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Date:	Wed, 6 Aug 2014 15:48:32 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

On Tue, Aug 05, 2014 at 05:51:01PM -0700, Paul E. McKenney wrote:
> 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 <paulmck@...ux.vnet.ibm.com>
> > > 
> > > 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.

And if not, it is not all that hard to handle this case.

							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)
> > > 
> > > .
> > > 
> > 

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