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