Re: [PATCH tip/core/rcu 1/4] rcu: Expedite deboost in case of deferred quiescent state

["Paul E. McKenney" <paulmck@kernel.org>]

On Wed, Jan 27, 2021 at 10:42:35AM +0800, Boqun Feng wrote:
> Hi Paul,
> 
> On Tue, Jan 19, 2021 at 08:32:33PM -0800, paulmck@kernel.org wrote:
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> > 
> > Historically, a task that has been subjected to RCU priority boosting is
> > deboosted at rcu_read_unlock() time.  However, with the advent of deferred
> > quiescent states, if the outermost rcu_read_unlock() was invoked with
> > either bottom halves, interrupts, or preemption disabled, the deboosting
> > will be delayed for some time.  During this time, a low-priority process
> > might be incorrectly running at a high real-time priority level.
> > 
> > Fortunately, rcu_read_unlock_special() already provides mechanisms for
> > forcing a minimal deferral of quiescent states, at least for kernels
> > built with CONFIG_IRQ_WORK=y.  These mechanisms are currently used
> > when expedited grace periods are pending that might be blocked by the
> > current task.  This commit therefore causes those mechanisms to also be
> > used in cases where the current task has been or might soon be subjected
> > to RCU priority boosting.  Note that this applies to all kernels built
> > with CONFIG_RCU_BOOST=y, regardless of whether or not they are also
> > built with CONFIG_PREEMPT_RT=y.
> > 
> > This approach assumes that kernels build for use with aggressive real-time
> > applications are built with CONFIG_IRQ_WORK=y.  It is likely to be far
> > simpler to enable CONFIG_IRQ_WORK=y than to implement a fast-deboosting
> > scheme that works correctly in its absence.
> > 
> > While in the area, alphabetize the rcu_preempt_deferred_qs_handler()
> > function's local variables.
> > 
> > Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > Cc: Scott Wood <swood@redhat.com>
> > Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> >  kernel/rcu/tree_plugin.h | 26 ++++++++++++++------------
> >  1 file changed, 14 insertions(+), 12 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 8b0feb2..fca31c6 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -660,9 +660,9 @@ static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
> >  static void rcu_read_unlock_special(struct task_struct *t)
> >  {
> >  	unsigned long flags;
> > +	bool irqs_were_disabled;
> >  	bool preempt_bh_were_disabled =
> >  			!!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
> > -	bool irqs_were_disabled;
> >  
> >  	/* NMI handlers cannot block and cannot safely manipulate state. */
> >  	if (in_nmi())
> > @@ -671,30 +671,32 @@ static void rcu_read_unlock_special(struct task_struct *t)
> >  	local_irq_save(flags);
> >  	irqs_were_disabled = irqs_disabled_flags(flags);
> >  	if (preempt_bh_were_disabled || irqs_were_disabled) {
> > -		bool exp;
> > +		bool expboost; // Expedited GP in flight or possible boosting.
> >  		struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> >  		struct rcu_node *rnp = rdp->mynode;
> >  
> > -		exp = (t->rcu_blocked_node &&
> > -		       READ_ONCE(t->rcu_blocked_node->exp_tasks)) ||
> > -		      (rdp->grpmask & READ_ONCE(rnp->expmask));
> > +		expboost = (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_tasks)) ||
> > +			   (rdp->grpmask & READ_ONCE(rnp->expmask)) ||
> > +			   (IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled &&
> > +			    t->rcu_blocked_node);
> 
> I take it that you check whether possible boosting is in progress via
> the last expression of "||", ie:
> 
> 	(IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled &&
> 	t->rcu_blocked_node)
> 
> if so, I don't see the point of using the new "expboost" in the
> raise_softirq_irqoff() branch, because if in_irq() is false, we only
> raise softirq if irqs_were_disabled is false (otherwise, we may take the
> risk of doing a wakeup with a pi or rq lock held, IIRC), and the
> boosting part of the "expboost" above is only true if irqs_were_disabled
> is true, so using expboost makes no different here.

I started out with two local variables, one for each side of the ||,
but this looked nicer.

> >  		// Need to defer quiescent state until everything is enabled.
> > -		if (use_softirq && (in_irq() || (exp && !irqs_were_disabled))) {
> > +		if (use_softirq && (in_irq() || (expboost && !irqs_were_disabled))) {
> >  			// Using softirq, safe to awaken, and either the
> > -			// wakeup is free or there is an expedited GP.
> > +			// wakeup is free or there is either an expedited
> > +			// GP in flight or a potential need to deboost.
> 
> and this comment will be incorrect, we won't enter here solely because
> there is a potential need to deboost.

You are quite right, given the !irqs_were_disabled.

> That said, why the boosting condition has a "irqs_were_disabled" in it?
> What if a task gets boosted because of RCU boosting, and exit the RCU
> read-side c.s. with irq enabled and there is no expedited GP in flight,
> will the task get deboosted quickly enough?

Because if !irqs_were_disabled, there will be a local_bh_enable() or
a preempt_enable(), give or take preempt_enable_no_resched(), and those
will both get the scheduler involved because of the set_tsk_need_resched()
and set_preempt_need_resched().  There is thus no need for the irq_work
unless irqs_were_disabled.

I am not all that worried about preempt_enable_no_resched() because
it is a problem for RT even in the absence of RCU priority boosting.
And the current uses appear to be in things that one would not use while
running an RT workload.

> Maybe I'm missing some subtle?

Or maybe I am.  Thoughts?

							Thanx, Paul

> Regards,
> Boqun
> 
> >  			raise_softirq_irqoff(RCU_SOFTIRQ);
> >  		} else {
> >  			// Enabling BH or preempt does reschedule, so...
> > -			// Also if no expediting, slow is OK.
> > -			// Plus nohz_full CPUs eventually get tick enabled.
> > +			// Also if no expediting and no possible deboosting,
> > +			// slow is OK.  Plus nohz_full CPUs eventually get
> > +			// tick enabled.
> >  			set_tsk_need_resched(current);
> >  			set_preempt_need_resched();
> >  			if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
> > -			    !rdp->defer_qs_iw_pending && exp && cpu_online(rdp->cpu)) {
> > +			    expboost && !rdp->defer_qs_iw_pending && cpu_online(rdp->cpu)) {
> >  				// Get scheduler to re-evaluate and call hooks.
> >  				// If !IRQ_WORK, FQS scan will eventually IPI.
> > -				init_irq_work(&rdp->defer_qs_iw,
> > -					      rcu_preempt_deferred_qs_handler);
> > +				init_irq_work(&rdp->defer_qs_iw, rcu_preempt_deferred_qs_handler);
> >  				rdp->defer_qs_iw_pending = true;
> >  				irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
> >  			}
> > -- 
> > 2.9.5
> >