linux-kernel - Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu

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Date:	Sat, 9 Aug 2014 18:38:29 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

On Sat, Aug 09, 2014 at 08:33:55PM +0200, Peter Zijlstra wrote:
> 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.

You want me not to disable irqs before invoking rcu_preempt_qs() from
rcu_preempt_note_context_switch(), I get that.  But right now, they
really are disabled courtesy of the local_irq_save() before the call
to rcu_preempt_qs() from rcu_preempt_note_context_switch().

> > 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.

It might be better to avoid storing to rcu_sched_data.passed_quiesce when
it is already 1, though the difference would be quite hard to measure.
In that case, this would work nicely:

static void rcu_preempt_qs(int cpu)
{
	if (rdp->passed_quiesce == 0) {
		trace_rcu_grace_period(TPS("rcu_preempt"), rdp->gpnum, TPS("cpuqs"));
	> 	__this_cpu_write(rcu_sched_data.passed_quiesce, 1);
	}
	current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
}

> 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.

This bit gets set from the scheduler-clock interrupt, so disabling
interrupts is the standard approach to avoid confusion.  Might be possible
to avoid it in this case, or make it less frequent, or whatever.  As I
said, I haven't thought much about it since the initial implementation
some years back, so worth worrying about again.

							Thanx, Paul

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