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    SubjectRe: Bug in scheduler when using rt_mutex
    On Thu, Jan 20, 2011 at 11:07:27AM +0100, Mike Galbraith wrote:
    > On Thu, 2011-01-20 at 17:07 +0800, Yong Zhang wrote:
    > > On Thu, Jan 20, 2011 at 4:37 PM, Mike Galbraith <efault@gmx.de> wrote:
    > > > On Thu, 2011-01-20 at 15:06 +0800, Yong Zhang wrote:
    > > >> On Thu, Jan 20, 2011 at 2:12 PM, Mike Galbraith <efault@gmx.de> wrote:
    > > >> > If the task returns as a sleeper, place entity() will be called when it
    > > >> > is awakened, so it's sleep credit will be clipped as usual. So vruntime
    > > >> > can be much less than min_vruntime at class exit time, and it doesn't
    > > >> > matter, clipping on wakeup after re-entry takes care of it.. if that's
    > > >> > what you were thinking about.
    > > >>
    > > >> For a sleep task which stay in sched_fair before it's waked:
    > > >> try_to_wake_up()
    > > >> ttwu_activate()
    > > >> activate_task()
    > > >> enqueue_task_fair()
    > > >> enqueue_entity()
    > > >> place_entity() <== clip vruntime
    > > >>
    > > >> For a sleep task which promote to sched_rt when it's sleep:
    > > >> rt_mutex_setprio()
    > > >> check_class_changed()
    > > >> switch_from_fair() <== vruntime -= min_vruntime
    > > >> try_to_wake_up()
    > > >> ...run then stay on rq
    > > >> rt_mutex_setprio()
    > > >> enqueue_task_fair() <==vruntime += min_vruntime
    > > >>
    > > >> The difference is that in the second case, place_entity() is not
    > > >> called, but wrt sched_fair, the task is a WAKEUP task.
    > > >> Then we place this task in sched_fair before where it should be.
    > > >
    > > > D'oh. You're right, he needs to be clipped before he leaves.
    > >
    > > Exactly we should clip it when it comes back, because it still could
    > > sleep for some time after it leaves ;)
    >
    > That's ok, we don't and aren't supposed to care what happens while he's
    > gone. But we do have to make sure that vruntime is sane either when he
    > leaves, or when he comes back. Seems to me the easiest is clip when he
    > leaves to cover him having slept a long time before leaving, then coming
    > back on us as a runner. If he comes back as a sleeper, he'll be clipped
    > again anyway, so all is well.

    OK, fair enough.

    >
    > sched_fork() should probably zero child's vruntime too, so non-fair
    > children can't enter fair_class with some bogus lag they never had.

    Yeah,
    So I think Peter's new patch is taking care of all the issue now.

    Thanks,
    Yong


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