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 12:03:05 +0800
From:   Cheng Chao <cs.os.kernel@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, oleg@...hat.com, 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

Peter, thank you.

on 09/12/2016 07:41 PM, Peter Zijlstra wrote:
> On Mon, Sep 12, 2016 at 01:37:27PM +0200, Peter Zijlstra wrote:
>> So what you're saying is that migration_stop_cpu() doesn't work because
>> wait_for_completion() dequeues the task.
>>
>> True I suppose. Not sure I like your solution, nor your implementation
>> of the solution much though.
>>
>> I would much prefer an unconditional cond_resched() there, but also, I
>> think we should do what __migrate_swap_task() does, and set wake_cpu.
>>
>> So something like so..
>>
>> ---
>>  kernel/sched/core.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index ddd5f48551f1..ade772aa9610 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -1063,8 +1063,12 @@ static int migration_cpu_stop(void *data)
>>  	 * holding rq->lock, if p->on_rq == 0 it cannot get enqueued because
>>  	 * we're holding p->pi_lock.
>>  	 */
>> -	if (task_rq(p) == rq && task_on_rq_queued(p))
>> -		rq = __migrate_task(rq, p, arg->dest_cpu);
>> +	if (task_rq(p) == rq) {
>> +		if (task_on_rq_queued(p))
>> +			rq = __migrate_task(rq, p, arg->dest_cpu);
>> +		else
>> +			p->wake_cpu = arg->dest_cpu;
>> +	}
>>  	raw_spin_unlock(&rq->lock);
>>  	raw_spin_unlock(&p->pi_lock);
>>  
> 
> And this, too narrow a constraint do git diff made it go away.
> 

yes, set wake_cpu is better when try_to_wake_up(). 
Peter, Is it as a new patch?

> ---
>  kernel/stop_machine.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index ae6f41fb9cba..637798d6b554 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -121,6 +121,11 @@ 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;
> +	/*
> +	 * In case @cpu == smp_proccessor_id() we can avoid a sleep+wakeup
> +	 * by doing a preemption.
> +	 */
> +	cond_resched();
>  	wait_for_completion(&done.completion);
>  	return done.ret;
>  }
> 

I agree to use cond_resched(). https://lkml.org/lkml/2016/9/12/1228

for CONFIG_PREEMPT=y and CONFIG_PREEMPT_VOLUNTARY=y, this patch seems unnecessary.
1. when CONFIG_PREEMPT=y
  stop_one_cpu()
  ->cpu_stop_queue_work()
    ->spin_unlock_irqrestore() (preempt_enable() calls __preempt_schedule())

2. when CONFIG_PREEMPT_VOLUNTARY=y
  stop_one_cpu()
  ->wait_for_completion()
    ->...
      ->might_sleep() (calls _cond_resched()


so we really don't need "if defined(CONFIG_PREEMPT_NONE)"?

I also think without the "if defined(CONFIG_PREEMPT_NONE)", 
the code is more clean,and the logic is still ok.



diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 4a1ca5f..87464a2 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -126,6 +126,15 @@ 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)
+       /*
+        * In case @cpu == smp_proccessor_id() we can avoid a sleep+wakeup
+        * by doing a preemption.
+        */
+       cond_resched();
+#endif
+
        wait_for_completion(&done.completion);
        return done.ret;
 }