• Arianna Avanzini's avatar
    block, cgroup: implement policy-specific per-blkcg data · e48453c3
    Arianna Avanzini authored
    The block IO (blkio) controller enables the block layer to provide service
    guarantees in a hierarchical fashion. Specifically, service guarantees
    are provided by registered request-accounting policies. As of now, a
    proportional-share and a throttling policy are available. They are
    implemented, respectively, by the CFQ I/O scheduler and the blk-throttle
    subsystem. Unfortunately, as for adding new policies, the current
    implementation of the block IO controller is only halfway ready to allow
    new policies to be plugged in. This commit provides a solution to make
    the block IO controller fully ready to handle new policies.
    In what follows, we first describe briefly the current state, and then
    list the changes made by this commit.
    The throttling policy does not need any per-cgroup information to perform
    its task. In contrast, the proportional share policy uses, for each cgroup,
    both the weight assigned by the user to the cgroup, and a set of dynamically-
    computed weights, one for each device.
    The first, user-defined weight is stored in the blkcg data structure: the
    block IO controller allocates a private blkcg data structure for each
    cgroup in the blkio cgroups hierarchy (regardless of which policy is active).
    In other words, the block IO controller internally mirrors the blkio cgroups
    with private blkcg data structures.
    On the other hand, for each cgroup and device, the corresponding dynamically-
    computed weight is maintained in the following, different way. For each device,
    the block IO controller keeps a private blkcg_gq structure for each cgroup in
    blkio. In other words, block IO also keeps one private mirror copy of the blkio
    cgroups hierarchy for each device, made of blkcg_gq structures.
    Each blkcg_gq structure keeps per-policy information in a generic array of
    dynamically-allocated 'dedicated' data structures, one for each registered
    policy (so currently the array contains two elements). To be inserted into the
    generic array, each dedicated data structure embeds a generic blkg_policy_data
    structure. Consider now the array contained in the blkcg_gq structure
    corresponding to a given pair of cgroup and device: one of the elements
    of the array contains the dedicated data structure for the proportional-share
    policy, and this dedicated data structure contains the dynamically-computed
    weight for that pair of cgroup and device.
    The generic strategy adopted for storing per-policy data in blkcg_gq structures
    is already capable of handling new policies, whereas the one adopted with blkcg
    structures is not, because per-policy data are hard-coded in the blkcg
    structures themselves (currently only data related to the proportional-
    share policy).
    This commit addresses the above issues through the following changes:
    . It generalizes blkcg structures so that per-policy data are stored in the same
      way as in blkcg_gq structures.
      Specifically, it lets also the blkcg structure store per-policy data in a
      generic array of dynamically-allocated dedicated data structures. We will
      refer to these data structures as blkcg dedicated data structures, to
      distinguish them from the dedicated data structures inserted in the generic
      arrays kept by blkcg_gq structures.
      To allow blkcg dedicated data structures to be inserted in the generic array
      inside a blkcg structure, this commit also introduces a new blkcg_policy_data
      structure, which is the equivalent of blkg_policy_data for blkcg dedicated
      data structures.
    . It adds to the blkcg_policy structure, i.e., to the descriptor of a policy, a
      cpd_size field and a cpd_init field, to be initialized by the policy with,
      respectively, the size of the blkcg dedicated data structures, and the
      address of a constructor function for blkcg dedicated data structures.
    . It moves the CFQ-specific fields embedded in the blkcg data structure (i.e.,
      the fields related to the proportional-share policy), into a new blkcg
      dedicated data structure called cfq_group_data.
    Signed-off-by: default avatarPaolo Valente <paolo.valente@unimore.it>
    Signed-off-by: default avatarArianna Avanzini <avanzini.arianna@gmail.com>
    Acked-by: default avatarTejun Heo <tj@kernel.org>
    Cc: Jens Axboe <axboe@fb.com>
    Signed-off-by: default avatarJens Axboe <axboe@fb.com>
blk-cgroup.c 31.2 KB