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

[chengchao]

Oled, thank you for moving this patch on.

on 09/07/2016 08:35 PM, Oleg Nesterov wrote:
> On 09/07, chengchao wrote:
>>
>> Oleg, thank you very much.
>>
>> on 09/06/2016 11:22 PM, Oleg Nesterov wrote:
>>> On 09/06, chengchao wrote:
>>>>
>>>> the key point is for CONFIG_PREEMPT_NONE=y,
>>>> ...
>>>> it is too much overhead for one task(fork()+exec()), isn't it?
>>>
>>> Yes, yes, I see, this is suboptimal. Not sure we actually do care,
>>> but yes, perhaps another helper which migrates the current task makes
>>> sense, I dunno.
>>
>> for CONFIG_PREEMPT_NONE=y, this patch wants the stopper thread can migrate the current
>> successfully instead of doing nothing.
> 
> I understand the intent. But I am not sure this optimization makes
> sense.
> 

For CONFIG_PREEMPT_NONE=y, when sched_exec() needs migration, sched_exec() calls
stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg).

If stopper thread can not migrate for us,why should we call stop_one_cpu() here? 
It just makes the task TASK_UNINTERRUPTIBLE, wakes up the stopper thread, executes the 
migration_cpu_stop, and the stopper thread wakes up the task.

But in fact, all above works are almost unuseful, the reason is that the migration_cpu_stop
doesn't migrate for us. why? the migration_cpu_stop() needs the task is TASK_ON_RQ_QUEUED 
before it calls __migrate_task().

This patch can make the task TASK_RUNNING instead of TASK_UNINTERRUPTIBLE,
so the migration_cpu_stop() can migrate happily.

Does this optimization make sense now?

Any different opinions are always welcome.

>> int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
>> {
>>         struct cpu_stop_done done;
>>         struct cpu_stop_work work = { .fn = fn, .arg = arg, .done = &done };
>>
>>         cpu_stop_init_done(&done, 1);
>>         if (!cpu_stop_queue_work(cpu, &work))
>>                 return -ENOENT;
>>
>> #if defined(CONFIG_PREEMPT_NONE)
>> 	   /*
>>          * let the stopper thread runs as soon as possible,
>>          * and keep current TASK_RUNNING.
>>          */
>> 	scheudle();
>> #endif
>>         wait_for_completion(&done.completion);
>>         return done.ret;
>> }
> 
> Agreed this looks better, although I'd suggest _cond_resche().
> 
> Again, I am not sure this makes sense, I leave this to maintainers.
> 

You have done much works for this patch. Thanks again. 

> Oleg.
> 
>