workqueue: make get_work_pool_id() cheaper

get_work_pool_id() currently first obtains pool using get_work_pool()
and then return pool->id.  For an off-queue work item, this involves
obtaining pool ID from worker->data, performing idr_find() to find the
matching pool and then returning its pool->id which of course is the
same as the one which went into idr_find().

Just open code WORK_STRUCT_CWQ case and directly return pool ID from
work->data.

tj: The original patch dropped on-queue work item handling and renamed
    the function to offq_work_pool_id().  There isn't much benefit in
    doing so.  Handling it only requires a single if() and we need at
    least BUG_ON(), which is also a branch, even if we drop on-queue
    handling.  Open code WORK_STRUCT_CWQ case and keep the function in
    line with get_work_pool().  Rewrote the description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -611,9 +611,13 @@ static struct worker_pool *get_work_pool(struct work_struct *work)
  */
 static int get_work_pool_id(struct work_struct *work)
 {
-	struct worker_pool *pool = get_work_pool(work);
+	unsigned long data = atomic_long_read(&work->data);
+
+	if (data & WORK_STRUCT_CWQ)
+		return ((struct cpu_workqueue_struct *)
+			(data & WORK_STRUCT_WQ_DATA_MASK))->pool->id;
 
-	return pool ? pool->id : WORK_OFFQ_POOL_NONE;
+	return data >> WORK_OFFQ_POOL_SHIFT;
 }
 
 static void mark_work_canceling(struct work_struct *work)