Examining a kobject hierarchy

The core data structure in this investigation is the kobject. In the diagrams that follow, kobjects will be represented by the small symbol you see to the right. The upper rectangle represents the kobject's parent field, while the other two are its entries in the doubly-linked list that implements a kset. Not all kobjects belong to a kset, so those links will often be empty.

At the root of the block subsystem hierarchy is a subsystem called block_subsys; it is defined in drivers/block/genhd.c. As you'll recall from The Zen of Kobjects, a subsystem is a very simple structure, consisting of a semaphore and a kset. The kset will define, in its ktype field, what type of kobjects it will contain; for block_subsys, this field is set to ktype_block. Pictorially, we can show this structure as seen on the right.

Each kset contains its own kobject, and block_subsys is no exception. In this case, the kobject's parent field is explicitly set to NULL (indicated by the ground symbol in the picture). As a result, this kobject will be represented in the top level of the sysfs hierarchy; it is the kobject which lurks behind /sys/block.

A block subsystem is not very interesting without disks. In the block hierarchy, disks are defined by a struct gendisk, which can be found in <include/linux/genhd.h>. The gendisk interface is described in this article. For our purposes, we will represent a gendisk as seen on the left; note that it has the inevitable embedded kobject inside it. A gendisk's kobject does not have an explicit type pointer; its membership in the block_subsys kset takes care of that. But its parent and kset pointers both point to the kobject within block_subsys, and the kset pointers are there too. The result, for a system with two disks, would be a structure that looks like this:

Things do not end there, however; a gendisk structure is a complicated thing. It contains, among other things, an array of partition entries (of type struct hd_struct), each of which has embedded within it, yes, a kobject. The parent of each partition is the disk which contains it. It would have been possible to implement the list of partitions as a kset, but things weren't done that way. Partitions are a relatively static item, and their ordering matters, so they were done as a simple array. We depict that array as seen on the right.

As you can see, the kobject type of a partition is ktype_part. This type implements the attributes you will see in the sysfs entries for each partition, including the starting block number and size.

Another item associated with each gendisk is its I/O request queue. The queue, too, contains a kobject (of type queue_ktype) whose parent is the associated gendisk. The I/O scheduler ("elevator") in use with an I/O request queue is also represented in the hierarchy. The scheduler's kobject's type depends on which scheduler is being used; the (default) anticipatory scheduler uses as_ktype. The resulting piece of the puzzle looks as portrayed on the left.

The request queue and I/O scheduler information in sysfs is currently read-only. There is no reason, however, why sysfs attributes could not be used to change I/O scheduling parameters on the fly. The selectable I/O scheduler patch uses sysfs attributes to change I/O schedulers completely, for example.

Putting it all together

The data structure as described is the full implementation of the /sys/block subtree of sysfs. The full sysfs tree contains rather more than this, of course. For each gendisk which shows up under /sys/block, there will be a separate entry under /sys/devices which describes the underlying hardware. Internally, the link between the two is contained in the driverfs_dev field of the gendisk structure. In sysfs, that link is represented as a symbolic link between the two sub-trees.

Hopefully this series of pictures helps in the visualization of a portion of the sysfs tree and the device model data structure that implements it. The device model brings a great deal of apparent complexity, but, once the underlying concepts are grasped, the whole thing is approachable.