Re: [RFC 06/10] rcu/hotplug: Make rcutree_dead_cpu() parallel

[Paul E. McKenney]

On Wed, Aug 24, 2022 at 01:26:01PM -0400, Joel Fernandes wrote:
> 
> 
> On 8/24/2022 12:20 PM, Paul E. McKenney wrote:
> > On Wed, Aug 24, 2022 at 09:53:11PM +0800, Pingfan Liu wrote:
> >> On Tue, Aug 23, 2022 at 11:01 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >>>
> >>> On Tue, Aug 23, 2022 at 09:50:56AM +0800, Pingfan Liu wrote:
> >>>> On Sun, Aug 21, 2022 at 07:45:28PM -0700, Paul E. McKenney wrote:
> >>>>> On Mon, Aug 22, 2022 at 10:15:16AM +0800, Pingfan Liu wrote:
> >>>>>> In order to support parallel, rcu_state.n_online_cpus should be
> >>>>>> atomic_dec()
> >>>>>>
> >>>>>> Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> >>>>>
> >>>>> I have to ask...  What testing have you subjected this patch to?
> >>>>>
> >>>>
> >>>> This patch subjects to [1]. The series aims to enable kexec-reboot in
> >>>> parallel on all cpu. As a result, the involved RCU part is expected to
> >>>> support parallel.
> >>>
> >>> I understand (and even sympathize with) the expectation.  But results
> >>> sometimes diverge from expectations.  There have been implicit assumptions
> >>> in RCU about only one CPU going offline at a time, and I am not sure
> >>> that all of them have been addressed.  Concurrent CPU onlining has
> >>> been looked at recently here:
> >>>
> >>> https://docs.google.com/document/d/1jymsaCPQ1PUDcfjIKm0UIbVdrJAaGX-6cXrmcfm0PRU/edit?usp=sharing
> >>>
> >>> You did us atomic_dec() to make rcu_state.n_online_cpus decrementing be
> >>> atomic, which is good.  Did you look through the rest of RCU's CPU-offline
> >>> code paths and related code paths?
> >>
> >> I went through those codes at a shallow level, especially at each
> >> cpuhp_step hook in the RCU system.
> > 
> > And that is fine, at least as a first step.
> > 
> >> But as you pointed out, there are implicit assumptions about only one
> >> CPU going offline at a time, I will chew the google doc which you
> >> share.  Then I can come to a final result.
> > 
> > Boqun Feng, Neeraj Upadhyay, Uladzislau Rezki, and I took a quick look,
> > and rcu_boost_kthread_setaffinity() seems to need some help.  As it
> > stands, it appears that concurrent invocations of this function from the
> > CPU-offline path will cause all but the last outgoing CPU's bit to be
> > (incorrectly) set in the cpumask_var_t passed to set_cpus_allowed_ptr().
> > 
> > This should not be difficult to fix, for example, by maintaining a
> > separate per-leaf-rcu_node-structure bitmask of the concurrently outgoing
> > CPUs for that rcu_node structure.  (Similar in structure to the
> > ->qsmask field.)
> > 
> > There are probably more where that one came from.  ;-)
> 
> Should rcutree_dying_cpu() access to rnp->qsmask have a READ_ONCE() ? I was
> thinking grace period initialization or qs reporting paths racing with that. Its
> just tracing, still :)

Looks like it should be regardless of Pingfan's patches, given that
the grace-period kthread might report a quiescent state concurrently.

Good catch!

							Thanx, Paul