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

Paul E. McKenney
2025-02-19T14:58:37Z

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.

> > 3. Your ideas here! ;-)
>
> :-)

Thanx, Paul