[PATCH v2] sched/core: introduce sched_core_idle_cpu()

[PATCH v2] sched/core: introduce sched_core_idle_cpu()

[Cruz Zhao]

As core scheduling introduced, a new state of idle is defined as
force idle, running idle task but nr_running greater than zero.

If a cpu is in force idle state, idle_cpu() will return zero. This
result makes sense in certain scenarios, e.g., load balance. but
this will cause error in other scenarios, e.g., tick_irq_exit()
will not change ts->idle_active back to 1 because idle_cpu() returns
0 when force idle.

To solve this problem, we introduce sched_core_idle_cpu(), which
returns 1 when force idle. We audit all users of idle_cpu(), and
change idle_cpu() into sched_core_idle_cpu() in the following
functions:
  - showacpu()
  - rcu_is_rcuc_kthread_starving()
  - tick_irq_exit()

v1-->v2: replace idle_cpu() with sched_core_idle_cpu() in function
showacpu() and rcu_is_rcuc_kthread_starving()

Signed-off-by: Cruz Zhao <CruzZhao@linux.alibaba.com>
---
 drivers/tty/sysrq.c     |  2 +-
 include/linux/sched.h   |  2 ++
 kernel/rcu/tree_stall.h |  2 +-
 kernel/sched/core.c     | 13 +++++++++++++
 kernel/softirq.c        |  2 +-
 5 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index b6e70c5cfa17..8a6586800385 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -232,7 +232,7 @@ static void showacpu(void *dummy)
 	unsigned long flags;
 
 	/* Idle CPUs have no interesting backtrace. */
-	if (idle_cpu(smp_processor_id())) {
+	if (sched_core_idle_cpu(smp_processor_id())) {
 		pr_info("CPU%d: backtrace skipped as idling\n", smp_processor_id());
 		return;
 	}
diff --git a/include/linux/sched.h b/include/linux/sched.h
index b09a83bfad8b..73e61c0f10a7 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2430,9 +2430,11 @@ extern void sched_core_free(struct task_struct *tsk);
 extern void sched_core_fork(struct task_struct *p);
 extern int sched_core_share_pid(unsigned int cmd, pid_t pid, enum pid_type type,
 				unsigned long uaddr);
+extern int sched_core_idle_cpu(int cpu);
 #else
 static inline void sched_core_free(struct task_struct *tsk) { }
 static inline void sched_core_fork(struct task_struct *p) { }
+static inline int sched_core_idle_cpu(int cpu) { return idle_cpu(cpu); }
 #endif
 
 extern void sched_set_stop_task(int cpu, struct task_struct *stop);
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index b10b8349bb2a..6169faf30ecd 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -418,7 +418,7 @@ static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp
 		return false;
 
 	cpu = task_cpu(rcuc);
-	if (cpu_is_offline(cpu) || idle_cpu(cpu))
+	if (cpu_is_offline(cpu) || sched_core_idle_cpu(cpu))
 		return false;
 
 	j = jiffies - READ_ONCE(rdp->rcuc_activity);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 71c1a0f232b4..c80088956987 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7286,6 +7286,19 @@ struct task_struct *idle_task(int cpu)
 	return cpu_rq(cpu)->idle;
 }
 
+#ifdef CONFIG_SCHED_CORE
+int sched_core_idle_cpu(int cpu)
+{
+	struct rq *rq = cpu_rq(cpu);
+
+	if (sched_core_enabled(rq) && rq->curr == rq->idle)
+		return 1;
+
+	return idle_cpu(cpu);
+}
+
+#endif
+
 #ifdef CONFIG_SMP
 /*
  * This function computes an effective utilization for the given CPU, to be
diff --git a/kernel/softirq.c b/kernel/softirq.c
index c8a6913c067d..98b98991ce45 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -630,7 +630,7 @@ static inline void tick_irq_exit(void)
 	int cpu = smp_processor_id();
 
 	/* Make sure that timer wheel updates are propagated */
-	if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) {
+	if ((sched_core_idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) {
 		if (!in_hardirq())
 			tick_nohz_irq_exit();
 	}
-- 
2.27.0

Re: [PATCH v2] sched/core: introduce sched_core_idle_cpu()

[Frederic Weisbecker]

Le Sun, Jun 25, 2023 at 02:28:15AM +0800, Cruz Zhao a écrit :
> As core scheduling introduced, a new state of idle is defined as
> force idle, running idle task but nr_running greater than zero.
> 
> If a cpu is in force idle state, idle_cpu() will return zero. This
> result makes sense in certain scenarios, e.g., load balance. but
> this will cause error in other scenarios, e.g., tick_irq_exit()
> will not change ts->idle_active back to 1 because idle_cpu() returns
> 0 when force idle.
> 
> To solve this problem, we introduce sched_core_idle_cpu(), which
> returns 1 when force idle. We audit all users of idle_cpu(), and
> change idle_cpu() into sched_core_idle_cpu() in the following
> functions:
>   - showacpu()
>   - rcu_is_rcuc_kthread_starving()
>   - tick_irq_exit()
> 
> v1-->v2: replace idle_cpu() with sched_core_idle_cpu() in function
> showacpu() and rcu_is_rcuc_kthread_starving()
> 
> Signed-off-by: Cruz Zhao <CruzZhao@linux.alibaba.com>
> ---
>  drivers/tty/sysrq.c     |  2 +-
>  include/linux/sched.h   |  2 ++
>  kernel/rcu/tree_stall.h |  2 +-
>  kernel/sched/core.c     | 13 +++++++++++++
>  kernel/softirq.c        |  2 +-
>  5 files changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index b6e70c5cfa17..8a6586800385 100644
> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -232,7 +232,7 @@ static void showacpu(void *dummy)
>  	unsigned long flags;
>  
>  	/* Idle CPUs have no interesting backtrace. */
> -	if (idle_cpu(smp_processor_id())) {
> +	if (sched_core_idle_cpu(smp_processor_id())) {
>  		pr_info("CPU%d: backtrace skipped as idling\n", smp_processor_id());

Actually perhaps an idle injection's backtrace is worth dumping. I guess
it might accidentally produce lockups and it's worth knowing the source then.

Though I don't have a strong opinion on that...

>  		return;
>  	}
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index b10b8349bb2a..6169faf30ecd 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -418,7 +418,7 @@ static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp
>  		return false;
>  
>  	cpu = task_cpu(rcuc);
> -	if (cpu_is_offline(cpu) || idle_cpu(cpu))
> +	if (cpu_is_offline(cpu) || sched_core_idle_cpu(cpu))

An idle injection could possibly starve the RCU boost kthread, and then it's
worth knowing about it. I would suggest keeping idle_cpu() here.

>  		return false;
>  
>  	j = jiffies - READ_ONCE(rdp->rcuc_activity);
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index c8a6913c067d..98b98991ce45 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -630,7 +630,7 @@ static inline void tick_irq_exit(void)
>  	int cpu = smp_processor_id();
>  
>  	/* Make sure that timer wheel updates are propagated */
> -	if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) {
> +	if ((sched_core_idle_cpu(cpu) && !need_resched()) ||  tick_nohz_full_cpu(cpu)) {

That one looks good.

Thanks!

Re: [PATCH v2] sched/core: introduce sched_core_idle_cpu()

[Joel Fernandes]

On 6/25/23 17:28, Frederic Weisbecker wrote:
>>   drivers/tty/sysrq.c     |  2 +-
>>   include/linux/sched.h   |  2 ++
>>   kernel/rcu/tree_stall.h |  2 +-
>>   kernel/sched/core.c     | 13 +++++++++++++
>>   kernel/softirq.c        |  2 +-
>>   5 files changed, 18 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
>> index b6e70c5cfa17..8a6586800385 100644
>> --- a/drivers/tty/sysrq.c
>> +++ b/drivers/tty/sysrq.c
>> @@ -232,7 +232,7 @@ static void showacpu(void *dummy)
>>   	unsigned long flags;
>>   
>>   	/* Idle CPUs have no interesting backtrace. */
>> -	if (idle_cpu(smp_processor_id())) {
>> +	if (sched_core_idle_cpu(smp_processor_id())) {
>>   		pr_info("CPU%d: backtrace skipped as idling\n", smp_processor_id());
> Actually perhaps an idle injection's backtrace is worth dumping. I guess
> it might accidentally produce lockups and it's worth knowing the source then.
> 
> Though I don't have a strong opinion on that...
> 
>>   		return;
>>   	}
>> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
>> index b10b8349bb2a..6169faf30ecd 100644
>> --- a/kernel/rcu/tree_stall.h
>> +++ b/kernel/rcu/tree_stall.h
>> @@ -418,7 +418,7 @@ static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp
>>   		return false;
>>   
>>   	cpu = task_cpu(rcuc);
>> -	if (cpu_is_offline(cpu) || idle_cpu(cpu))
>> +	if (cpu_is_offline(cpu) || sched_core_idle_cpu(cpu))
> An idle injection could possibly starve the RCU boost kthread, and then it's
> worth knowing about it. I would suggest keeping idle_cpu() here.
> 

Actually I think it should just be idle_cpu() for rcu_is_rcuc_kthread_starving() 
and showacpu() because "force idling" is different from "idling".

Force idling happens because there is something incompatible on the sibling 
runqueue in the core. That just makes the 2 runqueues on the core appear to be a 
single runqueue. The concept of "force idling" is more closer to "preemption of 
tasks on a single runqueue".

Considering that, I would vote for only converting the tick user. If force 
idling happens for too long, it'd be good to know that as Frederic also mentioned.

thanks,

  - Joel