_mostly;
+static bool softlockup_initialized __read_mostly;
 static u64 __read_mostly sample_period;
 
 static DEFINE_PER_CPU(unsigned long, watchdog_touch_ts);
-static DEFINE_PER_CPU(struct task_struct *, softlockup_watchdog);
 static DEFINE_PER_CPU(struct hrtimer, watchdog_hrtimer);
 static DEFINE_PER_CPU(bool, softlockup_touch_sync);
 static DEFINE_PER_CPU(bool, soft_watchdog_warn);
@@ -335,6 +330,25 @@ static void watchdog_interrupt_count(void)
 	__this_cpu_inc(hrtimer_interrupts);
 }
 
+/*
+ * The watchdog thread function - touches the timestamp.
+ *
+ * It only runs once every sample_period seconds (4 seconds by
+ * default) to reset the softlockup timestamp. If this gets delayed
+ * for more than 2*watchdog_thresh seconds then the debug-printout
+ * triggers in watchdog_timer_fn().
+ */
+static int softlockup_fn(void *data)
+{
+	__this_cpu_write(soft_lockup_hrtimer_cnt,
+			 __this_cpu_read(hrtimer_interrupts));
+	__touch_watchdog();
+
+	return 0;
+}
+
+static DEFINE_PER_CPU(struct cpu_stop_work, softlockup_stop_work);
+
 /* watchdog kicker functions */
 static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 {
@@ -350,7 +364,9 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 	watchdog_interrupt_count();
 
 	/* kick the softlockup detector */
-	wake_up_process(__this_cpu_read(softlockup_watchdog));
+	stop_one_cpu_nowait(smp_processor_id(),
+			softlockup_fn, NULL,
+			this_cpu_ptr(&softlockup_stop_work));
 
 	/* .. and repeat */
 	hrtimer_forward_now(hrtimer, ns_to_ktime(sample_period));
@@ -448,17 +464,12 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 	return HRTIMER_RESTART;
 }
 
-static void watchdog_set_prio(unsigned int policy, unsigned int prio)
-{
-	struct sched_param param = { .sched_priority = prio };
-
-	sched_setscheduler(current, policy, &param);
-}
-
 static void watchdog_enable(unsigned int cpu)
 {
 	struct hrtimer *hrtimer = this_cpu_ptr(&watchdog_hrtimer);
 
+	WARN_ON_ONCE(cpu != smp_processor_id());
+
 	/*
 	 * Start the timer first to prevent the NMI watchdog triggering
 	 * before the timer has a chance to fire.
@@ -473,15 +484,14 @@ static void watchdog_enable(unsigned int cpu)
 	/* Enable the perf event */
 	if (watchdog_enabled & NMI_WATCHDOG_ENABLED)
 		watchdog_nmi_enable(cpu);
-
-	watchdog_set_prio(SCHED_FIFO, MAX_RT_PRIO - 1);
 }
 
 static void watchdog_disable(unsigned int cpu)
 {
 	struct hrtimer *hrtimer = this_cpu_ptr(&watchdog_hrtimer);
 
-	watchdog_set_prio(SCHED_NORMAL, 0);
+	WARN_ON_ONCE(cpu != smp_processor_id());
+
 	/*
 	 * Disable the perf event first. That prevents that a large delay
 	 * between disabling the timer and disabling the perf event causes
@@ -491,77 +501,63 @@ static void watchdog_disable(unsigned int cpu)
 	hrtimer_cancel(hrtimer);
 }
 
-static void watchdog_cleanup(unsigned int cpu, bool online)
+static int softlockup_stop_fn(void *data)
 {
-	watchdog_disable(cpu);
+	watchdog_disable(smp_processor_id());
+	return 0;
 }
 
-static int watchdog_should_run(unsigned int cpu)
+static void softlockup_stop_all(void)
 {
-	return __this_cpu_read(hrtimer_interrupts) !=
-		__this_cpu_read(soft_lockup_hrtimer_cnt);
+	int cpu;
+
+	if (!softlockup_initialized)
+		return;
+
+	for_each_cpu(cpu, &watchdog_allowed_mask)
+		smp_call_on_cpu(cpu, softlockup_stop_fn, NULL, false);
+
+	cpumask_clear(&watchdog_allowed_mask);
 }
 
-/*
- * The watchdog thread function - touches the timestamp.
- *
- * It only runs once every sample_period seconds (4 seconds by
- * default) to reset the softlockup timestamp. If this gets delayed
- * for more than 2*watchdog_thresh seconds then the debug-printout
- * triggers in watchdog_timer_fn().
- */
-static void watchdog(unsigned int cpu)
+static int softlockup_start_fn(void *data)
 {
-	__this_cpu_write(soft_lockup_hrtimer_cnt,
-			 __this_cpu_read(hrtimer_interrupts));
-	__touch_watchdog();
+	watchdog_enable(smp_processor_id());
+	return 0;
 }
 
-static struct smp_hotplug_thread watchdog_threads = {
-	.store			= &softlockup_watchdog,
-	.thread_should_run	= watchdog_should_run,
-	.thread_fn		= watchdog,
-	.thread_comm		= "watchdog/%u",
-	.setup			= watchdog_enable,
-	.cleanup		= watchdog_cleanup,
-	.park			= watchdog_disable,
-	.unpark			= watchdog_enable,
-};
-
-static void softlockup_update_smpboot_threads(void)
+static void softlockup_start_all(void)
 {
-	lockdep_assert_held(&watchdog_mutex);
-
-	if (!softlockup_threads_initialized)
-		return;
+	int cpu;
 
-	smpboot_update_cpumask_percpu_thread(&watchdog_threads,
-					     &watchdog_allowed_mask);
+	cpumask_copy(&watchdog_allowed_mask, &watchdog_cpumask);
+	for_each_cpu(cpu, &watchdog_allowed_mask)
+		smp_call_on_cpu(cpu, softlockup_start_fn, NULL, false);
 }
 
-/* Temporarily park all watchdog threads */
-static void softlockup_park_all_threads(void)
+int lockup_detector_online_cpu(unsigned int cpu)
 {
-	cpumask_clear(&watchdog_allowed_mask);
-	softlockup_update_smpboot_threads();
+	watchdog_enable(cpu);
+	return 0;
 }
 
-/* Unpark enabled threads */
-static void softlockup_unpark_threads(void)
+int lockup_detector_offline_cpu(unsigned int cpu)
 {
-	cpumask_copy(&watchdog_allowed_mask, &watchdog_cpumask);
-	softlockup_update_smpboot_threads();
+	watchdog_disable(cpu);
+	return 0;
 }
 
 static void lockup_detector_reconfigure(void)
 {
 	cpus_read_lock();
 	watchdog_nmi_stop();
-	softlockup_park_all_threads();
+
+	softlockup_stop_all();
 	set_sample_period();
 	lockup_detector_update_enable();
 	if (watchdog_enabled && watchdog_thresh)
-		softlockup_unpark_threads();
+		softlockup_start_all();
+
 	watchdog_nmi_start();
 	cpus_read_unlock();
 	/*
@@ -580,8 +576,6 @@ static void lockup_detector_reconfigure(void)
  */
 static __init void lockup_detector_setup(void)
 {
-	int ret;
-
 	/*
 	 * If sysctl is off and watchdog got disabled on the command line,
 	 * nothing to do here.
@@ -592,24 +586,13 @@ static __init void lockup_detector_setup(void)
 	    !(watchdog_enabled && watchdog_thresh))
 		return;
 
-	ret = smpboot_register_percpu_thread_cpumask(&watchdog_threads,
-						     &watchdog_allowed_mask);
-	if (ret) {
-		pr_err("Failed to initialize soft lockup detector threads\n");
-		return;
-	}
-
 	mutex_lock(&watchdog_mutex);
-	softlockup_threads_initialized = true;
 	lockup_detector_reconfigure();
+	softlockup_initialized = true;
 	mutex_unlock(&watchdog_mutex);
 }
 
 #else /* CONFIG_SOFTLOCKUP_DETECTOR */
-static inline int watchdog_park_threads(void) { return 0; }
-static inline void watchdog_unpark_threads(void) { }
-static inline int watchdog_enable_all_cpus(void) { return 0; }
-static inline void watchdog_disable_all_cpus(void) { }
 static void lockup_detector_reconfigure(void)
 {
 	cpus_read_lock();