workqueue: Fix hung time report of worker pools

The workqueue watchdog prints a warning when there is no progress in
a worker pool. Where the progress means that the pool started processing
a pending work item.

Note that it is perfectly fine to process work items much longer.
The progress should be guaranteed by waking up or creating idle
workers.

show_one_worker_pool() prints state of non-idle worker pool. It shows
a delay since the last pool->watchdog_ts.

The timestamp is updated when a first pending work is queued in
__queue_work(). Also it is updated when a work is dequeued for
processing in worker_thread() and rescuer_thread().

The delay is misleading when there is no pending work item. In this
case it shows how long the last work item is being proceed. Show
zero instead. There is no stall if there is no pending work.

Fixes: 82607adc ("workqueue: implement lockup detector")
Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -5000,10 +5000,16 @@ static void show_one_worker_pool(struct worker_pool *pool)
 	struct worker *worker;
 	bool first = true;
 	unsigned long flags;
+	unsigned long hung = 0;
 
 	raw_spin_lock_irqsave(&pool->lock, flags);
 	if (pool->nr_workers == pool->nr_idle)
 		goto next_pool;
+
+	/* How long the first pending work is waiting for a worker. */
+	if (!list_empty(&pool->worklist))
+		hung = jiffies_to_msecs(jiffies - pool->watchdog_ts) / 1000;
+
 	/*
 	 * Defer printing to avoid deadlocks in console drivers that
 	 * queue work while holding locks also taken in their write
@@ -5012,9 +5018,7 @@ static void show_one_worker_pool(struct worker_pool *pool)
 	printk_deferred_enter();
 	pr_info("pool %d:", pool->id);
 	pr_cont_pool_info(pool);
-	pr_cont(" hung=%us workers=%d",
-		jiffies_to_msecs(jiffies - pool->watchdog_ts) / 1000,
-		pool->nr_workers);
+	pr_cont(" hung=%lus workers=%d", hung, pool->nr_workers);
 	if (pool->manager)
 		pr_cont(" manager: %d",
 			task_pid_nr(pool->manager->task));