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    SubjectPreempt & how long it takes to interrupt (was Re: [2.4.17/18pre] VM and swap - it's really unusable)
    Hi!

    > > I preempt leads to disaster than Linux can't do SMP. Are you saying that's
    > > the case?
    >
    > There is a difference. Preempt have the same locking requirements as
    > SMP, but there's also _timing_ requirements.
    ...
    > > The preempt patch is really "SMP on UP". If pre-empt shows up a problem,
    > > then it's a problem SMP users will see too. If we can't take advantage of
    > > the existing SMP locking infrastructure to improve latency and interactive
    > > feel on UP machines, than SMP for linux DOES NOT WORK.
    >
    > One example where preempt may break and SMP does not:
    >
    > Consider driver code. Critical data structures is protected by
    > spinlocks,
    > but some of the access to the hardware device itself is outside those
    > locks (I can prove that the other processors can't get there with
    > the driver in that state anyway)
    >
    > Now, hardware access has timing requirements. That works on SMP because
    > you don't loose the CPU to anything but interrupts, and they are fast.
    > You get it back almost immediately. The device in question times out
    > after a much longer interval.

    So... how long do you have to stay in interrupt for it to be a bug?

    There's *no* requirement that says "it may not take second to handle
    an interrupt". Actually I guess that some nasty conditions (UHCI needs
    reset?) may take that long in interrupt. Oh and actually few releases
    ago, console switching was done from interrupt and it *did* take 2
    seconds for me.

    If someone assumes interrupts are "short", he has broken code already.

    Pavel
    --
    (about SSSCA) "I don't say this lightly. However, I really think that the U.S.
    no longer is classifiable as a democracy, but rather as a plutocracy." --hpa
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