Merge branch 'for-4.5' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq

Pull workqueue update from Tejun Heo:
 "Workqueue changes for v4.5.  One cleanup patch and three to improve
  the debuggability.

  Workqueue now has a stall detector which dumps workqueue state if any
  worker pool hasn't made forward progress over a certain amount of time
  (30s by default) and also triggers a warning if a workqueue which can
  be used in memory reclaim path tries to wait on something which can't
  be.

  These should make workqueue hangs a lot easier to debug."

* 'for-4.5' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq:
  workqueue: simplify the apply_workqueue_attrs_locked()
  workqueue: implement lockup detector
  watchdog: introduce touch_softlockup_watchdog_sched()
  workqueue: warn if memory reclaim tries to flush !WQ_MEM_RECLAIM workqueue

Documentation/kernel-parameters.txt

@@ -4140,6 +4140,15 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			or other driver-specific files in the
 			Documentation/watchdog/ directory.
 
+	workqueue.watchdog_thresh=
+			If CONFIG_WQ_WATCHDOG is configured, workqueue can
+			warn stall conditions and dump internal state to
+			help debugging.  0 disables workqueue stall
+			detection; otherwise, it's the stall threshold
+			duration in seconds.  The default value is 30 and
+			it can be updated at runtime by writing to the
+			corresponding sysfs file.
+
 	workqueue.disable_numa
 			By default, all work items queued to unbound
 			workqueues are affine to the NUMA nodes they're

include/linux/sched.h

@@ -377,6 +377,7 @@ extern void scheduler_tick(void);
 extern void sched_show_task(struct task_struct *p);
 
 #ifdef CONFIG_LOCKUP_DETECTOR
+extern void touch_softlockup_watchdog_sched(void);
 extern void touch_softlockup_watchdog(void);
 extern void touch_softlockup_watchdog_sync(void);
 extern void touch_all_softlockup_watchdogs(void);
@@ -387,6 +388,9 @@ extern unsigned int  softlockup_panic;
 extern unsigned int  hardlockup_panic;
 void lockup_detector_init(void);
 #else
+static inline void touch_softlockup_watchdog_sched(void)
+{
+}
 static inline void touch_softlockup_watchdog(void)
 {
 }

include/linux/workqueue.h

@@ -618,4 +618,10 @@ static inline int workqueue_sysfs_register(struct workqueue_struct *wq)
 { return 0; }
 #endif	/* CONFIG_SYSFS */
 
+#ifdef CONFIG_WQ_WATCHDOG
+void wq_watchdog_touch(int cpu);
+#else	/* CONFIG_WQ_WATCHDOG */
+static inline void wq_watchdog_touch(int cpu) { }
+#endif	/* CONFIG_WQ_WATCHDOG */
+
 #endif

kernel/sched/clock.c

@@ -354,7 +354,7 @@ void sched_clock_idle_wakeup_event(u64 delta_ns)
 		return;
 
 	sched_clock_tick();
-	touch_softlockup_watchdog();
+	touch_softlockup_watchdog_sched();
 }
 EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
 

kernel/time/tick-sched.c

@@ -143,7 +143,7 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
 	 * when we go busy again does not account too much ticks.
 	 */
 	if (ts->tick_stopped) {
-		touch_softlockup_watchdog();
+		touch_softlockup_watchdog_sched();
 		if (is_idle_task(current))
 			ts->idle_jiffies++;
 	}
@@ -430,7 +430,7 @@ static void tick_nohz_update_jiffies(ktime_t now)
 	tick_do_update_jiffies64(now);
 	local_irq_restore(flags);
 
-	touch_softlockup_watchdog();
+	touch_softlockup_watchdog_sched();
 }
 
 /*
@@ -717,7 +717,7 @@ static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now, int
 	update_cpu_load_nohz(active);
 
 	calc_load_exit_idle();
-	touch_softlockup_watchdog();
+	touch_softlockup_watchdog_sched();
 	/*
 	 * Cancel the scheduled timer and restore the tick
 	 */

kernel/watchdog.c

@@ -20,6 +20,7 @@
 #include <linux/smpboot.h>
 #include <linux/sched/rt.h>
 #include <linux/tick.h>
+#include <linux/workqueue.h>
 
 #include <asm/irq_regs.h>
 #include <linux/kvm_para.h>
@@ -225,7 +226,15 @@ static void __touch_watchdog(void)
 	__this_cpu_write(watchdog_touch_ts, get_timestamp());
 }
 
-void touch_softlockup_watchdog(void)
+/**
+ * touch_softlockup_watchdog_sched - touch watchdog on scheduler stalls
+ *
+ * Call when the scheduler may have stalled for legitimate reasons
+ * preventing the watchdog task from executing - e.g. the scheduler
+ * entering idle state.  This should only be used for scheduler events.
+ * Use touch_softlockup_watchdog() for everything else.
+ */
+void touch_softlockup_watchdog_sched(void)
 {
 	/*
 	 * Preemption can be enabled.  It doesn't matter which CPU's timestamp
@@ -233,6 +242,12 @@ void touch_softlockup_watchdog(void)
 	 */
 	raw_cpu_write(watchdog_touch_ts, 0);
 }
+
+void touch_softlockup_watchdog(void)
+{
+	touch_softlockup_watchdog_sched();
+	wq_watchdog_touch(raw_smp_processor_id());
+}
 EXPORT_SYMBOL(touch_softlockup_watchdog);
 
 void touch_all_softlockup_watchdogs(void)
@@ -246,6 +261,7 @@ void touch_all_softlockup_watchdogs(void)
 	 */
 	for_each_watchdog_cpu(cpu)
 		per_cpu(watchdog_touch_ts, cpu) = 0;
+	wq_watchdog_touch(-1);
 }
 
 #ifdef CONFIG_HARDLOCKUP_DETECTOR

kernel/workqueue.c

@@ -148,6 +148,8 @@ struct worker_pool {
 	int			id;		/* I: pool ID */
 	unsigned int		flags;		/* X: flags */
 
+	unsigned long		watchdog_ts;	/* L: watchdog timestamp */
+
 	struct list_head	worklist;	/* L: list of pending works */
 	int			nr_workers;	/* L: total number of workers */
 
@@ -1083,6 +1085,8 @@ static void pwq_activate_delayed_work(struct work_struct *work)
 	struct pool_workqueue *pwq = get_work_pwq(work);
 
 	trace_workqueue_activate_work(work);
+	if (list_empty(&pwq->pool->worklist))
+		pwq->pool->watchdog_ts = jiffies;
 	move_linked_works(work, &pwq->pool->worklist, NULL);
 	__clear_bit(WORK_STRUCT_DELAYED_BIT, work_data_bits(work));
 	pwq->nr_active++;
@@ -1385,6 +1389,8 @@ static void __queue_work(int cpu, struct workqueue_struct *wq,
 		trace_workqueue_activate_work(work);
 		pwq->nr_active++;
 		worklist = &pwq->pool->worklist;
+		if (list_empty(worklist))
+			pwq->pool->watchdog_ts = jiffies;
 	} else {
 		work_flags |= WORK_STRUCT_DELAYED;
 		worklist = &pwq->delayed_works;
@@ -2157,6 +2163,8 @@ static int worker_thread(void *__worker)
 			list_first_entry(&pool->worklist,
 					 struct work_struct, entry);
 
+		pool->watchdog_ts = jiffies;
+
 		if (likely(!(*work_data_bits(work) & WORK_STRUCT_LINKED))) {
 			/* optimization path, not strictly necessary */
 			process_one_work(worker, work);
@@ -2240,6 +2248,7 @@ static int rescuer_thread(void *__rescuer)
 					struct pool_workqueue, mayday_node);
 		struct worker_pool *pool = pwq->pool;
 		struct work_struct *work, *n;
+		bool first = true;
 
 		__set_current_state(TASK_RUNNING);
 		list_del_init(&pwq->mayday_node);
@@ -2256,9 +2265,14 @@ static int rescuer_thread(void *__rescuer)
 		 * process'em.
 		 */
 		WARN_ON_ONCE(!list_empty(scheduled));
-		list_for_each_entry_safe(work, n, &pool->worklist, entry)
-			if (get_work_pwq(work) == pwq)
+		list_for_each_entry_safe(work, n, &pool->worklist, entry) {
+			if (get_work_pwq(work) == pwq) {
+				if (first)
+					pool->watchdog_ts = jiffies;
 				move_linked_works(work, scheduled, &n);
+			}
+			first = false;
+		}
 
 		if (!list_empty(scheduled)) {
 			process_scheduled_works(rescuer);
@@ -2316,6 +2330,37 @@ static int rescuer_thread(void *__rescuer)
 	goto repeat;
 }
 
+/**
+ * check_flush_dependency - check for flush dependency sanity
+ * @target_wq: workqueue being flushed
+ * @target_work: work item being flushed (NULL for workqueue flushes)
+ *
+ * %current is trying to flush the whole @target_wq or @target_work on it.
+ * If @target_wq doesn't have %WQ_MEM_RECLAIM, verify that %current is not
+ * reclaiming memory or running on a workqueue which doesn't have
+ * %WQ_MEM_RECLAIM as that can break forward-progress guarantee leading to
+ * a deadlock.
+ */
+static void check_flush_dependency(struct workqueue_struct *target_wq,
+				   struct work_struct *target_work)
+{
+	work_func_t target_func = target_work ? target_work->func : NULL;
+	struct worker *worker;
+
+	if (target_wq->flags & WQ_MEM_RECLAIM)
+		return;
+
+	worker = current_wq_worker();
+
+	WARN_ONCE(current->flags & PF_MEMALLOC,
+		  "workqueue: PF_MEMALLOC task %d(%s) is flushing !WQ_MEM_RECLAIM %s:%pf",
+		  current->pid, current->comm, target_wq->name, target_func);
+	WARN_ONCE(worker && (worker->current_pwq->wq->flags & WQ_MEM_RECLAIM),
+		  "workqueue: WQ_MEM_RECLAIM %s:%pf is flushing !WQ_MEM_RECLAIM %s:%pf",
+		  worker->current_pwq->wq->name, worker->current_func,
+		  target_wq->name, target_func);
+}
+
 struct wq_barrier {
 	struct work_struct	work;
 	struct completion	done;
@@ -2525,6 +2570,8 @@ void flush_workqueue(struct workqueue_struct *wq)
 		list_add_tail(&this_flusher.list, &wq->flusher_overflow);
 	}
 
+	check_flush_dependency(wq, NULL);
+
 	mutex_unlock(&wq->mutex);
 
 	wait_for_completion(&this_flusher.done);
@@ -2697,6 +2744,8 @@ static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr)
 		pwq = worker->current_pwq;
 	}
 
+	check_flush_dependency(pwq->wq, work);
+
 	insert_wq_barrier(pwq, barr, work, worker);
 	spin_unlock_irq(&pool->lock);
 
@@ -3069,6 +3118,7 @@ static int init_worker_pool(struct worker_pool *pool)
 	pool->cpu = -1;
 	pool->node = NUMA_NO_NODE;
 	pool->flags |= POOL_DISASSOCIATED;
+	pool->watchdog_ts = jiffies;
 	INIT_LIST_HEAD(&pool->worklist);
 	INIT_LIST_HEAD(&pool->idle_list);
 	hash_init(pool->busy_hash);
@@ -3601,7 +3651,6 @@ static int apply_workqueue_attrs_locked(struct workqueue_struct *wq,
 					const struct workqueue_attrs *attrs)
 {
 	struct apply_wqattrs_ctx *ctx;
-	int ret = -ENOMEM;
 
 	/* only unbound workqueues can change attributes */
 	if (WARN_ON(!(wq->flags & WQ_UNBOUND)))
@@ -3612,16 +3661,14 @@ static int apply_workqueue_attrs_locked(struct workqueue_struct *wq,
 		return -EINVAL;
 
 	ctx = apply_wqattrs_prepare(wq, attrs);
+	if (!ctx)
+		return -ENOMEM;
 
 	/* the ctx has been prepared successfully, let's commit it */
-	if (ctx) {
-		apply_wqattrs_commit(ctx);
-		ret = 0;
-	}
-
+	apply_wqattrs_commit(ctx);
 	apply_wqattrs_cleanup(ctx);
 
-	return ret;
+	return 0;
 }
 
 /**
@@ -4308,7 +4355,9 @@ void show_workqueue_state(void)
 
 		pr_info("pool %d:", pool->id);
 		pr_cont_pool_info(pool);
-		pr_cont(" workers=%d", pool->nr_workers);
+		pr_cont(" hung=%us workers=%d",
+			jiffies_to_msecs(jiffies - pool->watchdog_ts) / 1000,
+			pool->nr_workers);
 		if (pool->manager)
 			pr_cont(" manager: %d",
 				task_pid_nr(pool->manager->task));
@@ -5167,6 +5216,154 @@ static void workqueue_sysfs_unregister(struct workqueue_struct *wq)
 static void workqueue_sysfs_unregister(struct workqueue_struct *wq)	{ }
 #endif	/* CONFIG_SYSFS */
 
+/*
+ * Workqueue watchdog.
+ *
+ * Stall may be caused by various bugs - missing WQ_MEM_RECLAIM, illegal
+ * flush dependency, a concurrency managed work item which stays RUNNING
+ * indefinitely.  Workqueue stalls can be very difficult to debug as the
+ * usual warning mechanisms don't trigger and internal workqueue state is
+ * largely opaque.
+ *
+ * Workqueue watchdog monitors all worker pools periodically and dumps
+ * state if some pools failed to make forward progress for a while where
+ * forward progress is defined as the first item on ->worklist changing.
+ *
+ * This mechanism is controlled through the kernel parameter
+ * "workqueue.watchdog_thresh" which can be updated at runtime through the
+ * corresponding sysfs parameter file.
+ */
+#ifdef CONFIG_WQ_WATCHDOG
+
+static void wq_watchdog_timer_fn(unsigned long data);
+
+static unsigned long wq_watchdog_thresh = 30;
+static struct timer_list wq_watchdog_timer =
+	TIMER_DEFERRED_INITIALIZER(wq_watchdog_timer_fn, 0, 0);
+
+static unsigned long wq_watchdog_touched = INITIAL_JIFFIES;
+static DEFINE_PER_CPU(unsigned long, wq_watchdog_touched_cpu) = INITIAL_JIFFIES;
+
+static void wq_watchdog_reset_touched(void)
+{
+	int cpu;
+
+	wq_watchdog_touched = jiffies;
+	for_each_possible_cpu(cpu)
+		per_cpu(wq_watchdog_touched_cpu, cpu) = jiffies;
+}
+
+static void wq_watchdog_timer_fn(unsigned long data)
+{
+	unsigned long thresh = READ_ONCE(wq_watchdog_thresh) * HZ;
+	bool lockup_detected = false;
+	struct worker_pool *pool;
+	int pi;
+
+	if (!thresh)
+		return;
+
+	rcu_read_lock();
+
+	for_each_pool(pool, pi) {
+		unsigned long pool_ts, touched, ts;
+
+		if (list_empty(&pool->worklist))
+			continue;
+
+		/* get the latest of pool and touched timestamps */
+		pool_ts = READ_ONCE(pool->watchdog_ts);
+		touched = READ_ONCE(wq_watchdog_touched);
+
+		if (time_after(pool_ts, touched))
+			ts = pool_ts;
+		else
+			ts = touched;
+
+		if (pool->cpu >= 0) {
+			unsigned long cpu_touched =
+				READ_ONCE(per_cpu(wq_watchdog_touched_cpu,
+						  pool->cpu));
+			if (time_after(cpu_touched, ts))
+				ts = cpu_touched;
+		}
+
+		/* did we stall? */
+		if (time_after(jiffies, ts + thresh)) {
+			lockup_detected = true;
+			pr_emerg("BUG: workqueue lockup - pool");
+			pr_cont_pool_info(pool);
+			pr_cont(" stuck for %us!\n",
+				jiffies_to_msecs(jiffies - pool_ts) / 1000);
+		}
+	}
+
+	rcu_read_unlock();
+
+	if (lockup_detected)
+		show_workqueue_state();
+
+	wq_watchdog_reset_touched();
+	mod_timer(&wq_watchdog_timer, jiffies + thresh);
+}
+
+void wq_watchdog_touch(int cpu)
+{
+	if (cpu >= 0)
+		per_cpu(wq_watchdog_touched_cpu, cpu) = jiffies;
+	else
+		wq_watchdog_touched = jiffies;
+}
+
+static void wq_watchdog_set_thresh(unsigned long thresh)
+{
+	wq_watchdog_thresh = 0;
+	del_timer_sync(&wq_watchdog_timer);
+
+	if (thresh) {
+		wq_watchdog_thresh = thresh;
+		wq_watchdog_reset_touched();
+		mod_timer(&wq_watchdog_timer, jiffies + thresh * HZ);
+	}
+}
+
+static int wq_watchdog_param_set_thresh(const char *val,
+					const struct kernel_param *kp)
+{
+	unsigned long thresh;
+	int ret;
+
+	ret = kstrtoul(val, 0, &thresh);
+	if (ret)
+		return ret;
+
+	if (system_wq)
+		wq_watchdog_set_thresh(thresh);
+	else
+		wq_watchdog_thresh = thresh;
+
+	return 0;
+}
+
+static const struct kernel_param_ops wq_watchdog_thresh_ops = {
+	.set	= wq_watchdog_param_set_thresh,
+	.get	= param_get_ulong,
+};
+
+module_param_cb(watchdog_thresh, &wq_watchdog_thresh_ops, &wq_watchdog_thresh,
+		0644);
+
+static void wq_watchdog_init(void)
+{
+	wq_watchdog_set_thresh(wq_watchdog_thresh);
+}
+
+#else	/* CONFIG_WQ_WATCHDOG */
+
+static inline void wq_watchdog_init(void) { }
+
+#endif	/* CONFIG_WQ_WATCHDOG */
+
 static void __init wq_numa_init(void)
 {
 	cpumask_var_t *tbl;
@@ -5290,6 +5487,9 @@ static int __init init_workqueues(void)
 	       !system_unbound_wq || !system_freezable_wq ||
 	       !system_power_efficient_wq ||
 	       !system_freezable_power_efficient_wq);
+
+	wq_watchdog_init();
+
 	return 0;
 }
 early_initcall(init_workqueues);

lib/Kconfig.debug

@@ -812,6 +812,17 @@ config BOOTPARAM_HUNG_TASK_PANIC_VALUE
 	default 0 if !BOOTPARAM_HUNG_TASK_PANIC
 	default 1 if BOOTPARAM_HUNG_TASK_PANIC
 
+config WQ_WATCHDOG
+	bool "Detect Workqueue Stalls"
+	depends on DEBUG_KERNEL
+	help
+	  Say Y here to enable stall detection on workqueues.  If a
+	  worker pool doesn't make forward progress on a pending work
+	  item for over a given amount of time, 30s by default, a
+	  warning message is printed along with dump of workqueue
+	  state.  This can be configured through kernel parameter
+	  "workqueue.watchdog_thresh" and its sysfs counterpart.
+
 endmenu # "Debug lockups and hangs"
 
 config PANIC_ON_OOPS