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Subject: Re: [RFC] Thread Migration Preemption - v4
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Oleg Nesterov <oleg@tv-sign.ru>, linux-kernel@vger.kernel.org,
       Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>
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Date: Sat, 14 Jul 2007 21:14:11 +0200
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Would have been nice if you'd CC'ed me :-/

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.