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

[Z qiang]

>
> 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)) {
> >
> >