Re: [PATCH RFC tip/core/rcu 1/2] rcu: Defer reporting RCU-preempt quiescent states when disabled

[Paul E. McKenney]

On Sun, Jul 01, 2018 at 05:35:53PM -0700, Joel Fernandes wrote:
> On Sun, Jul 01, 2018 at 03:27:49PM -0700, Paul E. McKenney wrote:
> [...]
> > > > +/*
> > > > + * Report a deferred quiescent state if needed and safe to do so.
> > > > + * As with rcu_preempt_need_deferred_qs(), "safe" involves only
> > > > + * not being in an RCU read-side critical section.  The caller must
> > > > + * evaluate safety in terms of interrupt, softirq, and preemption
> > > > + * disabling.
> > > > + */
> > > > +static void rcu_preempt_deferred_qs(struct task_struct *t)
> > > > +{
> > > > +	unsigned long flags;
> > > > +
> > > > +	if (!rcu_preempt_need_deferred_qs(t))
> > > > +		return;
> > > > +	local_irq_save(flags);
> > > > +	rcu_preempt_deferred_qs_irqrestore(t, flags);
> > > > +}
> > > > +
> > > > +/*
> > > > + * Handle special cases during rcu_read_unlock(), such as needing to
> > > > + * notify RCU core processing or task having blocked during the RCU
> > > > + * read-side critical section.
> > > > + */
> > > > +static void rcu_read_unlock_special(struct task_struct *t)
> > > > +{
> > > > +	unsigned long flags;
> > > > +	bool preempt_bh_were_disabled = !!(preempt_count() & ~HARDIRQ_MASK);
> > > > +	bool irqs_were_disabled;
> > > > +
> > > > +	/* NMI handlers cannot block and cannot safely manipulate state. */
> > > > +	if (in_nmi())
> > > > +		return;
> > > > +
> > > > +	local_irq_save(flags);
> > > > +	irqs_were_disabled = irqs_disabled_flags(flags);
> > > > +	if ((preempt_bh_were_disabled || irqs_were_disabled) &&
> > > > +	    t->rcu_read_unlock_special.b.blocked) {
> > > > +		/* Need to defer quiescent state until everything is enabled. */
> > > > +		raise_softirq_irqoff(RCU_SOFTIRQ);
> > > > +		local_irq_restore(flags);
> > > > +		return;
> > > > +	}
> > > > +	rcu_preempt_deferred_qs_irqrestore(t, flags);
> > > > +}
> > > > +
> > > >  /*
> > > >   * Dump detailed information for all tasks blocking the current RCU
> > > >   * grace period on the specified rcu_node structure.
> > > > @@ -737,10 +784,20 @@ static void rcu_preempt_check_callbacks(void)
> > > >  	struct rcu_state *rsp = &rcu_preempt_state;
> > > >  	struct task_struct *t = current;
> > > >  
> > > > -	if (t->rcu_read_lock_nesting == 0) {
> > > > -		rcu_preempt_qs();
> > > > +	if (t->rcu_read_lock_nesting > 0 ||
> > > > +	    (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) {
> > > > +		/* No QS, force context switch if deferred. */
> > > > +		if (rcu_preempt_need_deferred_qs(t))
> > > > +			resched_cpu(smp_processor_id());
> > > 
> > > 
> > > Hi Paul,
> > > 
> > > I had a similar idea of checking the preempt_count() sometime back but didn't
> > > believe this path can be called with preempt enabled (for some reason ;-)).
> > > Now that I've convinced myself that's possible, what do you think about
> > > taking advantage of the opportunity to report a RCU-sched qs like below from
> > > rcu_check_callbacks ?
> > > 
> > > Did some basic testing, can roll into a patch later if you're Ok with it.
> > 
> > The problem here is that the code patch above cannot be called
> > with CONFIG_PREEMPT_COUNT=n, but the code below can.  And if
> > CONFIG_PREEMPT_COUNT=n, the return value from preempt_count() can be
> > misleading.
> > 
> > Or am I missing something here?
> 
> That is true! so then I could also test if PREEMPT_RCU is enabled like you're
> doing in the other path.
> 
> thanks!
> 
> ---8<-----------------------
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index fb440baf8ac6..03a460921dca 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2683,6 +2683,12 @@ void rcu_check_callbacks(int user)
>  		rcu_note_voluntary_context_switch(current);
> 
>  	} else if (!in_softirq()) {
> +		/*
> +		 * Report RCU-sched qs if not in an RCU-sched read-side
> +		 * critical section.
> +		 */
> +		if (IS_ENABLED(PREEMPT_RCU) && !(preempt_count() & PREEMPT_MASK))

For more precision, s/PREEMPT_RCU/CONFIG_PREEMPT_COUNT/

Hmmm...  I recently queued a patch that redefines the RCU-bh update-side
API in terms of the consolidated RCU implementation, so this "else"
clause no longer exists.  One approach would be to fold this condition
(with the addition of SOFTIRQ_MASK) into the previous "if" condition,
but that would call rcu_note_voluntary_context_switch() at bad times.
So maybe this becomes a new "else if" clause.

Another complication is an upcoming step that redefines the RCU-sched
update-side API in terms of the consolidated RCU implementation, which
will likely restructure this "if" statement yet again.

So I will try to fold this idea in (with attribution).  If I don't get
it in place in a week or two, please remind me.  Of course, one good way
to remind me is to supply a patch against whatever this turns into.  ;-)

							Thanx, Paul

> +			rcu_sched_qs();
> 
>  		/*
>  		 * Get here if this CPU did not take its interrupt from
>