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    SubjectRe: [PATCH] sched: Don't allow priority switch to realtime when the task doesn't belong to init_task_group and when CONFIG_RT_GROUP_SCHED isn't set
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    On Mon, 2008-11-24 at 09:28 +0530, Bharata B Rao wrote:
    > On Sun, Nov 23, 2008 at 02:11:16AM +0100, Peter Zijlstra wrote:
    > > On Thu, 2008-11-20 at 11:48 +0530, Bharata B Rao wrote:
    > > > Applies on 2.6.28-rc5.
    > > >
    > > > With CONFIG_RT_GROUP_SCHED not set, don't allow a task's priority switch
    > > > to realtime if the task isn't part of init_task_group.
    > > >
    > > > A task belonging to a fair group could use sched_setscheduler/sched_setparam
    > > > to become a realtime task. If such a task belongs to one of the
    > > > child groups of init_task_group and if CONFIG_RT_GROUP_SCHED is not set,
    > > > then it ends up getting queued in init_task_group's runqueue.
    > > > So we have a situation where, a task belongs to one group (child)
    > > > but ends in the runqueue of another group (init_task_group).
    > > > This does not look correct.
    > > >
    > > > Fix this by failing such priority change requests in sched_setscheduler()
    > > > and sched_setparam().
    > >
    > > NAK
    > >
    > > !RT_GROUP means the RT tasks should be fully invariant to any grouping
    > > configuration. This patch breaks that.
    > >
    > > Furthermore your justification for this is plain wrong, you write as if
    > > an RT tasks can belong to any grouping (in the !RT_GROUP case), by the
    >
    > Hmm... but that's I the impression I got when I started an RT
    > task in a subgroup and saw it listed in the tasks list of the cgroup.
    > So if RT tasks don't care about grouping (!RT_GROUP_SCHED), may be then
    > such tasks shoudn't be shown in the cgroup tasks list.

    Sure, do as you like upon the cgroup interface

    > And may be as Ingo
    > suggested they should be moved to init_task_group.

    Bzzt, wrong! They should not be moved to any group, they cannot be moved
    to any group, as they are group invariant.

    > Without that it makes one
    > think that a task is contributing to the cpu time of a group in which
    > it was started, but it in reality it is not.

    Until one thinks more and realizes RT tasks don't contribute anything to
    groups as they are not part of them.



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