From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753926AbaHEVzS (ORCPT ); Tue, 5 Aug 2014 17:55:18 -0400 Received: from e35.co.us.ibm.com ([32.97.110.153]:36817 "EHLO e35.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753621AbaHEVzQ (ORCPT ); Tue, 5 Aug 2014 17:55:16 -0400 Date: Tue, 5 Aug 2014 14:55:10 -0700 From: "Paul E. McKenney" To: Lai Jiangshan Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, mingo@kernel.org, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com Subject: Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks() Message-ID: <20140805215510.GT8101@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <53E029BB.2010200@cn.fujitsu.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14080521-6688-0000-0000-000003CACC23 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. 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)