From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755124AbaHFA01 (ORCPT ); Tue, 5 Aug 2014 20:26:27 -0400 Received: from cn.fujitsu.com ([59.151.112.132]:57021 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1751527AbaHFA00 (ORCPT ); Tue, 5 Aug 2014 20:26:26 -0400 X-IronPort-AV: E=Sophos;i="5.04,272,1406563200"; d="scan'208";a="34229925" Message-ID: <53E17687.7070705@cn.fujitsu.com> Date: Wed, 6 Aug 2014 08:27:51 +0800 From: Lai Jiangshan User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.9) Gecko/20100921 Fedora/3.1.4-1.fc14 Thunderbird/3.1.4 MIME-Version: 1.0 To: CC: Peter Zijlstra , , , , , , , , , , , , , , Subject: Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks() References: <20140731215445.GA21933@linux.vnet.ibm.com> <1406843709-23396-1-git-send-email-paulmck@linux.vnet.ibm.com> <53DEE1CD.4000705@cn.fujitsu.com> <20140804074620.GH9918@twins.programming.kicks-ass.net> <53DF41ED.2020508@cn.fujitsu.com> <20140804115043.GA31903@linux.vnet.ibm.com> <20140804122515.GR19379@twins.programming.kicks-ass.net> <20140804145648.GE3588@twins.programming.kicks-ass.net> <53E029BB.2010200@cn.fujitsu.com> <20140805215510.GT8101@linux.vnet.ibm.com> In-Reply-To: <20140805215510.GT8101@linux.vnet.ibm.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.167.226.103] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/06/2014 05:55 AM, Paul E. McKenney wrote: > On Tue, Aug 05, 2014 at 08:47:55AM +0800, Lai Jiangshan wrote: >> On 08/04/2014 10:56 PM, Peter Zijlstra wrote: >>> On Mon, Aug 04, 2014 at 02:25:15PM +0200, Peter Zijlstra wrote: >>>> On Mon, Aug 04, 2014 at 04:50:44AM -0700, Paul E. McKenney wrote: >>>>> OK, I will bite... >>>>> >>>>> What kinds of tasks are on a runqueue, but neither ->on_cpu nor >>>>> PREEMPT_ACTIVE? >>>> >>>> Userspace tasks, they don't necessarily get PREEMPT_ACTIVE when >>>> preempted. Now obviously you're not _that_ interested in userspace tasks >>>> for this, so that might be ok. >>>> >>>> But the main point was, you cannot use ->on_cpu or PREEMPT_ACTIVE >>>> without holding rq->lock. >>> >>> Hmm, maybe you can, we have the context switch in between setting >>> ->on_cpu and clearing PREEMPT_ACTIVE and vice-versa. >>> >>> The context switch (obviously) provides a full barrier, so we might be >>> able to -- with careful consideration -- read these two separate values >>> and construct something usable from them. >>> >>> Something like: >>> >>> task_preempt_count(tsk) & PREEMPT_ACTIVE >> the @tsk is running on_cpu, the above result is false. >>> smp_rmb(); >>> tsk->on_cpu >> now the @tsk is preempted, the above result also is false. >> >> so it is useless if we fetch the preempt_count and on_cpu in two separated >> instructions. Maybe it would work if we also take tsk->nivcsw in consideration. >> (I just noticed that tsk->n[i]vcsw are the version numbers for the tsk->on_cpu) >> >> bool task_on_cpu_or_preempted(tsk) >> { >> unsigned long saved_nivcsw; >> >> saved_nivcsw = task->nivcsw; >> if (tsk->on_cpu) >> return true; >> >> smp_rmb(); >> >> if (task_preempt_count(tsk) & PREEMPT_ACTIVE) >> return true; >> >> smp_rmb(); >> >> if (tsk->on_cpu || task->nivcsw != saved_nivcsw) >> return true; >> >> return false; >> } >> >>> >>> And because we set PREEMPT_ACTIVE before clearing on_cpu, this should >>> race the right way (err towards the inclusive side). >>> >>> Obviously that wants a big fat comment... > > How about the following? Non-nohz_full userspace tasks are already covered > courtesy of scheduling-clock interrupts, and this handles nohz_full usermode > tasks. synchronize_rcu_tasks() is called extremely rare. Is it right? So I think it is acceptable to interrupt the nohz_full usermode CPUs. > > Thoughts? > > Thanx, Paul > > ------------------------------------------------------------------------ > > rcu: Make TASKS_RCU handle nohx_full= CPUs > > Currently TASKS_RCU would ignore a CPU running a task in nohz_full= > usermode execution. There would be neither a context switch nor a > scheduling-clock interrupt to tell TASKS_RCU that the task in question > had passed through a quiescent state. The grace period would therefore > extend indefinitely. This commit therefore makes RCU's dyntick-idle > subsystem record the task_struct structure of the task that is running > in dyntick-idle mode on each CPU. The TASKS_RCU grace period can > then access this information and record a quiescent state on > behalf of any CPU running in dyntick-idle usermode. > > Signed-off-by: Paul E. McKenney > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > index f504f797c9c8..777aac3a34c0 100644 > --- a/include/linux/rcupdate.h > +++ b/include/linux/rcupdate.h > @@ -1140,5 +1140,14 @@ static inline void rcu_sysidle_force_exit(void) > > #endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */ > > +#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) > +struct task_struct *rcu_dynticks_task_cur(int cpu); > +#else /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */ > +static inline struct task_struct *rcu_dynticks_task_cur(int cpu) > +{ > + return NULL; > +} > +#endif /* #else #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */ > + > > #endif /* __LINUX_RCUPDATE_H */ > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 645a33efc0d4..86a0a7d5bbbd 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -526,6 +526,7 @@ static void rcu_eqs_enter_common(struct rcu_dynticks *rdtp, long long oldval, > atomic_inc(&rdtp->dynticks); > smp_mb__after_atomic(); /* Force ordering with next sojourn. */ > WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1); > + rcu_dynticks_task_enter(rdtp, current); > > /* > * It is illegal to enter an extended quiescent state while > @@ -642,6 +643,7 @@ void rcu_irq_exit(void) > static void rcu_eqs_exit_common(struct rcu_dynticks *rdtp, long long oldval, > int user) > { > + rcu_dynticks_task_exit(rdtp); > smp_mb__before_atomic(); /* Force ordering w/previous sojourn. */ > atomic_inc(&rdtp->dynticks); > /* CPUs seeing atomic_inc() must see later RCU read-side crit sects */ > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h > index 0f69a79c5b7d..1e79fa1b7cbf 100644 > --- a/kernel/rcu/tree.h > +++ b/kernel/rcu/tree.h > @@ -88,6 +88,9 @@ struct rcu_dynticks { > /* Process level is worth LLONG_MAX/2. */ > int dynticks_nmi_nesting; /* Track NMI nesting level. */ > atomic_t dynticks; /* Even value for idle, else odd. */ > +#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) > + struct task_struct *dynticks_tsk; > +#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */ > #ifdef CONFIG_NO_HZ_FULL_SYSIDLE > long long dynticks_idle_nesting; > /* irq/process nesting level from idle. */ > @@ -579,6 +582,9 @@ static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle, > static void rcu_bind_gp_kthread(void); > static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp); > static bool rcu_nohz_full_cpu(struct rcu_state *rsp); > +static void rcu_dynticks_task_enter(struct rcu_dynticks *rdtp, > + struct task_struct *t); > +static void rcu_dynticks_task_exit(struct rcu_dynticks *rdtp); > > #endif /* #ifndef RCU_TREE_NONCORE */ > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > index a86a363ea453..442d62edc564 100644 > --- a/kernel/rcu/tree_plugin.h > +++ b/kernel/rcu/tree_plugin.h > @@ -2852,3 +2852,29 @@ static void rcu_bind_gp_kthread(void) > set_cpus_allowed_ptr(current, cpumask_of(cpu)); > #endif /* #ifdef CONFIG_NO_HZ_FULL */ > } > + > +/* Record the current task on dyntick-idle entry. */ > +static void rcu_dynticks_task_enter(struct rcu_dynticks *rdtp, > + struct task_struct *t) > +{ > +#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) > + ACCESS_ONCE(rdtp->dynticks_tsk) = t; > +#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */ > +} > + > +/* Record no current task on dyntick-idle exit. */ > +static void rcu_dynticks_task_exit(struct rcu_dynticks *rdtp) > +{ > + rcu_dynticks_task_enter(rdtp, NULL); > +} > + > +#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) > +struct task_struct *rcu_dynticks_task_cur(int cpu) > +{ > + struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu); > + struct task_struct *t = ACCESS_ONCE(rdtp->dynticks_tsk); > + > + smp_read_barrier_depends(); /* Dereferences after fetch of "t". */ > + return t; > +} > +#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */ > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > index ad2a8df43757..6ad6af2ab028 100644 > --- a/kernel/rcu/update.c > +++ b/kernel/rcu/update.c > @@ -481,6 +481,28 @@ static void check_holdout_task(struct task_struct *t, > sched_show_task(current); > } > > +/* Check for nohz_full CPUs executing in userspace. */ > +static void check_no_hz_full_tasks(void) > +{ > +#ifdef CONFIG_NO_HZ_FULL > + int cpu; > + struct task_struct *t; > + > + for_each_online_cpu(cpu) { > + cond_resched(); > + rcu_read_lock(); > + t = rcu_dynticks_task_cur(cpu); > + if (t == NULL || is_idle_task(t)) { > + rcu_read_unlock(); > + continue; > + } > + if (ACCESS_ONCE(t->rcu_tasks_holdout)) > + ACCESS_ONCE(t->rcu_tasks_holdout) = 0; > + rcu_read_unlock(); > + } > +#endif /* #ifdef CONFIG_NO_HZ_FULL */ > +} > + > /* RCU-tasks kthread that detects grace periods and invokes callbacks. */ > static int __noreturn rcu_tasks_kthread(void *arg) > { > @@ -584,6 +606,7 @@ static int __noreturn rcu_tasks_kthread(void *arg) > lastreport = jiffies; > firstreport = true; > WARN_ON(signal_pending(current)); > + check_no_hz_full_tasks(); > rcu_read_lock(); > list_for_each_entry_rcu(t, &rcu_tasks_holdouts, > rcu_tasks_holdout_list) > > . >