On Tue, Jul 29, 2014 at 10:12:11AM +0200, Peter Zijlstra wrote:
> On Mon, Jul 28, 2014 at 03:56:12PM -0700, Paul E. McKenney wrote:
> > + /*
> > + * Each pass through the following loop scans the list
> > + * of holdout tasks, removing any that are no longer
> > + * holdouts. When the list is empty, we are done.
> > + */
> > + while (!list_empty(&rcu_tasks_holdouts)) {
> > + schedule_timeout_interruptible(HZ / 10);
> > + flush_signals(current);
> > + rcu_read_lock();
> > + list_for_each_entry_rcu(t, &rcu_tasks_holdouts,
> > + rcu_tasks_holdout_list) {
> > + if (smp_load_acquire(&t->rcu_tasks_holdout))
> > + continue;
> > + list_del_init(&t->rcu_tasks_holdout_list);
> > + /* @@@ need to check for usermode on CPU. */
> > + }
> > + rcu_read_unlock();
> > + }
>
> That's a potential CPU runtime sink.. imagine having to scan 100k tasks
> 10 times a second. Polling O(nr_tasks) is not good.
This only scans those tasks that are blocking the RCU-tasks grace period,
and this list should get shorter reasonably quickly as each task does
a voluntary context switch.
Of course, there is the do_each_thread() / while_each_thread() loop
that builds this list, and yes, that does look at each and every task,
as does the subsequent loop that waits for pre-existing partially exited
tasks to disappear from the list. As noted in an earlier email, I am
taking Steven at his word when he said that updates are very infrequent
and that he doesn't care about the latency and overhead of the updates.
Thanx, Paul
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