workqueue: Add workqueue_bh_budget_yield tracepoint

Bottom-Half (BH) workqueues execute work items in softirq context.
To prevent softirqs from starving user and kernel threads, bh_worker()
enforces execution limits (i.e., BH_WORKER_JIFFIES and BH_WORKER_RESTARTS).

When keep_working() is still true but either the time slice or restart
count is exhausted, bh_worker() yields execution and re-raises the
softirq via kick_bh_pool().

Currently, there is no observability into when a BH worker hits these
limits and is forced to yield.

Add the workqueue_bh_budget_yield tracepoint, emitted when bh_worker() exits
the processing loop with pending work items remaining. It records, the
worker pool ID, pool CPU, number of loop restarts consumed, a boolean flag
indicating whether the yield was due to a time slice timeout, and a boolean
flag indicating whether this is a high-priority BH pool.

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

1 files changed, 24 insertions, 2 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 53bb9ad56f49..5b70489a1e65 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3770,6 +3770,9 @@ static void bh_worker(struct worker *worker)
 	struct worker_pool *pool = worker->pool;
 	int nr_restarts = BH_WORKER_RESTARTS;
 	unsigned long end = jiffies + BH_WORKER_JIFFIES;
+	bool budget_exhausted = false;
+	bool timeout = false;
+	int restarts = 0;
 
 	worker_lock_callback(pool);
 	raw_spin_lock_irq(&pool->lock);
@@ -3792,8 +3795,23 @@ static void bh_worker(struct worker *worker)
 
 		if (assign_work(work, worker, NULL))
 			process_scheduled_works(worker);
-	} while (keep_working(pool) &&
-		 --nr_restarts && time_before(jiffies, end));
+
+		if (!keep_working(pool))
+			break;
+
+		if (!--nr_restarts) {
+			budget_exhausted = true;
+			break;
+		}
+
+		if (!time_before(jiffies, end)) {
+			budget_exhausted = true;
+			timeout = true;
+			break;
+		}
+
+		restarts++;
+	} while (1);
 
 	worker_set_flags(worker, WORKER_PREP);
 done:
@@ -3801,6 +3819,10 @@ done:
 	kick_pool(pool);
 	raw_spin_unlock_irq(&pool->lock);
 	worker_unlock_callback(pool);
+
+	if (budget_exhausted)
+		trace_workqueue_bh_budget_yield(pool, restarts, timeout,
+						pool->attrs->nice == HIGHPRI_NICE_LEVEL);
 }
 
 /*