Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

[Lai Jiangshan]

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 <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> 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)
> 
> .
> 

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