linux-kernel - Re: [PATCH 2/3] rcu/nocb: Remove NOCB deferred wakeup from rcutree_dead

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Date:   Thu, 20 May 2021 02:25:53 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 2/3] rcu/nocb: Remove NOCB deferred wakeup from
 rcutree_dead_cpu()

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.

Thanks!