Re: [patch 2/3]cfq-iosched: schedule dispatch for noidle queue

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Nov 08, 2010 at 10:07:18AM +0800, Shaohua Li wrote:
> A queue is idle at cfq_dispatch_requests(), but it gets noidle later. Unless
> other task explictly does unplug or all requests are drained, we will not
> deliever requests to the disk even cfq_arm_slice_timer doesn't make the
> queue idle. For example, cfq_should_idle() returns true because of
> service_tree->count == 1, and then other queues are added. Note, I didn't
> see obvious performance impacts so far with the patch, but just thought
> this could be a problem.
> 
> Signed-off-by: Shaohua Li <shaohua.li@intel.com>
> 
> ---
>  block/cfq-iosched.c |   23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
> 
> Index: linux/block/cfq-iosched.c
> ===================================================================
> --- linux.orig/block/cfq-iosched.c	2010-11-08 08:41:20.000000000 +0800
> +++ linux/block/cfq-iosched.c	2010-11-08 08:43:51.000000000 +0800
> @@ -3265,6 +3265,10 @@ cfq_should_preempt(struct cfq_data *cfqd
>  	if (cfq_class_rt(new_cfqq) && !cfq_class_rt(cfqq))
>  		return true;
>  
> +	/* An idle queue should not be idle now for some reason */
> +	if (RB_EMPTY_ROOT(&cfqq->sort_list) && !cfq_should_idle(cfqd, cfqq))
> +		return true;
> +
>  	if (!cfqd->active_cic || !cfq_cfqq_wait_request(cfqq))
>  		return false;
>  
> @@ -3508,8 +3512,25 @@ static void cfq_completed_request(struct
>  		}
>  	}
>  
> -	if (!cfqd->rq_in_driver)
> +	if (!cfqd->rq_in_driver) {
>  		cfq_schedule_dispatch(cfqd);
> +		return;
> +	}
> +	/*
> +	 * A queue is idle at cfq_dispatch_requests(), but it gets noidle
> +	 * later. We schedule a dispatch if the queue has no requests,
> +	 * otherwise the disk is actually in idle till all requests
> +	 * are finished even cfq_arm_slice_timer doesn't make the queue idle
> +	 * */

Why do we have to wait for all requests to finish in device? Will driver
most likely not ask for next request when 1-2 requests have completed
and at that time we should expire the queue if queue is no more marked
as "noidle"?

Vivek

> +	cfqq = cfqd->active_queue;
> +	if (!cfqq)
> +		return;
> +
> +	if (RB_EMPTY_ROOT(&cfqq->sort_list) && !cfq_should_idle(cfqd, cfqq) &&
> +	    (!cfqd->cfq_group_idle || cfqq->cfqg->nr_cfqq > 1)) {
> +		cfq_del_timer(cfqd, cfqq);
> +		cfq_schedule_dispatch(cfqd);
> +	}
>  }
>  
>  /*
>