linux-kernel - Re: [PATCH v2] sched/core: introduce sched_core_idle

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Date:   Mon, 26 Jun 2023 21:07:54 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Frederic Weisbecker <frederic@...nel.org>,
        Cruz Zhao <CruzZhao@...ux.alibaba.com>
Cc:     gregkh@...uxfoundation.org, jirislaby@...nel.org, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, paulmck@...nel.org,
        quic_neeraju@...cinc.com, josh@...htriplett.org,
        boqun.feng@...il.com, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, qiang1.zhang@...el.com, jstultz@...gle.com,
        clingutla@...eaurora.org, nsaenzju@...hat.com, tglx@...utronix.de,
        linux-kernel@...r.kernel.org,
        Vineeth Pillai <vineeth@...byteword.org>
Subject: Re: [PATCH v2] sched/core: introduce sched_core_idle_cpu()

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