RE: [PATCH] rcu: Avoid invalid wakeup for rcuc kthreads in RCU_KTHREAD_OFFCPU status

[Zhang, Qiang1]

On Tue, Nov 15, 2022 at 09:19:26PM +0800, Zqiang wrote:
> For CONFIG_PREEMPT_RT=y kernel, the "use_softirq=0" will be set, the
> RCU_SOFTIRQ processing is moved to per-CPU rcuc kthreads which created
> by smpboot_register_percpu_thread(). when CPU is going offline, the
> corresponding rcu_data.rcu_cpu_kthread_status is set RCU_KTHREAD_OFFCPU,
> and the rcuc kthreads enter TASK_PARKED state, kthreads in TASK_PARKED
> state only accept kthread_unpark() to wakeup.
> 
> Therefore, This commit avoid invoke wake_up_process() to rcuc kthreads
> in TASK_PARKED state.
> 
> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> ---
>  kernel/rcu/tree.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 3ccad468887e..49dd87356851 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);

>There is a tiny window where this can happen (between CPUHP_TEARDOWN_CPU
>and CPUHP_AP_SMPBOOT_THREADS) and it can't cause a spurious unpark because
>wake_up_process() only wakes up from TASK_[UN]INTERRUPTIBLE states. And even
>if it did, the KTHREAD_SHOULD_PARK bit would still be on.

Yes even if it did, because KTHREAD_SHOULD_PARK bit would still be on, this kthreads
will schedule out again.

>
>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, 
};

 /*
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index fcb5d696eb17..c4b9606968db 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -36,8 +36,9 @@ struct rcu_exp_work {
 #define RCU_KTHREAD_RUNNING  1
 #define RCU_KTHREAD_WAITING  2
 #define RCU_KTHREAD_OFFCPU   3
-#define RCU_KTHREAD_YIELDING 4
-#define RCU_KTHREAD_MAX      4
+#define RCU_KTHREAD_ONCPU    4
+#define RCU_KTHREAD_YIELDING 5
+#define RCU_KTHREAD_MAX      5


Thanks
Zqiang

>
>OTOH one cleanup that could be done is to make rcu_cpu_kthread_park() to use
>__this_cpu_write as it's guaranteed that cpu == smp_processor_id().
>
>Thanks.