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McKenney" Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com Subject: Re: [PATCH RFC tip/core/rcu 1/2] rcu: Defer reporting RCU-preempt quiescent states when disabled Message-ID: <20180701183828.GB111992@joelaf.mtv.corp.google.com> References: <20180627204835.GA25456@linux.vnet.ibm.com> <20180627204915.27253-1-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180627204915.27253-1-paulmck@linux.vnet.ibm.com> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Paul, On Wed, Jun 27, 2018 at 01:49:14PM -0700, Paul E. McKenney wrote: [...] > 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); > + 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. thanks. ---8<----------------------- diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index fb440baf8ac6..caa1e68f4168 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 (!(preempt_count() & PREEMPT_MASK)) + rcu_sched_qs(); /* * Get here if this CPU did not take its interrupt from