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    SubjectRe: [PATCH] sched/rt: fix SCHED_RR across cgroups
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    On Sat, May 19, 2012 at 6:11 AM, Dario Faggioli <raistlin@linux.it> wrote:
    > On Fri, 2012-05-18 at 17:13 -0700, Colin Cross wrote:
    >> Even with this patch, scheduling of SCHED_RR tasks in cgroups is a
    >> little odd.  Each cgroup is treated as a schedulable entity alongside
    >> the tasks in the same parent cgroup, and then the tasks inside the
    >> child cgroup round robin through the child cgroup's time slices.  So
    >> in the setup:
    >> root_cgroup
    >>    task 1
    >>    cgroup
    >>       task 2
    >>       task 3
    >>
    >> The RR will be:
    >> task 1, cgroup(task 2), task 1, cgroup(task 3), ...
    >>
    >> task 1 will run twice as often, for a full RR_TIMESLICE each time, as
    >> tasks 2 and 3.
    >>
    > That looks right to me...
    >
    >> Is that the way SCHED_RR is intended to interact with cgroups?
    >>
    > I would say it is. That's what you get because of putting task1 and
    > cgroup at the same level in the "hierarchy". I'm curious, what kind of
    > behaviour were you expecting?

    That behavior matches exactly with scheduling of normal tasks and
    cgroups with default cpu.shares, but doesn't match too well with what
    I can see of the posix SCHED_RR description, which suggests all the
    SCHED_RR threads go into a single queue. I was just curious if the
    behavior my patch restored was correct, since it can't be adjusted by
    tweaking any parameters like cpu.shares.

    > Of course, the actual schedule also depends on the real-time priority of
    > the various tasks (groups don't have a priority, they inherit it from
    > their tasks, or at least it was like this when I used to work with
    > it :-P), but I guess you're putting all the tasks in the same queue
    > (i.e., same rt-prio), is it that the case?

    Yes.
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