Re: [PATCH 1/2] rcu: Delete a redundant check in rcu_check_gp_kthread_starvation()

["Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>]

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@huawei.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.

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.


> 
>> -				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