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@xxxxxxxxxx>
>> ---
>> 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
