linux-kernel - Re: [PATCH 2/4] rcu/tasks: Handle new PF

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Date:   Thu, 26 Oct 2023 16:34:28 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Z qiang <qiang.zhang1211@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Neeraj Upadhyay <neeraj.upadhyay@....com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Uladzislau Rezki <urezki@...il.com>, rcu <rcu@...r.kernel.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>
Subject: Re: [PATCH 2/4] rcu/tasks: Handle new PF_IDLE semantics

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.