Messages in this thread Patch in this message | | | Date | Sun, 1 Jul 2018 10:40:45 -0700 | From | Joel Fernandes <> | Subject | Re: [PATCH RFC tip/core/rcu 1/2] rcu: Defer reporting RCU-preempt quiescent states when disabled |
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On Wed, Jun 27, 2018 at 01:49:14PM -0700, Paul E. McKenney wrote: > This commit defers reporting of RCU-preempt quiescent states at > rcu_read_unlock_special() time when any of interrupts, softirq, or > preemption are disabled. These deferred quiescent states are reported > at a later RCU_SOFTIRQ, context switch, idle entry, or CPU-hotplug > offline operation. Of course, if another RCU read-side critical > section has started in the meantime, the reporting of the quiescent > state will be further deferred. > > This also means that disabling preemption, interrupts, and/or > softirqs will act as an RCU-preempt read-side critical section. > This is enforced by checking preempt_count() as needed. > > Some special cases must be handled on an ad-hoc basis, for example, > context switch is a quiescent state even though both the scheduler and > do_exit() disable preemption. In these cases, additional calls to > rcu_preempt_deferred_qs() override the preemption disabling. Similar > logic overrides disabled interrupts in rcu_preempt_check_callbacks() > because in this case the quiescent state happened just before the > corresponding scheduling-clock interrupt. > > This change lifts a long-standing restriction that required that if > interrupts were disabled across a call to rcu_read_unlock() that the > matching rcu_read_lock() also be contained within that interrupts-disabled > region of code. Because the reporting of the corresponding RCU-preempt > quiescent state is now deferred until after interrupts have been enabled, > it is no longer possible for this situation to result in deadlocks > involving the scheduler's runqueue and priority-inheritance locks. > This may allow some code simplification that might reduce interrupt > latency a bit. Unfortunately, this would also defer deboosting a > low-priority task that had been subjected to RCU priority boosting, > so real-time-response considerations might well force this restriction > to remain in place. > > Because RCU-preempt grace periods are now blocked not only by RCU > read-side critical sections, but also by disabling of interrupts, > preemption, and softirqs, it will be possible to eliminate RCU-bh and > RCU-sched in favor of RCU-preempt in CONFIG_PREEMPT=y kernels. This may > require some additional plumbing to provide the network denial-of-service > guarantees that have been traditionally provided by RCU-bh. Once these > are in place, CONFIG_PREEMPT=n kernels will be able to fold RCU-bh > into RCU-sched. This would mean that all kernels would have but > one flavor of RCU, which would open the door to significant code > cleanup. > > Moving to a single flavor of RCU would also have the beneficial effect > of reducing the NOCB kthreads by at least a factor of two. > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> [...] > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > index c1b17f5b9361..ff5c70eae47d 100644 > --- a/kernel/rcu/tree_plugin.h > +++ b/kernel/rcu/tree_plugin.h > @@ -371,6 +371,9 @@ static void rcu_preempt_note_context_switch(bool preempt) > * behalf of preempted instance of __rcu_read_unlock(). > */ > rcu_read_unlock_special(t); > + rcu_preempt_deferred_qs(t); > + } else { > + rcu_preempt_deferred_qs(t); > } > > /* > @@ -464,54 +467,51 @@ static bool rcu_preempt_has_tasks(struct rcu_node *rnp) > } > > /* > - * 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. > + * Report deferred quiescent states. The deferral time can > + * be quite short, for example, in the case of the call from > + * rcu_read_unlock_special(). > */ > -static void rcu_read_unlock_special(struct task_struct *t) > +static void > +rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags) > { > bool empty_exp; > bool empty_norm; > bool empty_exp_now; > - unsigned long flags; > struct list_head *np; > bool drop_boost_mutex = false; > struct rcu_data *rdp; > struct rcu_node *rnp; > union rcu_special special; > > - /* NMI handlers cannot block and cannot safely manipulate state. */ > - if (in_nmi()) > - return; > - > - local_irq_save(flags); > - > /* > * If RCU core is waiting for this CPU to exit its critical section, > * report the fact that it has exited. Because irqs are disabled, > * t->rcu_read_unlock_special cannot change. > */ > special = t->rcu_read_unlock_special; > + rdp = this_cpu_ptr(rcu_state_p->rda); > + if (!special.s && !rdp->deferred_qs) { > + local_irq_restore(flags); > + return; > + } > if (special.b.need_qs) { > rcu_preempt_qs(); > t->rcu_read_unlock_special.b.need_qs = false; > - if (!t->rcu_read_unlock_special.s) { > + if (!t->rcu_read_unlock_special.s && !rdp->deferred_qs) { > local_irq_restore(flags); > return; > } > } > > /* > - * Respond to a request for an expedited grace period, but only if > - * we were not preempted, meaning that we were running on the same > - * CPU throughout. If we were preempted, the exp_need_qs flag > - * would have been cleared at the time of the first preemption, > - * and the quiescent state would be reported when we were dequeued. > + * Respond to a request by an expedited grace period for a > + * quiescent state from this CPU. Note that requests from > + * tasks are handled when removing the task from the > + * blocked-tasks list below. > */ > - if (special.b.exp_need_qs) { > - WARN_ON_ONCE(special.b.blocked); > + if (special.b.exp_need_qs || rdp->deferred_qs) { > t->rcu_read_unlock_special.b.exp_need_qs = false; > - rdp = this_cpu_ptr(rcu_state_p->rda); > + rdp->deferred_qs = false; > rcu_report_exp_rdp(rcu_state_p, rdp, true); > if (!t->rcu_read_unlock_special.s) { > local_irq_restore(flags); > @@ -519,19 +519,6 @@ static void rcu_read_unlock_special(struct task_struct *t) > } > } > > - /* Hardware IRQ handlers cannot block, complain if they get here. */ > - if (in_irq() || in_serving_softirq()) { > - lockdep_rcu_suspicious(__FILE__, __LINE__, > - "rcu_read_unlock() from irq or softirq with blocking in critical section!!!\n"); > - pr_alert("->rcu_read_unlock_special: %#x (b: %d, enq: %d nq: %d)\n", > - t->rcu_read_unlock_special.s, > - t->rcu_read_unlock_special.b.blocked, > - t->rcu_read_unlock_special.b.exp_need_qs, > - t->rcu_read_unlock_special.b.need_qs); > - local_irq_restore(flags); > - return; > - } > - > /* Clean up if blocked during RCU read-side critical section. */ > if (special.b.blocked) { > t->rcu_read_unlock_special.b.blocked = false; > @@ -602,6 +589,66 @@ static void rcu_read_unlock_special(struct task_struct *t) > } > } > > +/* > + * Is a deferred quiescent-state pending, and are we also not in > + * an RCU read-side critical section? It is the caller's responsibility > + * to ensure it is otherwise safe to report any deferred quiescent > + * states. The reason for this is that it is safe to report a > + * quiescent state during context switch even though preemption > + * is disabled. This function cannot be expected to understand these > + * nuances, so the caller must handle them. > + */ > +static bool rcu_preempt_need_deferred_qs(struct task_struct *t) > +{ > + return (this_cpu_ptr(&rcu_preempt_data)->deferred_qs || > + READ_ONCE(t->rcu_read_unlock_special.s)) && > + !t->rcu_read_lock_nesting; > +} > + > +/* > + * 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);
Would it be better to just test for those bits just to be safe the higher order bits don't bleed in, such as PREEMPT_NEED_RESCHED, something like the following based on the 'dev' branch?
thanks,
- Joel
---8<-----------------------
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index dfeca11c9fe7..ca7cfdf422f1 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -626,7 +626,8 @@ static void rcu_preempt_deferred_qs(struct task_struct *t) static void rcu_read_unlock_special(struct task_struct *t) { unsigned long flags; - bool preempt_bh_were_disabled = !!(preempt_count() & ~HARDIRQ_MASK); + bool preempt_bh_were_disabled = !!(preempt_count() & + (PREEMPT_MASK | SOFTIRQ_MASK)); bool irqs_were_disabled; /* NMI handlers cannot block and cannot safely manipulate state. */
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