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.
Thanks!
> + * So resort to cpu_dying_mask, whose changes has already been visible.
> + */
> + if (outgoingcpu != -1)
> + cpumask_andnot(cm, cm, cpu_dying_mask);
> if (cpumask_empty(cm))
> cpumask_copy(cm, housekeeping_cpumask(HK_TYPE_RCU));
> set_cpus_allowed_ptr(t, cm);
> --
> 2.31.1
>
