Re: [PATCH v4 3/3] sched: optimize migration by forcing rmb() and updating to be called once

[Byungchul Park]

On Tue, Nov 10, 2015 at 01:16:47PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 10, 2015 at 10:09:05AM +0900, Byungchul Park wrote:
> > On Mon, Nov 09, 2015 at 02:29:14PM +0100, Peter Zijlstra wrote:
> > > On Sat, Oct 24, 2015 at 01:16:21AM +0900, byungchul.park@xxxxxxx wrote:
> > > > +++ b/kernel/sched/core.c
> > > > @@ -1264,6 +1264,8 @@ EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr);
> > > >  
> > > >  void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
> > > >  {
> > > > +	unsigned int prev_cpu = task_cpu(p);
> > > > +
> > > >  #ifdef CONFIG_SCHED_DEBUG
> > > >  	/*
> > > >  	 * We should never call set_task_cpu() on a blocked task,
> > > > @@ -1289,15 +1291,14 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
> > > >  #endif
> > > >  
> > > >  	trace_sched_migrate_task(p, new_cpu);
> > > > +	__set_task_cpu(p, new_cpu);
> > > >  
> > > > -	if (task_cpu(p) != new_cpu) {
> > > > +	if (prev_cpu != new_cpu) {
> > > >  		if (p->sched_class->migrate_task_rq)
> > > > -			p->sched_class->migrate_task_rq(p, new_cpu);
> > > > +			p->sched_class->migrate_task_rq(p, prev_cpu);
> > > >  		p->se.nr_migrations++;
> > > >  		perf_event_task_migrate(p);
> > > >  	}
> > > > -
> > > > -	__set_task_cpu(p, new_cpu);
> > > >  }
> > > 
> > > I don't think this is safe, see the comment in __set_task_cpu(). We want
> > > that to be last.
> > 
> > I am sorry but I don't understand what you said. I checked the comment in 
> > __set_task_cpu().
> > 
> > 	/*
> > 	 * After ->cpu is set up to a new value, task_rq_lock(p, ...) can be
> > 	 * successfuly executed on another CPU. We must ensure that updates of
> > 	 * per-task data have been completed by this moment.
> > 	 */
> > 
> > Of course, ->cpu should be set up to a new value for task_rq_lock() to be
> > executed successfully on another CPU. Is this the case? Is there something
> > i missed? I think it would be ok if task->pi_lock can work correctly within
> > "if" statement in set_task_cpu(). Is there problem to do that?
> 
> So the problem is that as soon as that ->cpu store comes through, the
> other rq->lock can happen, even though we might still hold a rq->lock
> thinking we're serialized.
> 
> Take for instance move_queued_tasks(), it does:
> 
> 	dequeue_task(rq, p, 0);
> 	p->on_rq = TASK_ON_RQ_MIGRATING;
> 	set_task_cpu(p, new_cpu) {
> 	  __set_task_cpu();
> 
> ^^^ here holding rq->lock is insufficient and the below:
> 
> 	  p->sched_class->migrate_task_rq()

Thank you for explaning in detail, but this's why i asked you.
Yes, rq->lock is insufficient in this place as you said, but
should migrate_task_rq() be serialized by rq->lock? I might have
agreed with you if the migrate_task_rq() should be serialized by
rq->lock, but I think it's not the case. I think it would be of
if task->pi_lock can work correcly within *if statement* in 
set_task_cpu(). Wrong?

> 
> would no longer be serialized by rq->lock.
> 
> 	}
> 	raw_spin_unlock(&rq->lock);
> 
> 
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