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

[Peter Zijlstra <peterz@infradead.org>]

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On Fri, Aug 08, 2014 at 01:58:26PM -0700, Paul E. McKenney wrote:
> 
> > And on that, you probably should change rcu_sched_rq() to read:
> > 
> > 	this_cpu_inc(rcu_sched_data.passed_quiesce);
> > 
> > That avoids touching the per-cpu data offset.
> 
> Hmmm...  Interrupts are disabled,

No they are not, __schedule()->rcu_note_context_switch()->rcu_sched_qs()
is only called with preemption disabled.

We only disable IRQs later, where we take the rq->lock.

> so no need to further disable
> interrupts.  Storing 1 works fine, no need to increment.  If I followed
> the twisty per_cpu passages correctly, my guess is that you would like
> me to do something like this:
> 
> 	__this_cpu_write(rcu_sched_data.passed_quiesce, 1);
> 
> Does that work?

Yeah, should be more or less similar, the inc might be encoded shorter
due to not requiring an immediate, but who cares :-)

void rcu_sched_qs(int cpu)
{
	if (trace_rcu_grace_period_enabled()) {
		if (!__this_cpu_read(rcu_sched_data.passed_quiesce))
			trace_rcu_grace_period(...);
	}
	__this_cpu_write(rcu_sched_data.passed_quiesce, 1);
}

Would further avoid emitting the conditional in the normal case where
the tracepoint is inactive.

Steve does it make sense to have __DO_TRACE() emit __trace_##name() to
avoid the double static_branch thing?

> > 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.
> 
> Agreed, but sometimes RCU_READ_UNLOCK_NEED_QS is set.
> 
> That said, I should probably revisit RCU_READ_UNLOCK_NEED_QS.  A lot has
> changed since I wrote that code.

Sure, but a conditional testing RCU_READ_UNLOCK_NEED_QS is far cheaper
than poking the IRQ flags. That said, its not entirely clear to me why
that needs IRQs disabled at all, then again I didn't look long and I'm
sure its all subtle.

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