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Subject[PATCH 1/2] workqueues: shrink cpu_populated_map when CPU dies
When cpu_populated_map was introduced, it was supposed that cwq->thread can
survive after CPU_DEAD, that is why we never shrink cpu_populated_map.

This is not very nice, we can safely remove the already dead CPU from the map.
The only required change is that destroy_workqueue() must hold the hotplug lock
until it destroys all cwq->thread's, to protect the cpu_populated_map. We could
make the local copy of cpu mask and drop the lock, but sizeof(cpumask_t) may be
very large.

Also, fix the comment near queue_work(). Unless _cpu_down() happens we do
guarantee the cpu-affinity of the work_struct, and we have users which rely on
this.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- 25/kernel/workqueue.c~2_WQ_CPU_MAP 2008-02-15 16:59:18.000000000 +0300
+++ 25/kernel/workqueue.c 2008-02-16 19:33:03.000000000 +0300
@@ -158,8 +158,8 @@ static void __queue_work(struct cpu_work
*
* Returns 0 if @work was already on a queue, non-zero otherwise.
*
- * We queue the work to the CPU it was submitted, but there is no
- * guarantee that it will be processed by that CPU.
+ * We queue the work to the CPU it was submitted, but if CPU dies
+ * it can be processed by another CPU.
*/
int queue_work(struct workqueue_struct *wq, struct work_struct *work)
{
@@ -813,12 +813,12 @@ void destroy_workqueue(struct workqueue_
spin_lock(&workqueue_lock);
list_del(&wq->list);
spin_unlock(&workqueue_lock);
- put_online_cpus();

for_each_cpu_mask(cpu, *cpu_map) {
cwq = per_cpu_ptr(wq->cpu_wq, cpu);
cleanup_workqueue_thread(cwq, cpu);
}
+ put_online_cpus();

free_percpu(wq->cpu_wq);
kfree(wq);
@@ -836,7 +836,6 @@ static int __devinit workqueue_cpu_callb
action &= ~CPU_TASKS_FROZEN;

switch (action) {
-
case CPU_UP_PREPARE:
cpu_set(cpu, cpu_populated_map);
}
@@ -864,6 +863,12 @@ static int __devinit workqueue_cpu_callb
}
}

+ switch (action) {
+ case CPU_UP_CANCELED:
+ case CPU_DEAD:
+ cpu_clear(cpu, cpu_populated_map);
+ }
+
return NOTIFY_OK;
}



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