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

[Joel Fernandes]

On Wed, Aug 24, 2022 at 3:21 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> 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.

Thanks for confirming, I'll queue it into my next revision of the series.

 - Joel