Re: [PATCH 5/6 v4] cfq-iosched: CFQ group hierarchical scheduling and use_hierarchy interface

[Gui Jianfeng <guijianfeng@cn.fujitsu.com>]

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? 
For example, on valinna kernel, when we create a new cgroup, we check the
stats, say blkio.io_service_bytes, we will get "Total 0", and no disk
specific stats.
Currently, As soon as a cgroup issue one IO request, cfqg->blkg.dev will
be updated.

Thanks,
Gui

> 
> Thanks
> Vivek
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