Re: Xterm Hangs - Possible scheduler defect?

From: Chad N. Tindel (chad_at_tindel.net)
Date: 02/24/05

  • Next message: Greg KH: "Re: [PATCH] CKRM: 3/10 CKRM: Core ckrm, rcfs" 
    Date:	Thu, 24 Feb 2005 12:53:31 -0500
To: Mike Galbraith <EFAULT@gmx.de>

    > > Hmmm... Are you suggesting it is OK for a kernel to get nearly completely
    > > hosed and for not fully utilize all the processors in the system because
    > > of one SCHED_FIFO thread?
    >
    > Sure. You specifically directed the scheduler to run your thread at a
    > higher priority than anything else. The way I see it, you used root's
    > perogative to shoot himself in the foot. You could also have used root's
    > perogative to don steel toed shoes(set important kernel threads to a higher
    > priority) before pulling the trigger.

    No, I specifically directed the scheduler to run my thread at a higher
    priority than any other userspace application. The fact that I wrote it
    in userspace and not in kernel space implies that I am OK with the kernel
    stopping me sometimes when _it_ has work to do. If I wanted something
    higher priority than the kernel I would have written something in kernel
    space instead.

    > SCHED_FIFO thread are supposed to preempt
    > > all other userspace threads... not the kernel itself.
    >
    > Not so. The scheduler makes do distinction between user and kernel threads
    > of execution.

    That is SOOOO broken it isn't even funny.

    > If you think that's broken, you'll _love_ Ingo's IRQ threads. While testing
    > one of his recent (slightly buggy)unpriveleged-user-does-RT patches in an
    > IRQ threadified kernel, I ran a user SCHED_FIFO task at higher than the IRQ0
    > thread... if my box had been an embeded washing machine controller instead
    > of a desktop box, it'd have been a classic case of "No tickie no washie" :))

    Yeah, thats broken too.

    Perhaps I don't understand this philosophy you have where the kernel
    isn't more important than everything else. It seems to me like there needs
    to be a rigid hierarchy for scheduling, lest you get into deadlock problems:

    1. Kernel preempts all. There may be some hierarchy of kernel priorities
    too, but it isn't important here.
    2. SCHED_FIFO processes preempt all userspace applications.
    3. SCHED_RR.
    4. SCHED_OTHER.

    Under no circumstances should any single CPU-bound userspace thread completely
    hose a 64-way SMP box.

    Can somebody educate me on why it is correct to do it any other way?

    Chad
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  • Next message: Greg KH: "Re: [PATCH] CKRM: 3/10 CKRM: Core ckrm, rcfs"

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