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

Frederic Weisbecker
2025-02-15T22:23:48Z

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...

> > > > 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!

>
> > 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.

>
> 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.

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);

>
> 3. Your ideas here! ;-)

:-)

>
> Thanx, Paul