linux-kernel - Re: [PATCH 4/4] cfq-iosched: fix corner cases in idling logic

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Date:	Tue, 24 Nov 2009 09:42:22 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Jeff Moyer <jmoyer@...hat.com>
Subject: Re: [PATCH 4/4] cfq-iosched: fix corner cases in idling logic

On Tue, Nov 24, 2009 at 02:49:36PM +0100, Corrado Zoccolo wrote:
> Idling logic was disabled in some corner cases, leading to unfair share
> for noidle queues.
> * the idle timer was not armed if there were other requests in the
>   driver. unfortunately, those requests could come from other workloads,
>   or queues for which we don't enable idling. So we will check only
>   pending requests from the active queue
> * rq_noidle check on no-idle queue could disable the end of tree idle if
>   the last completed request was rq_noidle. Now, we will disable that
>   idle only if all the queues served in the no-idle tree had rq_noidle
>   requests.
> 
> Reported-by: Vivek Goyal <vgoyal@...hat.com>
> Signed-off-by: Corrado Zoccolo <czoccolo@...il.com>

Looks good to me.

A minor nit, "noidle_tree_requires_idle" is too long a string. A little
smaller string can be "idle_on_noidle_wl".

Acked-by: Vivek Goyal <vgoyal@...hat.com>

Thanks
Vivek

> ---
>  block/cfq-iosched.c |   32 ++++++++++++++++++++++----------
>  1 files changed, 22 insertions(+), 10 deletions(-)
> 
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 373e80f..d44f8a4 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -172,6 +172,7 @@ struct cfq_data {
>  	enum wl_prio_t serving_prio;
>  	enum wl_type_t serving_type;
>  	unsigned long workload_expires;
> +	bool noidle_tree_requires_idle;
>  
>  	/*
>  	 * Each priority tree is sorted by next_request position.  These
> @@ -1249,9 +1250,9 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)
>  		return;
>  
>  	/*
> -	 * still requests with the driver, don't idle
> +	 * still active requests from this queue, don't idle
>  	 */
> -	if (rq_in_driver(cfqd))
> +	if (cfqq->dispatched)
>  		return;
>  
>  	/*
> @@ -1487,6 +1488,7 @@ static void choose_service_tree(struct cfq_data *cfqd)
>  
>  	slice = max_t(unsigned, slice, CFQ_MIN_TT);
>  	cfqd->workload_expires = jiffies + slice;
> +	cfqd->noidle_tree_requires_idle = false;
>  }
>  
>  /*
> @@ -2606,17 +2608,27 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
>  			cfq_clear_cfqq_slice_new(cfqq);
>  		}
>  		/*
> -		 * If there are no requests waiting in this queue, and
> -		 * there are other queues ready to issue requests, AND
> -		 * those other queues are issuing requests within our
> -		 * mean seek distance, give them a chance to run instead
> -		 * of idling.
> +		 * Idling is not enabled on:
> +		 * - expired queues
> +		 * - idle-priority queues
> +		 * - async queues
> +		 * - queues with still some requests queued
> +		 * - when there is a close cooperator
>  		 */
>  		if (cfq_slice_used(cfqq) || cfq_class_idle(cfqq))
>  			cfq_slice_expired(cfqd, 1);
> -		else if (cfqq_empty && !cfq_close_cooperator(cfqd, cfqq) &&
> -			 sync && !rq_noidle(rq))
> -			cfq_arm_slice_timer(cfqd);
> +		else if (sync && cfqq_empty &&
> +			 !cfq_close_cooperator(cfqd, cfqq)) {
> +			cfqd->noidle_tree_requires_idle |= !rq_noidle(rq);
> +			/*
> +			 * Idling is enabled for SYNC_WORKLOAD.
> +			 * SYNC_NOIDLE_WORKLOAD idles at the end of the tree
> +			 * only if we processed at least one !rq_noidle request
> +			 */
> +			if (cfqd->serving_type == SYNC_WORKLOAD
> +			    || cfqd->noidle_tree_requires_idle)
> +				cfq_arm_slice_timer(cfqd);
> +		}
>  	}
>  
>  	if (!rq_in_driver(cfqd))
> -- 
> 1.6.2.5
> 
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