Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

Paul E. McKenney
2014-08-04T11:50:56Z

On Mon, Aug 04, 2014 at 04:18:53PM +0800, Lai Jiangshan wrote:
> On 08/04/2014 03:46 PM, Peter Zijlstra wrote:
> > On Mon, Aug 04, 2014 at 09:28:45AM +0800, Lai Jiangshan wrote:
> >> On 08/01/2014 05:55 AM, Paul E. McKenney wrote:
> >>> + rcu_read_lock();
> >>> + for_each_process_thread(g, t) {
> >>> + if (t != current && ACCESS_ONCE(t->on_rq) &&
> >>> + !is_idle_task(t)) {
> >>> + get_task_struct(t);
> >>> + t->rcu_tasks_nvcsw = ACCESS_ONCE(t->nvcsw);
> >>> + ACCESS_ONCE(t->rcu_tasks_holdout) = 1;
> >>> + list_add(&t->rcu_tasks_holdout_list,
> >>> + &rcu_tasks_holdouts);
> >>
> >> This loop will collect all the runnable tasks. It is too much tasks.
> >> Is it possible to collect only on_cpu tasks or PREEMPT_ACTIVE tasks?
> >> It seems hard to achieve it.
> >
> > Without taking the rq->lock you cannot do that race-free. And we're not
> > going to be taking rq->lock here.
>
> It is because we can't fetch task->on_cpu and preempt_count atomically
> so that rq->lock is required.
>
> 3 bleeding solutions:
>
> 1) Allocate one bit in preempt_count to stand for not_on_cpu ( = !task->on_cpu)
> 2) allocate one bit in nvcsw to stand for on_scheduled (or not_on_scheduled, see next)
> 3) introduce task->on_scheduled whose semantics is between on_cpu and on_rq,
> on_scheduled = scheduled on cpu or preempted, (not voluntary scheduled out)
>
> But the scheduler doesn't need neither of such things. So these is still no hope.

OK, I will bite...

What kinds of tasks are on a runqueue, but neither ->on_cpu nor
PREEMPT_ACTIVE?

Thanx, Paul