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    SubjectRe: realtime scheduling problems with 2.4 linux kernel >= 2.4.10
    On Sat, Jun 01, 2002 at 07:05:47PM +0200, Andi Kleen wrote:
    > Mike Kravetz <kravetz@us.ibm.com> writes:
    >
    > > This works fine for me on 2.4.17 with a SERIAL console. Could this
    > > be related to some differences (new features) in the VGA console?
    > > I am totally ignorant of how the consoles work.
    >
    > One possibility is that something relies on schedule_task() - keventd
    > doesn't run with realtime priority and can be starved.
    >
    > Seems to be the case indeed:
    >
    > /usr/src/linux/drivers/char% grep schedule_task *.c
    > console.c: schedule_task(&console_callback_tq);
    > ...
    >
    > the console switch does.

    Thanks Andi!

    Part of the 'problem' is the following in the 'sched_setscheduler'
    man page.

    " As a non-blocking end-less loop in a process scheduled
    under SCHED_FIFO or SCHED_RR will block all processes with
    lower priority forever, a software developer should always
    keep available on the console a shell scheduled under a
    higher static priority than the tested application. This
    will allow an emergency kill of tested real-time applica­
    tions that do not block or terminate as expected. As
    SCHED_FIFO and SCHED_RR processes can preempt other pro­
    cesses forever, only root processes are allowed to acti­
    vate these policies under Linux.
    "

    Seems that this tells people to leave a high priority real-
    time shell running on the console. However, if one can not
    get to the console, then there is no point in leaving a high
    priority shell running there. Part of the problem may be
    in the definition of 'console'. Different console implementations
    behave differently.

    Is this something we should 'fix'? I would envision a 'solution'
    for each console implementation. OR we could remove the above
    from the man page. :)

    Comments?
    --
    Mike
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