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    SubjectRe: [Devel] [RFC PATCH 0/4] Container Freezer: Reuse Suspend Freezer


    Matt Helsley wrote:
    > On Thu, 2008-04-03 at 16:49 -0700, Paul Menage wrote:
    >> On Thu, Apr 3, 2008 at 2:03 PM, <matthltc@us.ibm.com> wrote:
    >>> * "freezer.kill"
    >>>
    >>> writing <n> will send signal number <n> to all tasks
    >>>
    >> My first thought (not having looked at the code yet) is that sending a
    >> signal doesn't really have anything to do with freezing, so it
    >> shouldn't be in the same subsystem. Maybe a separate subsystem called
    >> "signal"?
    >>
    >> And more than that, it's not something that requires any particular
    >> per-process state, so there's no reason that the subsystem that
    >> provides the "kill" functionality shouldn't be able to be mounted in
    >> multiple hierarchies.
    >>
    >> How about if I added support for stateless subsystems, that could
    >> potentially be mounted in multiple hierarchies at once? They wouldn't
    >> need an entry in the css set, since they have no state.
    >
    > This seems reasonable to me. A quick look at Cedric's patches suggests
    > there's no need for such cgroup subsystems to be tied together -- the
    > signalling is all done internally to the freeze_task(), refrigerator(),
    > and thaw_process() functions from what I recall.
    >
    >>> * Usage :
    >>>
    >>> # mkdir /containers/freezer
    >>> # mount -t container -ofreezer freezer /containers/freezer
    >>> # mkdir /containers/freezer/0
    >>> # echo $some_pid > /containers/freezer/0/tasks
    >>>
    >>> to get status of the freezer subsystem :
    >>>
    >>> # cat /containers/freezer/0/freezer.freeze
    >>> RUNNING
    >>>
    >>> to freeze all tasks in the container :
    >>>
    >>> # echo 1 > /containers/freezer/0/freezer.freeze
    >>> # cat /containers/freezer/0/freezer.freeze
    >>> FREEZING
    >>> # cat /containers/freezer/0/freezer.freeze
    >>> FROZEN
    >> Could we separate this out into two files? One called "freeze" that's
    >> a 0/1 for whether we're intending to freeze the subsystem, and one
    >> called "frozen" that indicates whether it is frozen? And maybe a
    >> "state" file to report the RUNNING/FREEZING/FROZEN distinction in a
    >> human-readable way?
    >
    > 3 files seems like overkill. I think making them human-readable is good
    > and can be done with two files: "state" (read-only) and
    > "state-next" (read/write). Transitions between RUNNING and FROZEN are
    > obvious when state-next != state. I think the advantages are it's pretty
    > human-readable, you don't need separate strings and files for the
    > transitions, it's clear what's about to happen (IMHO), and it only
    > requires 2 files. Some examples:
    >
    > To initiate freezing:
    >
    > # cat /containers/freezer/0/freezer.state
    > RUNNING
    > # echo "FROZEN" > /containers/freezer/0/freezer.state-next
    > # cat /containers/freezer/0/freezer.state
    > RUNNING
    > # cat /containers/freezer/0/freezer.state-next
    > FROZEN
    > # sleep N
    > # cat /containers/freezer/0/freezer.state
    > FROZEN
    > # cat /containers/freezer/0/freezer.state-next
    > FROZEN
    >
    > So to cancel freezing you might see something like:
    >
    > # cat /containers/freezer/0/freezer.state
    > RUNNING
    > # cat /containers/freezer/0/freezer.state-next
    > FROZEN
    > # echo "RUNNING" > /containers/freezer/0/freezer.state-next
    > # cat /containers/freezer/0/freezer.state-next
    > RUNNING
    >
    > If you wanted to know if a group was transitioning:
    >
    > # diff /containers/freezer/0/freezer.state /containers/freezer/0/freezer.state-next
    >
    > Or:
    > # current=`cat /containers/freezer/0/freezer.state`
    > # next=`cat /containers/freezer/0/freezer.state-next`
    > # [ "$current" != "$next" ] && echo "Transitioning"
    > # [ "$current" == "RUNNING" -a "$next" == "FROZEN" ] && echo "Freezing"
    > # [ "$current" == "FROZEN" -a "$next" == "RUNNING" ] && echo "Thawing"
    > # [ "$current" == "RUNNING" -a "$next" == "RUNNING" ] && echo "No-op"
    > # [ "$current" == "FROZEN" -a "$next" == "FROZEN" ] && echo "No-op"

    First, I totally agree with Serge's comment (oh well, it's about my
    own suggestion, so I must) - for checkpoint/restart we'll need more
    states if we are to use the same subsystem.

    Second, my gut feeling is that a single, atomic operation to get the
    status is preferred over multiple (non-atomic) operations. In other
    words, I suggest a single state file instead of two. You can encode
    every possible transition in a single state. It's not that the kernel
    doesn't know what's going on inside, so it can just as well report it
    directly. I don't see the benefit of using two files.

    Oren.

    >
    > etc.
    >
    > Cheers,
    > -Matt Helsley
    >
    > _______________________________________________
    > Containers mailing list
    > Containers@lists.linux-foundation.org
    > https://lists.linux-foundation.org/mailman/listinfo/containers


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