[V2 for-next PATCH 7/7] workqueue: use hotcpu_notifier() for workqueue_cpu_down_callback()

[Lai Jiangshan <laijs@cn.fujitsu.com>]

workqueue_cpu_down_callback() is only used when HOTPLUG_CPU=y.
so it will be better if we use hotcpu_notifier() to register it.

When HOTPLUG_CPU=y, hotcpu_notifier() and cpu_notifier() are the same.

When HOTPLUG_CPU=n, if we use cpu_notifier(), workqueue_cpu_down_callback()
will be called(no-op) when booting and the memory of workqueue_cpu_down_callback()
and gcwq_unbind_fn() will be discard after boot.

But if we use hotcpu_notifier(), we can avoid the no-op calls of
workqueue_cpu_down_callback(). And the memory of workqueue_cpu_down_callback()
and gcwq_unbind_fn() will be discard at build time:

$ ls -l kernel/workqueue.o.cpu_notifier kernel/workqueue.o.hotcpu_notifier
-rw-rw-r-- 1 laijs laijs 484080 Sep 15 11:31 kernel/workqueue.o.cpu_notifier
-rw-rw-r-- 1 laijs laijs 478240 Sep 15 11:31 kernel/workqueue.o.hotcpu_notifier

$ size kernel/workqueue.o.cpu_notifier kernel/workqueue.o.hotcpu_notifier
   text	   data	    bss	    dec	    hex	filename
  18513	   2387	   1221	  22121	   5669	kernel/workqueue.o.cpu_notifier
  18082	   2355	   1221	  21658	   549a	kernel/workqueue.o.hotcpu_notifier

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
 kernel/workqueue.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 2adb316..ba2dbaa 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3815,7 +3815,7 @@ static int __init init_workqueues(void)
 		     WORK_CPU_LAST);
 
 	cpu_notifier(workqueue_cpu_up_callback, CPU_PRI_WORKQUEUE_UP);
-	cpu_notifier(workqueue_cpu_down_callback, CPU_PRI_WORKQUEUE_DOWN);
+	hotcpu_notifier(workqueue_cpu_down_callback, CPU_PRI_WORKQUEUE_DOWN);
 
 	/* initialize gcwqs */
 	for_each_gcwq_cpu(cpu) {
-- 
1.7.4.4