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workqueue: dump the last woken worker for stalled pools

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To identify the task most likely responsible for a stall, add
last_woken_worker (L: pool->lock) to worker_pool and record it in
kick_pool() just before wake_up_process().  This captures the idle
worker that was kicked to take over when the last running worker went to
sleep; if the pool is now stuck with no running worker, that task is the
prime suspect and its backtrace is dumped by show_pool_no_running_worker().

Using struct worker * rather than struct task_struct * avoids any
lifetime concern: workers are only destroyed via set_worker_dying()
which requires pool->lock, and set_worker_dying() clears
last_woken_worker when the dying worker matches.
show_cpu_pool_busy_workers() holds pool->lock while calling
sched_show_task(), so last_woken_worker is either NULL or points to a
live worker with a valid task.  More precisely, set_worker_dying() clears
last_woken_worker before setting WORKER_DIE, so a non-NULL
last_woken_worker means the kthread has not yet exited and worker->task
is still alive.

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
2026-07-01 04:05:00 -07:00
committed by Tejun Heo
parent f7dc933889
commit e73c290bd7
1 changed files with 27 additions and 2 deletions
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29
kernel/workqueue.c
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@@ -226,6 +226,8 @@ struct worker_pool {
/* L: hash of busy workers */
struct worker *manager; /* L: purely informational */
/* L: last worker woken by kick_pool() */
struct worker *last_woken_worker;
struct list_head workers; /* A: attached workers */
struct ida worker_ida; /* worker IDs for task name */
@@ -1318,6 +1320,9 @@ static bool kick_pool_pick(struct worker_pool *pool, struct task_struct **wakep)
}
}
#endif
/* Track the last idle worker woken, used for stall diagnostics. */
pool->last_woken_worker = worker;
*wakep = p;
return true;
}
@@ -2976,6 +2981,13 @@ static void set_worker_dying(struct worker *worker, struct list_head *list)
pool->nr_workers--;
pool->nr_idle--;
/*
* Clear last_woken_worker if it points to this worker, so that
* show_cpu_pool_busy_workers() cannot dereference a freed worker.
*/
if (pool->last_woken_worker == worker)
pool->last_woken_worker = NULL;
worker->flags |= WORKER_DIE;
list_move(&worker->entry, list);
@@ -7740,13 +7752,25 @@ static void show_pool_no_running_worker(struct worker_pool *pool)
idle_cpu(pool->cpu) ? "idle" : "busy",
pool->nr_workers, pool->nr_idle);
pr_info("The pool might have trouble waking an idle worker.\n");
/*
* last_woken_worker and its task are valid here: set_worker_dying()
* clears it under pool->lock before setting WORKER_DIE, so if
* last_woken_worker is non-NULL the kthread has not yet exited and
* worker->task is still alive.
*/
if (pool->last_woken_worker) {
pr_info("Backtrace of last woken worker:\n");
sched_show_task(pool->last_woken_worker->task);
} else {
pr_info("Last woken worker empty\n");
}
printk_deferred_exit();
}
/*
* Show running workers that might prevent the processing of pending work items.
* If no running worker is found, the pool may be stuck waiting for an idle
* worker to be woken, so report the pool state.
* worker to be woken, so report the pool state and the last woken worker.
*/
static void show_cpu_pool_busy_workers(struct worker_pool *pool)
{
@@ -7781,7 +7805,8 @@ static void show_cpu_pool_busy_workers(struct worker_pool *pool)
/*
* If no running worker was found, the pool is likely stuck. Print pool
* state.
* state and the backtrace of the last woken worker, which is the prime
* suspect for the stall.
*/
if (!found_running)
show_pool_no_running_worker(pool);