[2/4] rcu/tasks: Handle new PF_IDLE semantics

  The commit:

	cff9b2332ab7 ("kernel/sched: Modify initial boot task idle setup")

has changed the semantics of what is to be considered an idle task in
such a way that CPU boot code preceding the actual idle loop is excluded
from it.

This has however introduced new potential RCU-tasks stalls when either:

1) Grace period is started before init/0 had a chance to set PF_IDLE,
   keeping it stuck in the holdout list until idle ever schedules.

2) Grace period is started when some possible CPUs have never been
   online, keeping their idle tasks stuck in the holdout list until the
   CPU ever boots up.

3) Similar to 1) but with secondary CPUs: Grace period is started
   concurrently with secondary CPU booting, putting its idle task in
   the holdout list because PF_IDLE isn't yet observed on it. It stays
   then stuck in the holdout list until that CPU ever schedules. The
   effect is mitigated here by the hotplug AP thread that must run to
   bring the CPU up.

Fix this with handling the new semantics of PF_IDLE, keeping in mind
that it may or may not be set on an idle task. Take advantage of that to
strengthen the coverage of an RCU-tasks quiescent state within an idle
task, excluding the CPU boot code from it. Only the code running within
the idle loop is now a quiescent state, along with offline CPUs.

Fixes: cff9b2332ab7 ("kernel/sched: Modify initial boot task idle setup")
Suggested-by: Joel Fernandes <joel@joelfernandes.org>
Suggested-by: Paul E . McKenney" <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/rcu/tasks.h | 31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)
  

On Tue, Oct 24, 2023 at 11:46:23PM +0200, Frederic Weisbecker wrote:

> +/* Check for quiescent states since the pregp's synchronize_rcu() */
> +static bool rcu_tasks_is_holdout(struct task_struct *t)
> +{
> +	int cpu;
> +
> +	/* Has the task been seen voluntarily sleeping? */
> +	if (!READ_ONCE(t->on_rq))
> +		return false;
> +
> +	cpu = task_cpu(t);
> +
> +	/*
> +	 * Idle tasks within the idle loop or offline CPUs are RCU-tasks
> +	 * quiescent states. But CPU boot code performed by the idle task
> +	 * isn't a quiescent state.
> +	 */
> +	if (t == idle_task(cpu)) {
> +		if (is_idle_task(t))
> +			return false;
> +
> +		if (!rcu_cpu_online(cpu))
> +			return false;
> +	}

Hmm, why is this guarded by t == idle_task() ?

Notably, there is the idle-injection thing that uses FIFO tasks to run
'idle', see play_idle_precise(). This will (temporarily) get PF_IDLE on
tasks that are not idle_task().

> +
> +	return true;
> +}
> +
>  /* Per-task initial processing. */
>  static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
>  {
> -	if (t != current && READ_ONCE(t->on_rq) && !is_idle_task(t)) {
> +	if (t != current && rcu_tasks_is_holdout(t)) {
  

Le Wed, Oct 25, 2023 at 10:40:08AM +0200, Peter Zijlstra a écrit :
> On Tue, Oct 24, 2023 at 11:46:23PM +0200, Frederic Weisbecker wrote:
> 
> > +/* Check for quiescent states since the pregp's synchronize_rcu() */
> > +static bool rcu_tasks_is_holdout(struct task_struct *t)
> > +{
> > +	int cpu;
> > +
> > +	/* Has the task been seen voluntarily sleeping? */
> > +	if (!READ_ONCE(t->on_rq))
> > +		return false;
> > +
> > +	cpu = task_cpu(t);
> > +
> > +	/*
> > +	 * Idle tasks within the idle loop or offline CPUs are RCU-tasks
> > +	 * quiescent states. But CPU boot code performed by the idle task
> > +	 * isn't a quiescent state.
> > +	 */
> > +	if (t == idle_task(cpu)) {
> > +		if (is_idle_task(t))
> > +			return false;
> > +
> > +		if (!rcu_cpu_online(cpu))
> > +			return false;
> > +	}
> 
> Hmm, why is this guarded by t == idle_task() ?
> 
> Notably, there is the idle-injection thing that uses FIFO tasks to run
> 'idle', see play_idle_precise(). This will (temporarily) get PF_IDLE on
> tasks that are not idle_task().

Ah good point. So indeed the is_idle_task() test doesn't musn't be
guarded by t == idle_task(cpu). But rcu_cpu_online() has to, otherwise
if it's not an idle task, there is a risk that the task gets migrated out
by the time we observe the old CPU offline.

Thanks.

> 
> > +
> > +	return true;
> > +}
> > +
> >  /* Per-task initial processing. */
> >  static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
> >  {
> > -	if (t != current && READ_ONCE(t->on_rq) && !is_idle_task(t)) {
> > +	if (t != current && rcu_tasks_is_holdout(t)) {
> 
>
  

>
> Le Wed, Oct 25, 2023 at 10:40:08AM +0200, Peter Zijlstra a écrit :
> > On Tue, Oct 24, 2023 at 11:46:23PM +0200, Frederic Weisbecker wrote:
> >
> > > +/* Check for quiescent states since the pregp's synchronize_rcu() */
> > > +static bool rcu_tasks_is_holdout(struct task_struct *t)
> > > +{
> > > +   int cpu;
> > > +
> > > +   /* Has the task been seen voluntarily sleeping? */
> > > +   if (!READ_ONCE(t->on_rq))
> > > +           return false;
> > > +
> > > +   cpu = task_cpu(t);
> > > +
> > > +   /*
> > > +    * Idle tasks within the idle loop or offline CPUs are RCU-tasks
> > > +    * quiescent states. But CPU boot code performed by the idle task
> > > +    * isn't a quiescent state.
> > > +    */
> > > +   if (t == idle_task(cpu)) {
> > > +           if (is_idle_task(t))
> > > +                   return false;
> > > +
> > > +           if (!rcu_cpu_online(cpu))
> > > +                   return false;
> > > +   }
> >
> > Hmm, why is this guarded by t == idle_task() ?
> >
> > Notably, there is the idle-injection thing that uses FIFO tasks to run
> > 'idle', see play_idle_precise(). This will (temporarily) get PF_IDLE on
> > tasks that are not idle_task().
>
> Ah good point. So indeed the is_idle_task() test doesn't musn't be
> guarded by t == idle_task(cpu). But rcu_cpu_online() has to, otherwise
> if it's not an idle task, there is a risk that the task gets migrated out
> by the time we observe the old CPU offline.
>

If a fifo-tasks use play_idle_precise() to run idle and invoke
do_idle(), may cause
rcu-tasks to falsely report a rcu-tasks QS , when rcu_is_cpu_rrupt_from_idle()
return true in rcu_sched_clock_irq(), so should we also add a check for
"current == idle_task(task_cpu(current))" in the rcu_is_cpu_rrupt_from_idle() ?

Thanks
Zqiang


> Thanks.
>
> >
> > > +
> > > +   return true;
> > > +}
> > > +
> > >  /* Per-task initial processing. */
> > >  static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
> > >  {
> > > -   if (t != current && READ_ONCE(t->on_rq) && !is_idle_task(t)) {
> > > +   if (t != current && rcu_tasks_is_holdout(t)) {
> >
> >
  

On Thu, Oct 26, 2023 at 08:15:33PM +0800, Z qiang wrote:
> >
> > Le Wed, Oct 25, 2023 at 10:40:08AM +0200, Peter Zijlstra a écrit :
> > > On Tue, Oct 24, 2023 at 11:46:23PM +0200, Frederic Weisbecker wrote:
> > >
> > > > +/* Check for quiescent states since the pregp's synchronize_rcu() */
> > > > +static bool rcu_tasks_is_holdout(struct task_struct *t)
> > > > +{
> > > > +   int cpu;
> > > > +
> > > > +   /* Has the task been seen voluntarily sleeping? */
> > > > +   if (!READ_ONCE(t->on_rq))
> > > > +           return false;
> > > > +
> > > > +   cpu = task_cpu(t);
> > > > +
> > > > +   /*
> > > > +    * Idle tasks within the idle loop or offline CPUs are RCU-tasks
> > > > +    * quiescent states. But CPU boot code performed by the idle task
> > > > +    * isn't a quiescent state.
> > > > +    */
> > > > +   if (t == idle_task(cpu)) {
> > > > +           if (is_idle_task(t))
> > > > +                   return false;
> > > > +
> > > > +           if (!rcu_cpu_online(cpu))
> > > > +                   return false;
> > > > +   }
> > >
> > > Hmm, why is this guarded by t == idle_task() ?
> > >
> > > Notably, there is the idle-injection thing that uses FIFO tasks to run
> > > 'idle', see play_idle_precise(). This will (temporarily) get PF_IDLE on
> > > tasks that are not idle_task().
> >
> > Ah good point. So indeed the is_idle_task() test doesn't musn't be
> > guarded by t == idle_task(cpu). But rcu_cpu_online() has to, otherwise
> > if it's not an idle task, there is a risk that the task gets migrated out
> > by the time we observe the old CPU offline.
> >
> 
> If a fifo-tasks use play_idle_precise() to run idle and invoke
> do_idle(), may cause
> rcu-tasks to falsely report a rcu-tasks QS

Well, there can be a debate here: should we consider an idle injector as a real
task that we must wait for a voluntary schedule or should we treat it just like
an idle task?

Having that whole idle task quiescent state in RCU-tasks is quite a strange
semantic anyway. And in the long run, the purpose is to unify RCU-tasks and
RCU-tasks-RUDE with relying on ct_dynticks for idle quiescent states.

> , when rcu_is_cpu_rrupt_from_idle()
> return true in rcu_sched_clock_irq(), so should we also add a check for
> "current == idle_task(task_cpu(current))" in the rcu_is_cpu_rrupt_from_idle()
> ?

That looks fine OTOH. Whether idle injection or real idle,
rcu_is_cpu_rrupt_from_idle() is always a quiescent state in real RCU. Because
we know we have no RCU reader between ct_idle_enter() and ct_idle_exit().

Thanks.