linux-kernel - Re: [PATCH 1/2] rcu: Delete a redundant check in rcu_check_gp_kthread

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Date:   Thu, 13 Jul 2023 10:03:08 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     <paulmck@...nel.org>
CC:     Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>, <rcu@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] rcu: Delete a redundant check in
 rcu_check_gp_kthread_starvation()



On 2023/7/12 0:48, Paul E. McKenney wrote:
> On Tue, Jul 11, 2023 at 11:20:07AM +0800, Leizhen (ThunderTown) wrote:
>>
>>
>> On 2023/7/11 3:03, Paul E. McKenney wrote:
>>> On Wed, Jul 05, 2023 at 03:30:19PM +0800, Zhen Lei wrote:
>>>> The above condition "if (gpk)" already ensures that gp_kthread is created,
>>>> so the local variable 'cpu' cannot be negative here.
>>>>
>>>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
>>>> ---
>>>>  kernel/rcu/tree_stall.h | 12 +++++-------
>>>>  1 file changed, 5 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
>>>> index b10b8349bb2a48b..dcfaa3d5db2cbc7 100644
>>>> --- a/kernel/rcu/tree_stall.h
>>>> +++ b/kernel/rcu/tree_stall.h
>>>> @@ -537,13 +537,11 @@ static void rcu_check_gp_kthread_starvation(void)
>>>>  			pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name);
>>>>  			pr_err("RCU grace-period kthread stack dump:\n");
>>>>  			sched_show_task(gpk);
>>>> -			if (cpu >= 0) {
>>>
>>> I am not quite this trusting of the relation between the relationship
>>> between the existence of the grace-period khread and its CPU number
>>> being in range.  Let's please start with something like this:
>>>
>>> 			if (!WARN_ON_ONCE(cpu < 0)) {
>>>
>>> Please note that this is not just me.  See for example the use of the
>>> cpumask_check() function, albeit the opposite concern.
>>
>> git grep -wn "\->cpu" kernel/ include/
>> kernel/kthread.c:583:   to_kthread(p)->cpu = cpu;				//kthread_create_on_cpu()
>> kernel/sched/sched.h:2024:      WRITE_ONCE(task_thread_info(p)->cpu, cpu);	//__set_task_cpu()
>> include/linux/sched.h:2250:     return READ_ONCE(task_thread_info(p)->cpu);	//task_cpu()
>>
>> git grep -wn "\.cpu" kernel/ include/						//There is no task related, the search result is omitted.
>>
>> Therefore, there is only one path "set_task_cpu()-->__set_task_cpu()" that can dynamically
>> change the value of task_cpu(p). In fact, this guarantee has been made in set_task_cpu().
>> set_task_cpu
>> 	WARN_ON_ONCE(!cpu_online(new_cpu));
>> 	__set_task_cpu(p, new_cpu);
>>
>> In addition, task_struct has member 'on_rq'. Therefore, when a task leaves the scheduling
>> queue, setting the member 'cpu' to an invalid value will be thankless.
> 
> Thank you for digging into this!  Given that, as you say, we can dispense
> with the check.
> 
>> Sorry, these two patches was posted too quickly, and I'm still regretting that I should have
>> attached this to the commit description these days.
> 
> Please do resend the patches with this explanation in the commit log.
> And please don't worry about making the English pretty, as I can always
> wordsmith.

OK, thank you very much.

> 
> 							Thanx, Paul
> 
>>>> -				if (cpu_is_offline(cpu)) {
>>>> -					pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
>>>> -				} else  {
>>>> -					pr_err("Stack dump where RCU GP kthread last ran:\n");
>>>> -					dump_cpu_task(cpu);
>>>> -				}
>>>> +			if (cpu_is_offline(cpu)) {
>>>> +				pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
>>>> +			} else  {
>>>> +				pr_err("Stack dump where RCU GP kthread last ran:\n");
>>>> +				dump_cpu_task(cpu);
>>>>  			}
>>>>  			wake_up_process(gpk);
>>>>  		}
>>>> -- 
>>>> 2.25.1
>>>>
>>> .
>>>
>>
>> -- 
>> Regards,
>>   Zhen Lei
> .
> 

-- 
Regards,
  Zhen Lei