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

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Message-ID: <20091112230736.GD2936@redhat.com>
Date:	Thu, 12 Nov 2009 18:07:36 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Gui Jianfeng <guijianfeng@...fujitsu.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

On Wed, Nov 11, 2009 at 08:48:09AM +0800, Gui Jianfeng wrote:
> 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.
> 

Hi Gui,

This can happen only once during a one wrap around cycle of jiffies. That
too depends in case we are hitting jiffies+1 as 0 or not.

So I would not worry much about it right now.

In fact, not updating slice_end, will make idle or async queue slice last
much longer than it should have.

Thanks
Vivek

 
> 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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