Re: [PATCH v2] rcu-tasks: Make rude RCU-Tasks work well with CPU hotplug

[Joel Fernandes <joel@joelfernandes.org>]

> On Nov 29, 2022, at 10:18 AM, Paul E. McKenney <paulmck@kernel.org> wrote:
> 
> On Tue, Nov 29, 2022 at 06:25:04AM +0000, Zhang, Qiang1 wrote:
>>>> On Nov 28, 2022, at 11:54 PM, Zhang, Qiang1 <qiang1.zhang@intel.com> wrote:
>>> 
>>> On Mon, Nov 28, 2022 at 10:34:28PM +0800, Zqiang wrote:
>>>> Currently, invoke rcu_tasks_rude_wait_gp() to wait one rude
>>>> RCU-tasks grace period, if __num_online_cpus == 1, will return
>>>> directly, indicates the end of the rude RCU-task grace period.
>>>> suppose the system has two cpus, consider the following scenario:
>>>> 
>>>>   CPU0                                   CPU1 (going offline)
>>>>                         migration/1 task:
>>>>                                     cpu_stopper_thread
>>>>                                      -> take_cpu_down
>>>>                                         -> _cpu_disable
>>>>                              (dec __num_online_cpus)
>>>>                                         ->cpuhp_invoke_callback
>>>>                                               preempt_disable
>>>>                       access old_data0
>>>>          task1
>>>> del old_data0                                  .....
>>>> synchronize_rcu_tasks_rude()
>>>> task1 schedule out
>>>> ....
>>>> task2 schedule in
>>>> rcu_tasks_rude_wait_gp()
>>>>    ->__num_online_cpus == 1
>>>>      ->return
>>>> ....
>>>> task1 schedule in
>>>> ->free old_data0
>>>>                                               preempt_enable
>>>> 
>>>> when CPU1 dec __num_online_cpus and __num_online_cpus is equal one,
>>>> the CPU1 has not finished offline, stop_machine task(migration/1)
>>>> still running on CPU1, maybe still accessing 'old_data0', but the
>>>> 'old_data0' has freed on CPU0.
>>>> 
>>>> This commit add cpus_read_lock/unlock() protection before accessing
>>>> __num_online_cpus variables, to ensure that the CPU in the offline
>>>> process has been completed offline.
>>>> 
>>>> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
>>>> 
>>>> First, good eyes and good catch!!!
>>>> 
>>>> The purpose of that check for num_online_cpus() is not performance
>>>> on single-CPU systems, but rather correct operation during early boot.
>>>> So a simpler way to make that work is to check for RCU_SCHEDULER_RUNNING,
>>>> for example, as follows:
>>>> 
>>>>   if (rcu_scheduler_active != RCU_SCHEDULER_RUNNING &&
>>>>       num_online_cpus() <= 1)
>>>>       return;    // Early boot fastpath for only one CPU.
>>> 
>>> Hi Paul
>>> 
>>> During system startup, because the RCU_SCHEDULER_RUNNING is set after starting other CPUs, 
>>> 
>>>             CPU0                                                                       CPU1                                                                 
>>> 
>>> if (rcu_scheduler_active !=                                    
>>>   RCU_SCHEDULER_RUNNING &&
>>>          __num_online_cpus  == 1)                                               
>>>   return;                                                                         inc  __num_online_cpus
>>>                           (__num_online_cpus == 2)
>>> 
>>> CPU0 didn't notice the update of the __num_online_cpus variable by CPU1 in time
>>> Can we move rcu_set_runtime_mode() before smp_init()
>>> any thoughts?
>>> 
>>> Is anyone expected to do rcu-tasks operation before the scheduler is running? 
>> 
>> Not sure if such a scenario exists.
>> 
>>> Typically this requires the tasks to context switch which is a scheduler operation.
>>> 
>>> If the scheduler is not yet running, then I don’t think missing an update the __num_online_cpus matters since no one does a tasks-RCU synchronize.
>> 
>> Hi Joel
>> 
>> After the kernel_init task runs, before calling smp_init() to starting other CPUs, 
>> the scheduler haven been initialization, task context switching can occur.
> 
> Good catch, thank you both.  For some reason, I was thinking that the
> additional CPUs did not come online until later.
> 
> So how about this?
> 
>    if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
>        return;    // Early boot fastpath.
> 
> If this condition is true, there is only one CPU and no scheduler,
> thus no preemption.

Agreed. I was going to suggest exactly this :)

Ack.
(Replying by phone but feel free to add my reviewed by tag).

- Joel


> 
>                        Thanx, Paul
> 
>> Thanks
>> Zqiang
>> 
>>> 
>>> Or did I miss something?
>>> 
>>> Thanks.
>>> 
>>> 
>>> 
>>> 
>>> Thanks
>>> Zqiang
>>> 
>>>> 
>>>> This works because rcu_scheduler_active is set to RCU_SCHEDULER_RUNNING
>>>> long before it is possible to offline CPUs.
>>>> 
>>>> Yes, schedule_on_each_cpu() does do cpus_read_lock(), again, good eyes,
>>>> and it also unnecessarily does the schedule_work_on() the current CPU,
>>>> but the code calling synchronize_rcu_tasks_rude() is on high-overhead
>>>> code paths, so this overhead is down in the noise.
>>>> 
>>>> Until further notice, anyway.
>>>> 
>>>> So simplicity is much more important than performance in this code.
>>>> So just adding the check for RCU_SCHEDULER_RUNNING should fix this,
>>>> unless I am missing something (always possible!).
>>>> 
>>>>                           Thanx, Paul
>>>> 
>>>> ---
>>>> kernel/rcu/tasks.h | 20 ++++++++++++++++++--
>>>> 1 file changed, 18 insertions(+), 2 deletions(-)
>>>> 
>>>> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
>>>> index 4a991311be9b..08e72c6462d8 100644
>>>> --- a/kernel/rcu/tasks.h
>>>> +++ b/kernel/rcu/tasks.h
>>>> @@ -1033,14 +1033,30 @@ static void rcu_tasks_be_rude(struct work_struct *work)
>>>> {
>>>> }
>>>> 
>>>> +static DEFINE_PER_CPU(struct work_struct, rude_work);
>>>> +
>>>> // Wait for one rude RCU-tasks grace period.
>>>> static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
>>>> {
>>>> +    int cpu;
>>>> +    struct work_struct *work;
>>>> +
>>>> +    cpus_read_lock();
>>>>   if (num_online_cpus() <= 1)
>>>> -        return;    // Fastpath for only one CPU.
>>>> +        goto end;// Fastpath for only one CPU.
>>>> 
>>>>   rtp->n_ipis += cpumask_weight(cpu_online_mask);
>>>> -    schedule_on_each_cpu(rcu_tasks_be_rude);
>>>> +    for_each_online_cpu(cpu) {
>>>> +        work = per_cpu_ptr(&rude_work, cpu);
>>>> +        INIT_WORK(work, rcu_tasks_be_rude);
>>>> +        schedule_work_on(cpu, work);
>>>> +    }
>>>> +
>>>> +    for_each_online_cpu(cpu)
>>>> +        flush_work(per_cpu_ptr(&rude_work, cpu));
>>>> +
>>>> +end:
>>>> +    cpus_read_unlock();
>>>> }
>>>> 
>>>> void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
>>>> -- 
>>>> 2.25.1
>>>>