linux-kernel - Re: [PATCH 02/20] blkio: Change CFQ to use CFS like queue time stamps

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Date:	Wed, 11 Nov 2009 08:48:09 +0800
From:	Gui Jianfeng <guijianfeng@...fujitsu.com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	linux-kernel@...r.kernel.org, jens.axboe@...cle.com,
	nauman@...gle.com, dpshah@...gle.com, lizf@...fujitsu.com,
	ryov@...inux.co.jp, fernando@....ntt.co.jp, s-uchida@...jp.nec.com,
	taka@...inux.co.jp, jmoyer@...hat.com, balbir@...ux.vnet.ibm.com,
	righi.andrea@...il.com, m-ikeda@...jp.nec.com,
	akpm@...ux-foundation.org, riel@...hat.com,
	kamezawa.hiroyu@...fujitsu.com
Subject: Re: [PATCH 02/20] blkio: Change CFQ to use CFS like queue time stamps

Vivek Goyal wrote:
...
>  
> @@ -1245,10 +1429,10 @@ static int cfq_forced_dispatch(struct cfq_data *cfqd)
>  	struct cfq_queue *cfqq;
>  	int dispatched = 0;
>  
> -	while ((cfqq = cfq_rb_first(&cfqd->service_tree)) != NULL)
> +	while ((cfqq = cfq_get_next_queue(cfqd)) != NULL)
>  		dispatched += __cfq_forced_dispatch_cfqq(cfqq);
>  
> -	cfq_slice_expired(cfqd, 0);
> +	cfq_slice_expired(cfqd);
>  
>  	BUG_ON(cfqd->busy_queues);
>  
> @@ -1391,7 +1575,7 @@ static int cfq_dispatch_requests(struct request_queue *q, int force)
>  	    cfqq->slice_dispatch >= cfq_prio_to_maxrq(cfqd, cfqq)) ||
>  	    cfq_class_idle(cfqq))) {
>  		cfqq->slice_end = jiffies + 1;
> -		cfq_slice_expired(cfqd, 0);
> +		cfq_slice_expired(cfqd);

Hi Vivek,

I think here you should make sure that when updating cfqq->slice_end, cfqq->slice_end doesn't 
equal to 0. Because if cfqq->slice_end == 0, cfq_slice_expired() just charge for 1 jiffy, but
if cfqq->slice_end is updated when it equals to 0(first request still in the air), at that time
cfqq->slice_start == 0, and slice_used is charged as "jiffies - cfqq->slice_start". Following
patch fixes this bug.

Signed-off-by: Gui Jianfeng <guijianfeng@...fujitsu.com>
---
 block/cfq-iosched.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index f23d713..12afc14 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1999,7 +1999,8 @@ static int cfq_dispatch_requests(struct request_queue *q, int force)
 	if (cfqd->busy_queues > 1 && ((!cfq_cfqq_sync(cfqq) &&
 	    cfqq->slice_dispatch >= cfq_prio_to_maxrq(cfqd, cfqq)) ||
 	    cfq_class_idle(cfqq))) {
-		cfqq->slice_end = jiffies + 1;
+		if (cfqq->slice_end)
+			cfqq->slice_end = jiffies + 1;
 		cfq_slice_expired(cfqd);
 	}
 
-- 
1.5.4.rc3

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