linux-kernel - Re: [PATCH 5/8 v2] cfq-iosched: Introduce hierarchical scheduling with CFQ queue and group at the same level

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Message-ID: <4D070A1D.4060904@cn.fujitsu.com>
Date:	Tue, 14 Dec 2010 14:09:33 +0800
From:	Gui Jianfeng <guijianfeng@...fujitsu.com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	Jens Axboe <axboe@...nel.dk>, Corrado Zoccolo <czoccolo@...il.com>,
	Chad Talbott <ctalbott@...gle.com>,
	Nauman Rafique <nauman@...gle.com>,
	Divyesh Shah <dpshah@...gle.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/8 v2] cfq-iosched: Introduce hierarchical scheduling
 with CFQ queue and group at the same level

Vivek Goyal wrote:
...
>> -static struct cfq_group *cfq_get_next_cfqg(struct cfq_data *cfqd)
>> +static void choose_service_tree(struct cfq_data *cfqd, struct cfq_group *cfqg)
>>  {
>> -	struct cfq_rb_root *st = &cfqd->grp_service_tree;
>> -	struct cfq_group *cfqg;
>> -	struct cfq_entity *cfqe;
>> +	struct cfq_rb_root *st;
>> +	unsigned count;
>>  
>> -	if (RB_EMPTY_ROOT(&st->rb))
>> -		return NULL;
>> -	cfqe = cfq_rb_first_entity(st);
>> -	cfqg = cfqg_of_entity(cfqe);
>> -	BUG_ON(!cfqg);
>> -	update_min_vdisktime(st);
>> -	return cfqg;
>> +	if (!cfqg) {
>> +		cfqd->serving_prio = IDLE_WORKLOAD;
>> +		cfqd->workload_expires = jiffies + 1;
>> +		return;
>> +	}
> 
> I am wondering where do we use above code. Do we ever call
> choose_service_tree() with cfqg == NULL? Can't seem to figure out by
> looking at the code.
> 

Vivek,

This piece of code comes from original CFQ code. Think more about it, this
piece of code seems redundant. When cfq_choose_cfqg() is called in select_queue(),
there must be at least one backlogged CFQ queue waiting for dispatching, hence
there must be at least one backlogged CFQ group on service tree. So we never call
choose_service_tree() with cfqg == NULL.
I'd like to post a seperate patch to get rid of this piece.

Thanks,
Gui
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