linux-kernel - Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT

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Date:   Tue, 13 Sep 2016 10:45:27 +0800
From:   Cheng Chao <cs.os.kernel@...il.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     mingo@...nel.org, peterz@...radead.org, tj@...nel.org,
        akpm@...ux-foundation.org, chris@...is-wilson.co.uk,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful
 works for CONFIG_PREEMPT_NONE



on 09/12/2016 07:03 PM, Oleg Nesterov wrote:
> On 09/10, Cheng Chao wrote:
>>
>> @@ -126,6 +126,17 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
>>  	cpu_stop_init_done(&done, 1);
>>  	if (!cpu_stop_queue_work(cpu, &work))
>>  		return -ENOENT;
>> +
>> +#if defined(CONFIG_PREEMPT_NONE)
>> +	/*
>> +	 * Makes the stopper thread run as soon as possible.
>> +	 * And if the caller is TASK_RUNNING, keeps the caller TASK_RUNNING.
>> +	 * It's special useful for some callers which are expected to be
>> +	 * TASK_ON_RQ_QUEUED.
>> +	 * sched_exec does benefit from this improvement.
>> +	 */
>> +	schedule();
>> +#endif
>>  	wait_for_completion(&done.completion);
>>  	return done.ret;
>>  }
> 
> Cheng, I already tried twice to suggest to conditionalize this schedule,
> because it can only help if cpu == smp_processor_id, and you didn't reply.
> I still think _cond_resched() makes more sense.
> 
> I won't really argue if you prefer it this way. But did you see my emails?
>

I read them, thanks. because Peter didn't receive my mails before, it took me much time
to fix my mailbox, so I didn't reply on time.

Ok, even if cpu != smp_processor_id(), to call schedule() instead _cond_resched() can
give the caller a chance not to sleep. when the caller runs on the cpu again, it may 
likely find the completion is already done. 
then the stopper thread cpu_stop_signal_done() and the caller wait_for_completion() will
actually run very soon.

I think it is trivial improvement. using cond_resched()/_cond_resched() is better for 
readability, I choose the cond_resched().

thanks again.



 
> Oleg.
>