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