From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755152AbaHFAvL (ORCPT ); Tue, 5 Aug 2014 20:51:11 -0400 Received: from e34.co.us.ibm.com ([32.97.110.152]:59553 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751961AbaHFAvJ (ORCPT ); Tue, 5 Aug 2014 20:51:09 -0400 Date: Tue, 5 Aug 2014 17:51:02 -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: <20140806005101.GW8101@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <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> <53E177D9.2090105@cn.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <53E177D9.2090105@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: 14080600-1542-0000-0000-000003C93D8A Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 06, 2014 at 08:33:29AM +0800, Lai Jiangshan wrote: > 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. > > > > 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); > > What happen when the trampoline happens before rcu_eqs_exit_common()? > synchronize_sched() also skip this CPUs. I think, for all CPUs, only > real schedule is reliable. True, this prohibits tracing the point from the call to rcu_eqs_enter_common() to the transition to usermode. I am betting that this is OK, though. Thanx, Paul > > 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) > > > > . > > >