linux-kernel - Re: [PATCH 5/6 v4] cfq-iosched: CFQ group hierarchical scheduling and use

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Date:	Wed, 16 Feb 2011 09:44:39 +0800
From:	Gui Jianfeng <guijianfeng@...fujitsu.com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	Jens Axboe <axboe@...nel.dk>, Shaohua Li <shaohua.li@...el.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Chad Talbott <ctalbott@...gle.com>,
	Divyesh Shah <dpshah@...gle.com>
Subject: Re: [PATCH 5/6 v4] cfq-iosched: CFQ group hierarchical scheduling
 and use_hierarchy interface

Vivek Goyal wrote:
> On Tue, Feb 15, 2011 at 10:38:32AM +0800, Gui Jianfeng wrote:
>> Vivek Goyal wrote:
>>> On Sat, Feb 12, 2011 at 10:21:47AM +0800, Gui Jianfeng wrote:
>>> [..]
>>>>>> +static struct cfq_group *
>>>>>> +cfq_find_alloc_cfqg(struct cfq_data *cfqd, struct cgroup *cgroup, int create)
>>>>>> +{
>>>>>> +	struct blkio_cgroup *blkcg = cgroup_to_blkio_cgroup(cgroup);
>>>>>> +	struct cfq_group *cfqg = NULL;
>>>>>> +	void *key = cfqd;
>>>>>> +	struct backing_dev_info *bdi = &cfqd->queue->backing_dev_info;
>>>>>> +	unsigned int major, minor;
>>>>>> +
>>>>>> +	cfqg = cfqg_of_blkg(blkiocg_lookup_group(blkcg, key));
>>>>>> +	if (cfqg && !cfqg->blkg.dev && bdi->dev && dev_name(bdi->dev)) {
>>>>>> +		sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
>>>>>> +		cfqg->blkg.dev = MKDEV(major, minor);
>>>>>> +		goto done;
>>>>>> +	}
>>>>> Should we make this updation of this info hierarhical?
>>>> IMHO, it's fine to defer the updation when we really get the cfqg.
>>> But if cfqg is alrady present, we will never hit the allocation path 
>>> again. So if somebody creates 2-3 level deep hierarchy and does IO
>>> only in the children cgroup, parent cgroups will potentially not get
>>> device info updated hence no visible stats?
>> Ahh, I see your concern. But do we really need to show the stats even if
>> a cgroup doesn't issue any IO on a given device? 
> 
> I am assuming that once use_hierarchy=1, you are aggregating the stats
> in parent cgroups? So if a child services 5 IOs, these are accounted
> to parent group also when user_hier=1?
> 
> What happens in case of  memoy cgroup controller?

Hmm, it seems memcg aggregating stats in parent group.
But do we really need to do that in kernel? I think it's easier to do it in
userland, and it makes kernel much simpler.

Thanks,
Gui

> 
> Thanks
> Vivek
> 
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