workqueue: Reduce expensive locks for unbound workqueue

For unbound workqueue, pwqs usually map to just a few pools. Most of
the time, pwqs will be linked sequentially to wq->pwqs list by cpu
index.  Usually, consecutive CPUs have the same workqueue attribute
(e.g. belong to the same NUMA node). This makes pwqs with the same
pool cluster together in the pwq list.

Only do lock/unlock if the pool has changed in flush_workqueue_prep_pwqs().
This reduces the number of expensive lock operations.

The performance data shows this change boosts FIO by 65x in some cases
when multiple concurrent threads write to xfs mount points with fsync.

FIO Benchmark Details
- FIO version: v3.35
- FIO Options: ioengine=libaio,iodepth=64,norandommap=1,rw=write,
  size=128M,bs=4k,fsync=1
- FIO Job Configs: 64 jobs in total writing to 4 mount points (ramdisks
  formatted as xfs file system).
- Kernel Codebase: v6.12-rc5
- Test Platform: Xeon 8380 (2 sockets)

Reviewed-by: Tim Chen <[email protected]>
Signed-off-by: Wangyang Guo <[email protected]>
Reviewed-by: Lai Jiangshan <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: guowangy

kernel/workqueue.c

@@ -3833,16 +3833,28 @@ static bool flush_workqueue_prep_pwqs(struct workqueue_struct *wq,
 {
 	bool wait = false;
 	struct pool_workqueue *pwq;
+	struct worker_pool *current_pool = NULL;
 
 	if (flush_color >= 0) {
 		WARN_ON_ONCE(atomic_read(&wq->nr_pwqs_to_flush));
 		atomic_set(&wq->nr_pwqs_to_flush, 1);
 	}
 
+	/*
+	 * For unbound workqueue, pwqs will map to only a few pools.
+	 * Most of the time, pwqs within the same pool will be linked
+	 * sequentially to wq->pwqs by cpu index. So in the majority
+	 * of pwq iters, the pool is the same, only doing lock/unlock
+	 * if the pool has changed. This can largely reduce expensive
+	 * lock operations.
+	 */
 	for_each_pwq(pwq, wq) {
-		struct worker_pool *pool = pwq->pool;
-
-		raw_spin_lock_irq(&pool->lock);
+		if (current_pool != pwq->pool) {
+			if (likely(current_pool))
+				raw_spin_unlock_irq(&current_pool->lock);
+			current_pool = pwq->pool;
+			raw_spin_lock_irq(&current_pool->lock);
+		}
 
 		if (flush_color >= 0) {
 			WARN_ON_ONCE(pwq->flush_color != -1);
@@ -3859,9 +3871,11 @@ static bool flush_workqueue_prep_pwqs(struct workqueue_struct *wq,
 			pwq->work_color = work_color;
 		}
 
-		raw_spin_unlock_irq(&pool->lock);
 	}
 
+	if (current_pool)
+		raw_spin_unlock_irq(&current_pool->lock);
+
 	if (flush_color >= 0 && atomic_dec_and_test(&wq->nr_pwqs_to_flush))
 		complete(&wq->first_flusher->done);