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    SubjectRe: [patch] Re: autogroup: sched_setscheduler() fails
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    On Wed, 2011-01-12 at 09:07 +0530, Bharata B Rao wrote:
    > On Tue, Jan 11, 2011 at 07:48:06PM +0100, Mike Galbraith wrote:
    > > On Tue, 2011-01-11 at 22:40 +0530, Bharata B Rao wrote:
    > > > On Mon, Jan 10, 2011 at 05:42:26PM +0100, Mike Galbraith wrote:
    > > > > Index: linux-2.6/kernel/sched_autogroup.c
    > > > > ===================================================================
    > > > > --- linux-2.6.orig/kernel/sched_autogroup.c
    > > > > +++ linux-2.6/kernel/sched_autogroup.c
    > > > > @@ -73,6 +73,15 @@ static inline struct autogroup *autogrou
    > > > > ag->id = atomic_inc_return(&autogroup_seq_nr);
    > > > > ag->tg = tg;
    > > > > tg->autogroup = ag;
    > > > > +#ifdef CONFIG_RT_GROUP_SCHED
    > > > > + /*
    > > > > + * HACK: autogroup RT tasks run in the root task group.
    > > > > + * This fools __sched_setscheduler() into proceeding on
    > > > > + * so we can move the task to the appropriate runqueue
    > > > > + * upon scheduling policy change.
    > > > > + */
    > > > > + tg->rt_bandwidth.rt_runtime = RUNTIME_INF;
    > > > > +#endif
    > > > >
    > > > > return ag;
    > > > >
    > > > > @@ -143,6 +152,15 @@ autogroup_move_group(struct task_struct
    > > > > autogroup_kref_put(prev);
    > > > > }
    > > > >
    > > > > +static inline void
    > > > > +autogroup_setscheduler(struct task_struct *p, int on_rq)
    > > > > +{
    > > > > + if (p->sched_class->task_move_group)
    > > > > + p->sched_class->task_move_group(p, on_rq);
    > > > > + else
    > > > > + set_task_rq(p, task_cpu(p));
    > > > > +}
    > > > > +
    > > >
    > > > IIUC, with the above changes you are actually queing the task into
    > > > rt_rq of an autogroup. But the task's autogroup interface
    > > > (/proc/<pid>/autogroup) allows you to control the bandwidth of only
    > > > cfs_rq tasks, while the rt tasks in the group get RUNTIME_INF bandwidth.
    > > >
    > > > I think what we need is a real group change here (which is difficult I
    > > > think) and not just sched_class change.
    > >
    > > Unless I fscked up, set_task_rq() is the group change. As soon as the
    > > task's class changes, it'll be moved to the root_task_group.
    >
    > Ok, this is what I understand, may be Peter Z can confirm...
    >
    > set_task_rq() just changes the task's cfs_rq and rt_rq as per its
    > task_group().

    Exactly. When you call task_group() for an RT task, it returns
    &root_task_group, so a task making the transition to RT will be dequeued
    from it's autogroup cfs_rq, and enqueued in root_task_group's rt_rq.

    > The normal way to change a task's group is to first
    > change its cgroup pointer (task->cgroups, see cgroup_attach_task())
    > After this you change the runqueues by calling set_task_rq() which
    > now refers to new group's runqueues.

    Autogroup doesn't need to do cgroup stuff. If a task is in a cgroup,
    it's autogroup remains intact, but is ignored. Move it back to root,
    and it's autogroup will be used again.

    -Mike



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