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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Sun, 1 Jul 2018 18:25:39 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w61MPcRj47972384 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Sun, 1 Jul 2018 22:25:38 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E5E6AB2065; Sun, 1 Jul 2018 18:25:24 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B1829B2064; Sun, 1 Jul 2018 18:25:24 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.80.206.224]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Sun, 1 Jul 2018 18:25:24 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id CD07C16CA30F; Sun, 1 Jul 2018 15:27:49 -0700 (PDT) Date: Sun, 1 Jul 2018 15:27:49 -0700 From: "Paul E. McKenney" To: Joel Fernandes 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 Reply-To: paulmck@linux.vnet.ibm.com References: <20180627204835.GA25456@linux.vnet.ibm.com> <20180627204915.27253-1-paulmck@linux.vnet.ibm.com> <20180701183828.GB111992@joelaf.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180701183828.GB111992@joelaf.mtv.corp.google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18070122-0040-0000-0000-00000447DE7E X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009291; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01055174; UDB=6.00541172; IPR=6.00833096; MB=3.00021955; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-01 22:25:42 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18070122-0041-0000-0000-0000084DF9A2 Message-Id: <20180701222749.GD3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-01_07:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807010268 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Jul 01, 2018 at 11:38:28AM -0700, Joel Fernandes wrote: > 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. 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? Thanx, Paul > 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 >