[PATCH wq/for-3.9] workqueue: make get_work_pool_id() cheaper

[Tejun Heo <tj@kernel.org>]

From: Lai Jiangshan <laijs@cn.fujitsu.com>

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 |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -626,9 +626,13 @@ static struct worker_pool *get_work_pool
  */
 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);
 
-	return pool ? pool->id : WORK_OFFQ_POOL_NONE;
+	if (data & WORK_STRUCT_CWQ)
+		return ((struct cpu_workqueue_struct *)
+			(data & WORK_STRUCT_WQ_DATA_MASK))->pool->id;
+
+	return data >> WORK_OFFQ_POOL_SHIFT;
 }
 
 static void mark_work_canceling(struct work_struct *work)