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workqueue: Show all busy workers in stall diagnostics

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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@debian.org>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Breno Leitao
2026-03-05 08:15:40 -08:00
committed by Tejun Heo
parent e8e14ac7cf
commit 8823eaef45
1 changed files with 13 additions and 15 deletions
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28
kernel/workqueue.c
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@@ -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();