Re: [PATCH] sched/core: simpler function for sched_exec migration

[chengchao <chengchao@kedacom.com>]

Oleg, thank you.

the key point is for CONFIG_PREEMPT_NONE=y, if sched_exec needs migrate the current, 
migration_cpu_stop doesn't migrate the task(current) at all, it means that the stopper thread does some 
unuseful works in this scenario.
finally,the stopper thread calls cpu_stop_signal_done() to wake up this task, it calls select_task_rq() again, 
maybe select another different cpu. totally calls select_task_rq() two times(first at sched_exec())
plus one time(wake_up_new_task() also calls select_task_rq()).

it is too much overhead for one task(fork()+exec()), isn't it?



1.
sched_exec()
  ->stop_one_cpu()
    ->wait_for_completion().
     wait_for_completion() makes the current TASK_UNINTERRUPTIBLE and call schedule_timeout()

schedule_timeout(timeout)  timeout is MAX_SCHEDULE_TIMEOUT.
  ->schedule()
	deactivate_task(rq, current, DEQUEUE_SLEEP);
	current->on_rq = 0;

2.
migration_cpu_stop() checks the task_on_rq_queued(p), but the task p->on_rq is 0.

#define TASK_ON_RQ_QUEUED       1

static inline int task_on_rq_queued(struct task_struct *p)
{
        return p->on_rq == TASK_ON_RQ_QUEUED;
}


migration_cpu_stop()
...
        if (task_rq(p) == rq && task_on_rq_queued(p))   
                rq = __migrate_task(rq, p, arg->dest_cpu);
...


thanks again, any suggestions and more reviews are welcome.


on 09/05/2016 09:11 PM, Oleg Nesterov wrote:
> On 09/05, cheng chao wrote:
>>
>> @@ -2958,7 +2958,7 @@ void sched_exec(void)
>>  		struct migration_arg arg = { p, dest_cpu };
>>  
>>  		raw_spin_unlock_irqrestore(&p->pi_lock, flags);
>> -		stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg);
>> +		stop_one_cpu_sync(task_cpu(p), migration_cpu_stop, &arg);
>>  		return;
>>  	}
>>  unlock:
>> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
>> index 4a1ca5f..24f8637 100644
>> --- a/kernel/stop_machine.c
>> +++ b/kernel/stop_machine.c
>> @@ -130,6 +130,27 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
>>  	return done.ret;
>>  }
>>  
>> +/**
>> + * the caller keeps task_on_rq_queued, so it's more suitable for
>> + * sched_exec on the case when needs migration
>> + */
>> +void stop_one_cpu_sync(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
>> +{
>> +	struct cpu_stop_work work = { .fn = fn, .arg = arg, .done = NULL };
>> +
>> +	if (!cpu_stop_queue_work(cpu, &work))
>> +		return;
>> +
>> +#if defined(CONFIG_PREEMPT_NONE) || defined(CONFIG_PREEMPT_VOLUNTARY)
>> +	/*
>> +	 * CONFIG_PREEMPT doesn't need call schedule here, because
>> +	 * preempt_enable already does the similar thing when call
>> +	 * cpu_stop_queue_work
>> +	 */
>> +	schedule();
>> +#endif
>> +}
> 
> Honestly, I don't really understand the changelog, but this looks wrong.
> 
> stop_one_cpu_sync() assumes that cpu == smp_processor_id/task_cpu(current),
> and thus the stopper thread should preempt us at least after schedule()
> (if CONFIG_PREEMPT_NONE), so we do not need to synchronize.
> 
   yes. the stop_one_cpu_sync is not a good name, stop_one_cpu_schedule is better?  
there is nothing about synchronization.

> But this is not necessarily true? This task can migrate to another CPU
> before cpu_stop_queue_work() ?
>
  before sched_exec() calls stop_one_cpu()/cpu_stop_queue_work(), this task(current) cannot migrate 
to another cpu,because this task is running on the cpu.

 
> Oleg.
> 
>