linux-kernel - Re: [RFC] Thread Migration Preemption

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Date:	Sat, 14 Jul 2007 16:25:09 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Oleg Nesterov <oleg@...sign.ru>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC] Thread Migration Preemption - v4

* Peter Zijlstra (a.p.zijlstra@...llo.nl) wrote:
> Would have been nice if you'd CC'ed me :-/
> 
Sorry

> On Sat, 2007-07-14 at 14:42 -0400, Mathieu Desnoyers wrote:
> 
> > @@ -4891,10 +4948,42 @@ static int migration_thread(void *data)
> >  		list_del_init(head->next);
> >  
> >  		spin_unlock(&rq->lock);
> > -		__migrate_task(req->task, cpu, req->dest_cpu);
> > +		migrated = __migrate_task(req->task, cpu, req->dest_cpu);
> >  		local_irq_enable();
> > -
> > -		complete(&req->done);
> > +		if (!migrated) {
> > +			/*
> > +			 * If the process has not been migrated, let it run
> > +			 * until it reaches a migration_check() so it can
> > +			 * wake us up.
> > +			 */
> > +			spin_lock_irq(&rq->lock);
> > +			head = &rq->migration_queue;
> > +			list_add(&req->list, head);
> > +			if (req->task->se.on_rq
> > +					|| !task_migrate_count(req->task)) {
> > +				/*
> > +				 * The process is on the runqueue, it could
> > +				 * exit its critical section at any moment,
> > +				 * don't race with it and retry actively.
> > +				 * Also, if the thread is not on the runqueue
> > +				 * and has a zero migration count
> > +				 * (__migrate_task failed because cpus allowed
> > +				 * changed), just retry.
> > +				 */
> > +				spin_unlock_irq(&rq->lock);
> > +				continue;
> > +			}
> > +			set_tsk_thread_flag(req->task, TIF_NEED_MIGRATE);
> > +			set_current_state(TASK_INTERRUPTIBLE);
> > +			spin_unlock_irq(&rq->lock);
> > +			/*
> > +			 * Wait for the process currently in its critical
> > +			 * section.
> > +			 */
> > +			wake_up_process(req->task);
> > +			schedule();
> > +		} else
> > +			complete(&req->done);
> >  	}
> >  	__set_current_state(TASK_RUNNING);
> >  	return 0;
> 
> I'm not really liking this, this way we hold up all other migration
> requests.
> 

True. You approach of delayed migration helps on this.

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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