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.
