Re: [patch 4/4] sched: Distangle worker accounting from rq->lock

Tejun Heo
2011-06-23T15:07:27Z

Hello, again.

So, let's get it correct first.

On Wed, Jun 22, 2011 at 05:52:15PM -0000, Thomas Gleixner wrote:
> The worker accounting for cpu bound workers is plugged into the core
> scheduler code and the wakeup code. This is not a hard requirement and
> can be avoided by keeping track of the state in the workqueue code
> itself.
>
> Keep track of the sleeping state in the worker itself and call the
> notifier before entering the core scheduler. There might be false
> positives when the task is woken between that call and actually
> scheduling, but that's not really different from scheduling and being
> woken immediately after switching away. There is also no harm from
> updating nr_running when the task returns from scheduling instead of
> accounting it in the wakeup code.

I think false positives on schedule() should be safe. As said
earlier, the gap between ttwu and actually running is a bit worrisome
but it might be nothing, but please at least describe the behavior
change.

> Index: linux-2.6/kernel/workqueue.c
> ===================================================================
> --- linux-2.6.orig/kernel/workqueue.c
> +++ linux-2.6/kernel/workqueue.c
> @@ -137,6 +137,7 @@ struct worker {
> unsigned int flags; /* X: flags */
> int id; /* I: worker id */
> struct work_struct rebind_work; /* L: rebind worker to cpu */
> + int sleeping; /* None */

bool?

> -struct task_struct *wq_worker_sleeping(struct task_struct *task,
> - unsigned int cpu)
> +void wq_worker_sleeping(struct task_struct *task)
> {
> - struct worker *worker = kthread_data(task), *to_wakeup = NULL;
> - struct global_cwq *gcwq = get_gcwq(cpu);
> - atomic_t *nr_running = get_gcwq_nr_running(cpu);
> + struct worker *worker = kthread_data(task);
> + struct global_cwq *gcwq;
> + int cpu;
>
> if (worker->flags & WORKER_NOT_RUNNING)
> - return NULL;
> + return;

This doesn't look safe. It can race with trustee_thread() setting
WORKER_ROGUE. Let's just grab gcwq->lock on entry to
wq_worker_sleeping() for now; then, the schedule() trickery in
trustee_thread() can go away too. This also means we can remove the
weird sync rules from ->flags and ->idle_list and just use simple
gcwq->lock for those, which is pretty nice.

> - /* this can only happen on the local cpu */
> - BUG_ON(cpu != raw_smp_processor_id());
> + if (WARN_ON_ONCE(worker->sleeping))
> + return;

Re-entrance is prevented by the scheduler hook being called only for
non-premption schedule(). Maybe it's better to explain that in the
function comment?

Hmmm... Also, I think worker->sleeping should be cleared by
trustee_thread() when WORKER_ROGUE is set for the worker; otherwise,
it can get out of sync and the above WARN_ON_ONCE() will trigger.

Thanks.

--
tejun