On Tue, Sep 20, 2022 at 5:39 PM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote:
>
> On Thu, Sep 15, 2022 at 01:58:24PM +0800, Pingfan Liu wrote:
> > During offlining, the concurrent rcutree_offline_cpu() can not be aware
> > of each other through ->qsmaskinitnext. But cpu_dying_mask carries such
> > information at that point and can be utilized.
> >
> > Besides, a trivial change which removes the redudant call to
> > rcu_boost_kthread_setaffinity() in rcutree_dead_cpu() since
> > rcutree_offline_cpu() can fully serve that purpose.
> >
> > Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> > Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> > Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> > Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> > Cc: "Jason A. Donenfeld" <Jason@xxxxxxxxx>
> > To: rcu@xxxxxxxxxxxxxxx
> > ---
> > kernel/rcu/tree.c | 2 --
> > kernel/rcu/tree_plugin.h | 6 ++++++
> > 2 files changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 79aea7df4345..8a829b64f5b2 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2169,8 +2169,6 @@ int rcutree_dead_cpu(unsigned int cpu)
> > return 0;
> >
> > WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
> > - /* Adjust any no-longer-needed kthreads. */
> > - rcu_boost_kthread_setaffinity(rnp, -1);
> > // Stop-machine done, so allow nohz_full to disable tick.
> > tick_dep_clear(TICK_DEP_BIT_RCU);
> > return 0;
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index ef6d3ae239b9..e5afc63bd97f 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -1243,6 +1243,12 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
> > cpu != outgoingcpu)
> > cpumask_set_cpu(cpu, cm);
> > cpumask_and(cm, cm, housekeeping_cpumask(HK_TYPE_RCU));
> > + /*
> > + * For concurrent offlining, bit of qsmaskinitnext is not cleared yet.
>
> For clarification, the comment could be:
>
> While concurrently offlining, rcu_report_dead() can race, making
> ->qsmaskinitnext unstable. So rely on cpu_dying_mask which is stable
> and already contains all the currently offlining CPUs.
>
It is a neat description.
Thanks,
Pingfan
