• Matt Helsley's avatar
    container freezer: implement freezer cgroup subsystem · dc52ddc0
    Matt Helsley authored
    This patch implements a new freezer subsystem in the control groups
    framework.  It provides a way to stop and resume execution of all tasks in
    a cgroup by writing in the cgroup filesystem.
    The freezer subsystem in the container filesystem defines a file named
    freezer.state.  Writing "FROZEN" to the state file will freeze all tasks
    in the cgroup.  Subsequently writing "RUNNING" will unfreeze the tasks in
    the cgroup.  Reading will return the current state.
    * Examples of usage :
       # mkdir /containers/freezer
       # mount -t cgroup -ofreezer freezer  /containers
       # mkdir /containers/0
       # echo $some_pid > /containers/0/tasks
    to get status of the freezer subsystem :
       # cat /containers/0/freezer.state
    to freeze all tasks in the container :
       # echo FROZEN > /containers/0/freezer.state
       # cat /containers/0/freezer.state
       # cat /containers/0/freezer.state
    to unfreeze all tasks in the container :
       # echo RUNNING > /containers/0/freezer.state
       # cat /containers/0/freezer.state
    This is the basic mechanism which should do the right thing for user space
    task in a simple scenario.
    It's important to note that freezing can be incomplete.  In that case we
    return EBUSY.  This means that some tasks in the cgroup are busy doing
    something that prevents us from completely freezing the cgroup at this
    time.  After EBUSY, the cgroup will remain partially frozen -- reflected
    by freezer.state reporting "FREEZING" when read.  The state will remain
    "FREEZING" until one of these things happens:
    	1) Userspace cancels the freezing operation by writing "RUNNING" to
    		the freezer.state file
    	2) Userspace retries the freezing operation by writing "FROZEN" to
    		the freezer.state file (writing "FREEZING" is not legal
    		and returns EIO)
    	3) The tasks that blocked the cgroup from entering the "FROZEN"
    		state disappear from the cgroup's set of tasks.
    [akpm@linux-foundation.org: coding-style fixes]
    [akpm@linux-foundation.org: export thaw_process]
    Signed-off-by: default avatarCedric Le Goater <clg@fr.ibm.com>
    Signed-off-by: default avatarMatt Helsley <matthltc@us.ibm.com>
    Acked-by: default avatarSerge E. Hallyn <serue@us.ibm.com>
    Tested-by: default avatarMatt Helsley <matthltc@us.ibm.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
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