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    SubjectRe: [RFC/PATCH 01/17] sched: mix tasks and groups
    On Sun, Mar 09, 2008 at 06:08:51PM +0100, Peter Zijlstra wrote:
    > This patch allows tasks and groups to exist in the same cfs_rq. With this
    > change the CFS group scheduling follows a 1/(M+N) model from a 1/(1+N)
    > fairness model where M tasks and N groups exist at the cfs_rq level.
    >
    > [a.p.zijlstra@chello.nl: rt bits]
    > Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
    > Signed-off-by: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
    > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > ---
    > kernel/sched.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++--
    > kernel/sched_fair.c | 48 +++++++++++++++++++++++++++++++++++++++++++---
    > kernel/sched_rt.c | 15 ++++++++------
    > 3 files changed, 106 insertions(+), 11 deletions(-)
    >
    > Index: linux-2.6-2/kernel/sched.c
    > ===================================================================
    > --- linux-2.6-2.orig/kernel/sched.c
    > +++ linux-2.6-2/kernel/sched.c
    > @@ -273,18 +273,23 @@ struct task_group {
    > };
    >
    > #ifdef CONFIG_FAIR_GROUP_SCHED
    > +
    > +#ifdef CONFIG_USER_SCHED
    > /* Default task group's sched entity on each cpu */
    > static DEFINE_PER_CPU(struct sched_entity, init_sched_entity);
    > /* Default task group's cfs_rq on each cpu */
    > static DEFINE_PER_CPU(struct cfs_rq, init_cfs_rq) ____cacheline_aligned_in_smp;
    > +#endif


    I am trying to understand the change this brings for semantics of RT-scheduling.

    With this change, /cgroup will be seen as the parent group of all other
    groups (say: /cgroup/A, /cgroup/B etc). Is that correct?

    If so, the check in __rt_schedulable() needs a change as well, which assumes
    that all task groups form a flat hierarchy.

    For example: lets say that init_task_group (/cgroup in this case) had the
    default rt_bandwidth of 95% (global_rt_runtime()). A child group under it
    (/cgroup/A) is created. If user tries to assign it a rt-bandwidth of
    50%, then AFAICS, it will fail with current code, whereas it shouldn't
    (because by giving /cgroup/A 50% bandwidth, we are not really exceeding
    the globally allowed RT bandwidth of 95%, since /cgroup/A is a child of
    /cgroup).

    --
    Regards,
    vatsa


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