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

[Zhang, Qiang1]

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@xxxxxxxxx>
>
>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?

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
>