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    SubjectRe: new IRQ scalability changes in 2.3.48
    On Sun, Mar 12, 2000 at 09:53:29AM -0800, Linus Torvalds wrote:
    > Doing an __sti in interrupts will let higher-priority interrupts in. As it
    > always has.

    Previously, _any_ interrupt was let to progress. That is, hardware
    priorities were ignored by Linux.

    > I think it's probably stupid most of the time, and shows that something is
    > wrong (an interrupt should not take long enough for it to matter that we
    > do an __sti), but sometimes that kind of stupidity is a result of horrible
    > hardware latencies (eg the time it takes to read a sector from the IDE
    > disk using programmed IO).

    So if we have a hardware determined higher priority level IDE than level
    network, then it previously was the case that if the IDE driver was polite
    and did an __sti, the network was assured of a relatively low latency. However,
    the network now may need to wait until the programmed i/o completes.
    Probably programmed i/o should rather happen in soft_interrupt space, but
    with the current drivers it does not.

    > Quite regardless of how you do interrupts: it doesn't matter where youput
    > the ACK's, you always need to make sure that irq masking etc is correct,
    > and you must NOT allow a context switch while an interrupt handler is
    > still running.

    Ok. Ingo: does your "low latency" patch violate this rule?

    > You're making these problems up.

    Yes. I thought that the hardware worked like 8259s, but it is somewhat
    more sensible.

    > > I really dislike hardware interrupt priorities since they have nothing
    > > to do with the OS' ideas of importance and are complex to work with.
    > I agree. I think interrupt controllers should be simple masks, nothing
    > more, nothing less.
    > However, I'm not designing the hardware.

    It's amazing how stubbornly hardware folks refuse to consider OS in their

    Victor Yodaiken
    FSMLabs is a servicemark and a service of
    VJY Associates L.L.C, New Mexico.

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