Re: [PATCH 3/3] rcu/exp: Remove needless CPU up quiescent state report

Paul E. McKenney
2025-02-19T15:55:05Z

On Wed, Feb 19, 2025 at 06:58:36AM -0800, Paul E. McKenney wrote:
> On Sat, Feb 15, 2025 at 11:23:45PM +0100, Frederic Weisbecker wrote:
> > Le Sat, Feb 15, 2025 at 02:38:04AM -0800, Paul E. McKenney a écrit :
> > > On Fri, Feb 14, 2025 at 01:10:52PM +0100, Frederic Weisbecker wrote:
> > > > Le Fri, Feb 14, 2025 at 01:01:56AM -0800, Paul E. McKenney a écrit :
> > > > > On Fri, Feb 14, 2025 at 12:25:59AM +0100, Frederic Weisbecker wrote:
> > > > > > A CPU coming online checks for an ongoing grace period and reports
> > > > > > a quiescent state accordingly if needed. This special treatment that
> > > > > > shortcuts the expedited IPI finds its origin as an optimization purpose
> > > > > > on the following commit:
> > > > > >
> > > > > > 338b0f760e84 (rcu: Better hotplug handling for synchronize_sched_expedited()
> > > > > >
> > > > > > The point is to avoid an IPI while waiting for a CPU to become online
> > > > > > or failing to become offline.
> > > > > >
> > > > > > However this is pointless and even error prone for several reasons:
> > > > > >
> > > > > > * If the CPU has been seen offline in the first round scanning offline
> > > > > > and idle CPUs, no IPI is even tried and the quiescent state is
> > > > > > reported on behalf of the CPU.
> > > > > >
> > > > > > * This means that if the IPI fails, the CPU just became offline. So
> > > > > > it's unlikely to become online right away, unless the cpu hotplug
> > > > > > operation failed and rolled back, which is a rare event that can
> > > > > > wait a jiffy for a new IPI to be issued.
> > >
> > > But the expedited grace period might be preempted for an arbitrarily
> > > long period, especially if a hypervisor is in play. And we do drop
> > > that lock midway through...
> >
> > Well, then that delays the expedited grace period as a whole anyway...
>
> Fair enough. Part of this is the paranoia that has served me so well.
> But which can also cause the occasional problem. On the other hand,
> we really do occasionally lose things during CPU hotplug operations...
>
> > > > > > For all those reasons, remove this optimization that doesn't look worthy
> > > > > > to keep around.
> > > > >
> > > > > Thank you for digging into this!
> > > > >
> > > > > When I ran tests that removed the call to sync_sched_exp_online_cleanup()
> > > > > a few months ago, I got grace-period hangs [1]. Has something changed
> > > > > to make this safe?
> > > >
> > > > Hmm, but was it before or after "rcu: Fix get_state_synchronize_rcu_full()
> > > > GP-start detection" ?
> > >
> > > Before. There was also some buggy debug code in play. Also, to get the
> > > failure, it was necessary to make TREE03 disable preemption, as stock
> > > TREE03 has an empty sync_sched_exp_online_cleanup() function.
> > >
> > > I am rerunning the test with a WARN_ON_ONCE() after the early exit from
> > > the sync_sched_exp_online_cleanup(). Of course, lack of a failure does
> > > not necessairly indicate
> >
> > Cool, thanks!
>
> No failures. But might it be wise to put this WARN_ON_ONCE() in,
> let things go for a year or two, and complete the removal if it never
> triggers? Or is the lack of forward progress warning enough?
>
> > > > And if after do we know why?
> > >
> > > Here are some (possibly bogus) possibilities that came to mind:
> > >
> > > 1. There is some coming-online race that deprives the incoming
> > > CPU of an IPI, but nevertheless marks that CPU as blocking the
> > > current grace period.
> >
> > Arguably there is a tiny window between rcutree_report_cpu_starting()
> > and set_cpu_online() that could make ->qsmaskinitnext visible before
> > cpu_online() and therefore delay the IPI a bit. But I don't expect
> > more than a jiffy to fill up the gap. And if that's relevant, note that
> > only !PREEMPT_RCU is then "fixed" by sync_sched_exp_online_cleanup() here.
>
> Agreed. And I vaguely recall that there was some difference due to
> preemptible RCU's ability to clean up at the next rcu_read_unlock(),
> though more recently, possibly deferred.
>
> > > 2. Some strange scenario involves the CPU going offline for just a
> > > little bit, so that the IPI gets wasted on the outgoing due to
> > > neither of the "if" conditions in rcu_exp_handler() being true.
> > > The outgoing CPU just says "I need a QS", then leaves and
> > > comes back. (The expedited grace period doesn't retry because
> > > it believes that it already sent that IPI.)
> >
> > I don't think this is possible. Once the CPU enters CPUHP_TEARDOWN_CPU with
> > stop_machine, no more IPIs can be issued. The remaining ones are executed
> > at CPUHP_AP_SMPCFD_DYING, still in stop_machine. So this is the last call
> > for rcu_exp_handler() execution. And this last call has to be followed
> > by rcu_note_context_switch() between stop_machine and the final schedule to
> > idle. And that rcu_note_context_switch() must report the rdp exp context
> > switch.
>
> Makes sense to me.
>
> > One easy way to assert that is:
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 86935fe00397..40d6090a33f5 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -4347,6 +4347,12 @@ void rcutree_report_cpu_dead(void)
> > * may introduce a new READ-side while it is actually off the QS masks.
> > */
> > lockdep_assert_irqs_disabled();
> > + /*
> > + * CPUHP_AP_SMPCFD_DYING was the last call for rcu_exp_handler() execution.
> > + * The requested QS must have been reported on the last context switch
> > + * from stop machine to idle.
> > + */
> > + WARN_ON_ONCE(rdp->cpu_no_qs.b.exp);
> > // Do any dangling deferred wakeups.
> > do_nocb_deferred_wakeup(rdp);
>
> I fired off a 30-minute run of 100*TREE03 with this change also.

And no failures!

Thanx, Paul

> > > 3. Your ideas here! ;-)
> >
> > :-)
>
> Thanx, Paul