rcu: Avoid invalid wakeup for rcuc kthreads in RCU_KTHREAD_OFFCPU status

  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@intel.com>
---
 kernel/rcu/tree.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
  

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@intel.com>
> ---
>  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.

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.

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.
  

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@intel.com>
> ---
>  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.
  

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;

Thanks.
  

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.