Re: [PATCH 2/3] rcu/nocb: Remove NOCB deferred wakeup from rcutree_dead_cpu()

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, May 20, 2021 at 02:25:53AM +0200, Frederic Weisbecker wrote:
> On Wed, May 19, 2021 at 08:59:05AM -0700, Paul E. McKenney wrote:
> > On Wed, May 19, 2021 at 02:09:29AM +0200, Frederic Weisbecker wrote:
> > > At CPU offline time, we make sure to flush any pending wakeup for the
> > > nocb_gp kthread linked to the outgoing CPU.
> > > 
> > > Now we are making sure of that twice:
> > > 
> > > 1) From rcu_report_dead() when the outgoing CPU makes the very last
> > >    local cleanups by itself before switching offline.
> > > 
> > > 2) From rcutree_dead_cpu(). Here the offlining CPU has gone and is truly
> > >    now offline. Another CPU takes care of post-portem cleaning up and
> > >    check if the offline CPU had pending wakeup.
> > > 
> > > Both ways are fine but we have to choose one or the other because we
> > > don't need to repeat that action. Simply benefit from cache locality
> > > and keep only the first solution.
> > 
> > But between those two calls, the CPU takes a full pass through the
> > scheduler and heads into the idle loop.  What if there is a call_rcu()
> > along the way, and if this was the last online CPU in its rcuog kthread's
> > group of CPUs?  Wouldn't that callback be stranded until one of those
> > CPUs came back online?
> 
> Nope, rcu_report_dead() is called from the idle path right before
> arch_cpu_idle_dead(). There should be no call to the scheduler until the
> CPU comes back online.

You are of course correct.  I have pulled this one in as well, thank you!

							Thanx, Paul