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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: <20180702003553.GA95395@joelaf.mtv.corp.google.com> References: <20180627204835.GA25456@linux.vnet.ibm.com> <20180627204915.27253-1-paulmck@linux.vnet.ibm.com> <20180701183828.GB111992@joelaf.mtv.corp.google.com> <20180701222749.GD3593@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180701222749.GD3593@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 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