workqueue: Add workqueue_mayday and workqueue_rescued tracepoints

When a worker pool fails to create a new worker thread (e.g., typically
under severe memory pressure where memory reclaim paths depend on pending
work items), send_mayday() signals distress to the workqueue's rescuer
thread. The rescuer then takes over processing the pending work items via
assign_rescuer_work().

While pwq->stats[PWQ_STAT_MAYDAY] and pwq->stats[PWQ_STAT_RESCUED] track
these occurrences cumulatively, there is currently no event-driven
mechanism to observe exactly when mayday distress occurs or which work
items require rescue.

Add two new tracepoints namely workqueue_mayday and workqueue_rescued to
make distress and rescuer execution easily observable (e.g., via Ftrace
or eBPF).

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 22fb41b6ec26..53bb9ad56f49 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3127,6 +3127,7 @@ static void send_mayday(struct pool_workqueue *pwq)
 		list_add_tail(&pwq->mayday_node, &wq->maydays);
 		wake_up_process(wq->rescuer->task);
 		pwq->stats[PWQ_STAT_MAYDAY]++;
+		trace_workqueue_mayday(pwq);
 	}
 }
 
@@ -3633,6 +3634,7 @@ static bool assign_rescuer_work(struct pool_workqueue *pwq, struct worker *rescu
 	list_for_each_entry_safe_from(work, n, &pool->worklist, entry) {
 		if (get_work_pwq(work) == pwq && assign_work(work, rescuer, &n)) {
 			pwq->stats[PWQ_STAT_RESCUED]++;
+			trace_workqueue_rescued(pwq, work, work->func);
 			/* put the cursor for next search */
 			list_move_tail(&cursor->entry, &n->entry);
 			return true;