Re: [PATCH] RT: Add priority-queuing and priority-inheritance to workqueue infrastructure

Oleg Nesterov
2007-08-06T16:38:26Z

On 08/06, Peter Zijlstra wrote:
>
> On Mon, 2007-08-06 at 18:45 +0400, Oleg Nesterov wrote:
> > On 08/06, Peter Zijlstra wrote:
>
> > > > I suspect most of the barrier/flush semantics could be replaced with
> > > > completions from specific work items.
> >
> > Hm. But this is exactly how it works?
>
> Yes, barriers work by enqueueing work and waiting for that one work item
> to fall out, thereby knowing that all previous work has been completed.
>
> My point was that most flushes are there to wait for a previously
> enqueued work item, and might as well wait for that one.
>
> Let me try to illustrate: a regular pattern is, we enqueue work A and
> then flush the whole queue to ensure A is processed. So instead of
> enqueueing A, then B in the barrier code, and wait for B to pop out, we
> might as well wait for A to begin with.

This is a bit off-topic, but in that particular case we can do

if (cancel_work_sync(&A))
A->func(&A);

unless we want to execute ->func() on another CPU of course. It is easy to
implement flush_work() which waits for a single work_strcut, but it is not
so useful when it comes to schedule_on_each_cpu().

But I agree, flush_workqueue() should be avoided if possible. Not sure
it makes sense, but we can do something like below.

Oleg.

--- kernel/workqueue.c~ 2007-07-28 16:58:17.000000000 +0400
+++ kernel/workqueue.c 2007-08-06 20:33:25.000000000 +0400
@@ -572,25 +572,54 @@ EXPORT_SYMBOL(schedule_delayed_work_on);
*
* schedule_on_each_cpu() is very slow.
*/
+
+struct xxx
+{
+ atomic_t count;
+ struct completion done;
+ work_func_t func;
+};
+
+struct yyy
+{
+ struct work_struct work;
+ struct xxx *xxx;
+};
+
+static void yyy_func(struct work_struct *work)
+{
+ struct xxx *xxx = container_of(work, struct yyy, work)->xxx;
+ xxx->func(work);
+
+ if (atomic_dec_and_test(&xxx->count))
+ complete(&xxx->done);
+}
+
int schedule_on_each_cpu(work_func_t func)
{
int cpu;
- struct work_struct *works;
+ struct xxx xxx;
+ struct yyy *works;

- works = alloc_percpu(struct work_struct);
+ init_completion(&xxx.done);
+ xxx.func = func;
+
+ works = alloc_percpu(struct yyy);
if (!works)
return -ENOMEM;

preempt_disable(); /* CPU hotplug */
+ atomic_set(&xxx.count, num_online_cpus());
for_each_online_cpu(cpu) {
- struct work_struct *work = per_cpu_ptr(works, cpu);
+ struct yyy *yyy = per_cpu_ptr(works, cpu);

- INIT_WORK(work, func);
- set_bit(WORK_STRUCT_PENDING, work_data_bits(work));
- __queue_work(per_cpu_ptr(keventd_wq->cpu_wq, cpu), work);
+ yyy->xxx = &xxx;
+ INIT_WORK(&yyy->work, yyy_func);
+ set_bit(WORK_STRUCT_PENDING, work_data_bits(&yyy->work));
+ __queue_work(per_cpu_ptr(keventd_wq->cpu_wq, cpu), &yyy->work);
}
preempt_enable();
- flush_workqueue(keventd_wq);
+ wait_for_completion(&xxx.done);
free_percpu(works);
return 0;
}