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

[Joel Fernandes <joel@joelfernandes.org>]

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