On Wed, Nov 16, 2022 at 02:07:28PM +0000, Zhang, Qiang1 wrote:
> On Tue, Nov 15, 2022 at 09:19:26PM +0800, Zqiang wrote:
> >And more important! On unpark time RCU_KTHREAD_OFFCPU isn't cleared. Only the
> >rcuc kthread does it, and after your patch it couldn't be awaken to perform
> >that, unless rcuc is lucky enough to have rcu_data.rcu_cpu_has_work = 1
> >by the time it unparks and that isn't guaranteed. So rcuc may sleep forever.
>
> Thanks for review, yes I should register an unpark function to clear RCU_KTHREAD_OFFCPU.
> Is the following modification more appropriate?
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 3ccad468887e..a2248af0ccda 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2375,7 +2375,8 @@ static void rcu_wake_cond(struct task_struct *t, int status)
> * If the thread is yielding, only wake it when this
> * is invoked from idle
> */
> - if (t && (status != RCU_KTHREAD_YIELDING || is_idle_task(current)))
> + if (t && (status != RCU_KTHREAD_YIELDING || is_idle_task(current)) &&
> + status != RCU_KTHREAD_OFFCPU)
> wake_up_process(t);
> }
>
> @@ -2407,7 +2408,14 @@ static void invoke_rcu_core(void)
>
> static void rcu_cpu_kthread_park(unsigned int cpu)
> {
> - per_cpu(rcu_data.rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
> + WARN_ON_ONCE(cpu != smp_processor_id());
> + __this_cpu_write(rcu_data.rcu_cpu_kthread_status, RCU_KTHREAD_OFFCPU);
> +}
> +
> +static void rcu_cpu_kthread_unpark(unsigned int cpu)
> +{
> + WARN_ON_ONCE(cpu != smp_processor_id());
> + __this_cpu_write(rcu_data.rcu_cpu_kthread_status, RCU_KTHREAD_ONCPU);
> }
>
> static int rcu_cpu_kthread_should_run(unsigned int cpu)
> @@ -2460,6 +2468,7 @@ static struct smp_hotplug_thread rcu_cpu_thread_spec = {
> .thread_comm = "rcuc/%u",
> .setup = rcu_cpu_kthread_setup,
> .park = rcu_cpu_kthread_park,
> + .unpark = rcu_cpu_kthread_unpark,
>Well, personally I don't think it's worth the burden because wake_up_process()
>already does an early exit if it's not dealing with a TASK_[UN]INTERRUPTIBLE task and
>the window is so short and rare that it doesn't look like a good candidate for
>extra optimization;
Agree, in try_to_wake_up(), acquire rcuc task->pi_lock(there's almost no lock contention here)
and then check tasks state, for TASK_[UN]INTERRUPTIBLE task, not dealing and early exit.
Thanks
Zqiang
>
>Thanks.
