[PATCH v2 4/5] workqueue: Show all busy workers in stall diagnostics

[Breno Leitao]

show_cpu_pool_hog() only prints workers whose task is currently running
on the CPU (task_is_running()).  This misses workers that are busy
processing a work item but are sleeping or blocked — for example, a
worker that clears PF_WQ_WORKER and enters wait_event_idle().  Such a
worker still occupies a pool slot and prevents progress, yet produces
an empty backtrace section in the watchdog output.

This is happening on real arm64 systems, where
toggle_allocation_gate() IPIs every single CPU in the machine (which
lacks NMI), causing workqueue stalls that show empty backtraces because
toggle_allocation_gate() is sleeping in wait_event_idle().

Remove the task_is_running() filter so every in-flight worker in the
pool's busy_hash is dumped.  The busy_hash is protected by pool->lock,
which is already held.

Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 kernel/workqueue.c | 28 +++++++++++++---------------
 1 file changed, 13 insertions(+), 15 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 56d8af13843f8..09b9ad78d566c 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -7583,9 +7583,9 @@ MODULE_PARM_DESC(panic_on_stall_time, "Panic if stall exceeds this many seconds
 
 /*
  * Show workers that might prevent the processing of pending work items.
- * The only candidates are CPU-bound workers in the running state.
- * Pending work items should be handled by another idle worker
- * in all other situations.
+ * A busy worker that is not running on the CPU (e.g. sleeping in
+ * wait_event_idle() with PF_WQ_WORKER cleared) can stall the pool just as
+ * effectively as a CPU-bound one, so dump every in-flight worker.
  */
 static void show_cpu_pool_hog(struct worker_pool *pool)
 {
@@ -7596,19 +7596,17 @@ static void show_cpu_pool_hog(struct worker_pool *pool)
 	raw_spin_lock_irqsave(&pool->lock, irq_flags);
 
 	hash_for_each(pool->busy_hash, bkt, worker, hentry) {
-		if (task_is_running(worker->task)) {
-			/*
-			 * Defer printing to avoid deadlocks in console
-			 * drivers that queue work while holding locks
-			 * also taken in their write paths.
-			 */
-			printk_deferred_enter();
+		/*
+		 * Defer printing to avoid deadlocks in console
+		 * drivers that queue work while holding locks
+		 * also taken in their write paths.
+		 */
+		printk_deferred_enter();
 
-			pr_info("pool %d:\n", pool->id);
-			sched_show_task(worker->task);
+		pr_info("pool %d:\n", pool->id);
+		sched_show_task(worker->task);
 
-			printk_deferred_exit();
-		}
+		printk_deferred_exit();
 	}
 
 	raw_spin_unlock_irqrestore(&pool->lock, irq_flags);
@@ -7619,7 +7617,7 @@ static void show_cpu_pools_hogs(void)
 	struct worker_pool *pool;
 	int pi;
 
-	pr_info("Showing backtraces of running workers in stalled CPU-bound worker pools:\n");
+	pr_info("Showing backtraces of busy workers in stalled CPU-bound worker pools:\n");
 
 	rcu_read_lock();
 

-- 
2.47.3