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Date:	Fri, 8 Aug 2014 21:13:26 +0200
From:	Peter Zijlstra <>
To:	"Paul E. McKenney" <>
Subject: Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

So I think you can make the entire thing work with

If we have the sync thing do something like:

	for_each_task(t) {
		atomic_or(&t->rcu_attention, RCU_TASK);
		if (!t->on_rq) {
			if (atomic_test_and_clear(&t->rcu_attention, RCU_TASK))

	wait_event(&rcu_tasks_wq, !atomic_read(&rcu_tasks));

And then we have rcu_task_note_context_switch() (as called from
rcu_note_context_switch) do:

	/* we want actual context switches, ignore preemption */
	if (preempt_count() & PREEMPT_ACTIVE)

	/* if not marked for RCU attention, bail */
	if (!(atomic_read(&t->rcu_attention) & RCU_TASK))

	/* raced with sync_rcu_task(), all done */
	if (!atomic_test_and_clear(&t->rcu_attention, RCU_TASK))

	/* not the last.. */
	if (!atomic_dec_and_test(&rcu_tasks))


The idea is to share rcu_attention with rcu_preempt, such that we only
touch a single 'extra' cacheline in case RCU doesn't need to pay
attention to this task.

Also, it would be good if we can manage to squeeze this variable in a
cacheline that's already touched by the schedule() so as not to incur
undue overhead.

And on that, you probably should change rcu_sched_rq() to read:


That avoids touching the per-cpu data offset.

And it would be very good if we could avoid the unconditional IRQ flag
fiddling in rcu_preempt_note_context_switch(), them expensive, this
looks entirely feasibly in the 'normal' case where
t->rcu_read_unlock_special doesn't have RCU_READ_UNLOCK_NEED_QS set.
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