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SubjectRe: [RFC] rcu: Warn that rcu ktheads cannot be spawned
On Mon, Jun 24, 2019 at 05:27:32PM +0900, Byungchul Park wrote:
> Hello rcu folks,
>
> I thought it'd better to announce it if those spawnings fail because of
> !rcu_scheduler_fully_active.
>
> Of course, with the current code, it never happens though.
>
> Thoughts?

It seems in the right spirit, but with your patch a warning always fires.
rcu_prepare_cpu() is called multiple times, once from rcu_init() and then
from hotplug paths.

Warning splat stack looks like:

[ 0.398767] Call Trace:
[ 0.398775] rcu_init+0x6aa/0x724
[ 0.398779] start_kernel+0x220/0x4a2
[ 0.398780] ? copy_bootdata+0x12/0xac
[ 0.398782] secondary_startup_64+0xa4/0xb0


>
> Thanks,
> Byungchul
>
> ---8<---
> From 58a33a85c70f82c406319b4752af95cf6ceb73a3 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul.park@lge.com>
> Date: Mon, 24 Jun 2019 17:08:26 +0900
> Subject: [RFC] rcu: Warn that rcu ktheads cannot be spawned
>
> In case that rcu ktheads cannot be spawned due to
> !rcu_scheduler_fully_active, it'd be better to anounce it.
>
> While at it, because the return value of rcu_spawn_one_boost_kthread()
> is not used any longer, changed the return type from int to void.
>
> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> ---
> kernel/rcu/tree_plugin.h | 31 +++++++++++++++++++------------
> 1 file changed, 19 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 1102765..7d74193 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -1131,7 +1131,7 @@ static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
> * already exist. We only create this kthread for preemptible RCU.
> * Returns zero if all is well, a negated errno otherwise.
> */
> -static int rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
> +static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
> {
> int rnp_index = rnp - rcu_get_root();
> unsigned long flags;
> @@ -1139,25 +1139,24 @@ static int rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
> struct task_struct *t;
>
> if (!IS_ENABLED(CONFIG_PREEMPT_RCU))
> - return 0;
> + return;
>
> - if (!rcu_scheduler_fully_active || rcu_rnp_online_cpus(rnp) == 0)
> - return 0;
> + if (rcu_rnp_online_cpus(rnp) == 0)
> + return;
>
> rcu_state.boost = 1;
> if (rnp->boost_kthread_task != NULL)
> - return 0;
> + return;
> t = kthread_create(rcu_boost_kthread, (void *)rnp,
> "rcub/%d", rnp_index);
> if (IS_ERR(t))
> - return PTR_ERR(t);
> + return;
> raw_spin_lock_irqsave_rcu_node(rnp, flags);
> rnp->boost_kthread_task = t;
> raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> sp.sched_priority = kthread_prio;
> sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
> wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
> - return 0;
> }
>
> static void rcu_cpu_kthread_setup(unsigned int cpu)
> @@ -1264,8 +1263,12 @@ static void __init rcu_spawn_boost_kthreads(void)
> per_cpu(rcu_data.rcu_cpu_has_work, cpu) = 0;
> if (WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec), "%s: Could not start rcub kthread, OOM is now expected behavior\n", __func__))
> return;
> +
> + if (WARN_ON(!rcu_scheduler_fully_active))
> + return;
> +
> rcu_for_each_leaf_node(rnp)
> - (void)rcu_spawn_one_boost_kthread(rnp);
> + rcu_spawn_one_boost_kthread(rnp);
> }
>
> static void rcu_prepare_kthreads(int cpu)
> @@ -1273,9 +1276,11 @@ static void rcu_prepare_kthreads(int cpu)
> struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> struct rcu_node *rnp = rdp->mynode;
>
> + if (WARN_ON(!rcu_scheduler_fully_active))
> + return;
> +
> /* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */
> - if (rcu_scheduler_fully_active)
> - (void)rcu_spawn_one_boost_kthread(rnp);
> + rcu_spawn_one_boost_kthread(rnp);
> }
>
> #else /* #ifdef CONFIG_RCU_BOOST */
> @@ -2198,8 +2203,10 @@ static void rcu_spawn_one_nocb_kthread(int cpu)
> */
> static void rcu_spawn_cpu_nocb_kthread(int cpu)
> {
> - if (rcu_scheduler_fully_active)
> - rcu_spawn_one_nocb_kthread(cpu);
> + if (WARN_ON(!rcu_scheduler_fully_active))
> + return;
> +
> + rcu_spawn_one_nocb_kthread(cpu);
> }
>
> /*
> --
> 1.9.1
>

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