• Roman Gushchin's avatar
    cgroup: cgroup v2 freezer · 76f969e8
    Roman Gushchin authored
    Cgroup v1 implements the freezer controller, which provides an ability
    to stop the workload in a cgroup and temporarily free up some
    resources (cpu, io, network bandwidth and, potentially, memory)
    for some other tasks. Cgroup v2 lacks this functionality.
    This patch implements freezer for cgroup v2.
    Cgroup v2 freezer tries to put tasks into a state similar to jobctl
    stop. This means that tasks can be killed, ptraced (using
    PTRACE_SEIZE*), and interrupted. It is possible to attach to
    a frozen task, get some information (e.g. read registers) and detach.
    It's also possible to migrate a frozen tasks to another cgroup.
    This differs cgroup v2 freezer from cgroup v1 freezer, which mostly
    tried to imitate the system-wide freezer. However uninterruptible
    sleep is fine when all tasks are going to be frozen (hibernation case),
    it's not the acceptable state for some subset of the system.
    Cgroup v2 freezer is not supporting freezing kthreads.
    If a non-root cgroup contains kthread, the cgroup still can be frozen,
    but the kthread will remain running, the cgroup will be shown
    as non-frozen, and the notification will not be delivered.
    * PTRACE_ATTACH is not working because non-fatal signal delivery
    is blocked in frozen state.
    There are some interface differences between cgroup v1 and cgroup v2
    freezer too, which are required to conform the cgroup v2 interface
    design principles:
    1) There is no separate controller, which has to be turned on:
    the functionality is always available and is represented by
    cgroup.freeze and cgroup.events cgroup control files.
    2) The desired state is defined by the cgroup.freeze control file.
    Any hierarchical configuration is allowed.
    3) The interface is asynchronous. The actual state is available
    using cgroup.events control file ("frozen" field). There are no
    dedicated transitional states.
    4) It's allowed to make any changes with the cgroup hierarchy
    (create new cgroups, remove old cgroups, move tasks between cgroups)
    no matter if some cgroups are frozen.
    Signed-off-by: default avatarRoman Gushchin <guro@fb.com>
    Signed-off-by: default avatarTejun Heo <tj@kernel.org>
    No-objection-from-me-by: default avatarOleg Nesterov <oleg@redhat.com>
    Cc: kernel-team@fb.com