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[dm-devel] Re: [PATCH 05/24] io-controller: Modify cfq to make use of flat elevator fair queuing



Vivek Goyal wrote:
...
>  /*
>   * Check if new_cfqq should preempt the currently active queue. Return 0 for
> - * no or if we aren't sure, a 1 will cause a preempt.
> + * no or if we aren't sure, a 1 will cause a preemption attempt.
> + * Some of the preemption logic has been moved to common layer. Only cfq
> + * specific parts are left here.
>   */
>  static int
> -cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
> -		   struct request *rq)
> +cfq_should_preempt(struct request_queue *q, void *new_cfqq, struct request *rq)
>  {
> -	struct cfq_queue *cfqq;
> +	struct cfq_data *cfqd = q->elevator->elevator_data;
> +	struct cfq_queue *cfqq = elv_active_sched_queue(q->elevator);
>  
> -	cfqq = cfqd->active_queue;
>  	if (!cfqq)
>  		return 0;
>  
> -	if (cfq_slice_used(cfqq))
> +	if (elv_ioq_slice_used(cfqq->ioq))
>  		return 1;
>  
>  	if (cfq_class_idle(new_cfqq))
> @@ -2018,13 +1661,7 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
>  	if (rq_is_meta(rq) && !cfqq->meta_pending)
>  		return 1;
>  
> -	/*
> -	 * Allow an RT request to pre-empt an ongoing non-RT cfqq timeslice.
> -	 */
> -	if (cfq_class_rt(new_cfqq) && !cfq_class_rt(cfqq))
> -		return 1;
> -
> -	if (!cfqd->active_cic || !cfq_cfqq_wait_request(cfqq))
> +	if (!cfqd->active_cic || !elv_ioq_wait_request(cfqq->ioq))
>  		return 0;
>  
>  	/*

Hi Vivek,

cfq_should_preempt() will do the check "if (cfq_rq_close(cfqd, rq)) to see whether
it should preempt the current cfqq. From fairness point of view,  should we also 
check "fairness" value, if it's set fairness == 1, don't allow to preempt the current
cfqq?


-- 
Regards
Gui Jianfeng


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