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

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