workqueue: use hotcpu_notifier() for workqueue_cpu_down_callback()

workqueue_cpu_down_callback() is used only if HOTPLUG_CPU=y, so hotcpu_notifier() fits better than cpu_notifier(). 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 during boot to do nothing, and the memory of workqueue_cpu_down_callback() and gcwq_unbind_fn() will be discarded after boot. If we use hotcpu_notifier(), we can avoid the no-op call 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 tj: Updated description.

LJLai Jiangshan committed 14 years ago