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workqueue: trigger a single-CPU backtrace for stalled pools

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When a CPU pool is stalled with no running worker, the task occupying the
CPU may not be a workqueue worker at all.  Trigger a single-CPU backtrace
for the stalled CPU to capture what it is currently executing.

The CPU is snapshotted under pool->lock and the backtrace is triggered
after releasing the lock to avoid any potential issues with NMI delivery.

Skip the backtrace when the CPU is offline.  A pool disassociated by CPU
hotplug keeps its pool->cpu, and an NMI to an offline CPU is never acked,
so nmi_trigger_cpumask_backtrace() would busy-wait for its full timeout
in the watchdog's timer context.

Suggested-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Breno Leitao 2026-06-30 09:15:21 -07:00 committed by Tejun Heo
parent 7ddfa24d3f
commit f7dc933889
1 changed files with 13 additions and 1 deletions
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14
kernel/workqueue.c
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@ -7753,10 +7753,13 @@ static void show_cpu_pool_busy_workers(struct worker_pool *pool)
bool found_running = false;
struct worker *worker;
unsigned long irq_flags;
int bkt;
int cpu, bkt;
raw_spin_lock_irqsave(&pool->lock, irq_flags);
/* Snapshot cpu inside the lock to safely use it after unlock. */
cpu = pool->cpu;
hash_for_each(pool->busy_hash, bkt, worker, hentry) {
/* Skip workers that are not actively running on the CPU. */
if (!task_is_running(worker->task))
@ -7784,6 +7787,15 @@ static void show_cpu_pool_busy_workers(struct worker_pool *pool)
show_pool_no_running_worker(pool);
raw_spin_unlock_irqrestore(&pool->lock, irq_flags);
/*
* Trigger a backtrace on the stalled CPU to capture what it is
* currently executing. Skip an offline CPU, whose NMI is never acked
* and would make the backtrace busy-wait until it times out. Done
* after releasing the lock to avoid issues with NMI delivery.
*/
if (!found_running && cpu_online(cpu))
trigger_single_cpu_backtrace(cpu);
}
static void show_cpu_pools_busy_workers(void)