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

[Frederic Weisbecker]

Le Wed, Feb 19, 2025 at 06:58:36AM -0800, Paul E. McKenney a écrit :
> On Sat, Feb 15, 2025 at 11:23:45PM +0100, Frederic Weisbecker wrote:
> > > 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?

Hmm, what prevents a WARN_ON_ONCE() after the early exit of
sync_sched_exp_online_cleanup() to hit?

All it takes is for sync_sched_exp_online_cleanup() to execute between
sync_exp_reset_tree() and  __sync_rcu_exp_select_node_cpus() manage
to send an IPI.

But we can warn about the lack of forward progress after a few iterations
of the retry_ipi label in __sync_rcu_exp_select_node_cpus().

> 
> > > > 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.

Perhaps at the time but today at least I can't find any.

Thanks.