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.
>
>
