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    SubjectRe: floppy.c soft lockup
    On Wed, 06 Jun 2007 09:12:04 -0400 Mark Hounschell <markh@compro.net> wrote:

    > >
    > > As far as a 100% CPU bound task being a valid thing to do, it has been
    > > done for many years on SMP machines. Any kernel limitation on this
    > > surely must be considered a bug?
    > >
    >
    > Could someone authoritatively comment on this? Is a SCHED_RR/SCHED_FIFO
    > 100% Cpu bound process supported in an SMP env on Linux? (vanilla or -rt)

    It will kill the kernel, sorry.

    The only way in which we can fix that is to allow kernel threads to preempt
    rt-priority userspace threads. But if we were to do that (to benefit the
    few) it would cause _all_ people's rt-prio processes to experience glitches
    due to kernel activity, which we believe to be worse.

    So we're between a rock and a hard place here.

    If we really did want to solve this then I guess the kernel would need some
    new code to detect a 100%-busy rt-prio process and to then start premitting
    preemption of it for kernel thread activity. That detector would need to
    be smart enough to detect a number of 100%-busy rt-prio processes which are
    yielding to each other, and one rt-prio process which keeps forking others,
    etc. It might get tricky.
    -
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