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SubjectRe: [PATCH 1/3] Separate IRQ-stacks from 4K-stacks option
--Zwane Mwaikambo <zwane@linuxpower.ca> wrote (on Sunday, September 12, 2004 11:45:19 -0400):

> yOn Sun, 12 Sep 2004, Martin J. Bligh wrote:
>
>> --Andrea Arcangeli <andrea@novell.com> wrote (on Sunday, September 12, 2004 16:17:01 +0200):
>>
>> > On Fri, Sep 10, 2004 at 05:34:21PM +0200, Arjan van de Ven wrote:
>> >> disabling is actually not a bad idea; hard irq handlers run for a very short
>> >
>> > you mean hard irq handlers "should run" for a very short time. There can
>> > be slow hardware that needs a long time, and fast hardware that needs a
>> > short time, and in turn it makes perfect sense to allow nesting to give
>> > low latency to the "fast" onces, like it has always happened so far (not
>> > only in linux AFIK). Disabling nesting completely sounds a very bad
>> > idea to me, when "limiting nesting" can be achieved easily as confirmed
>> > by Alan too.
>>
>> IIRC, what we did in PTX was have 16 SPL levels, each interrupt was assigned
>> a prio, and higher prio interrupts could interrupt lower prio ones (but not
>> the same prio or higher). There's some support for that in the APIC, I think,
>> something like the high nybble is prio, and the low nybble is just an index.
>
> Currently we do use priorities on i386/APIC, albeit unintentionally by
> assigning higher IRQs higher vectors resulting in a higher priority.
> However interrupt priorities on non deterministic general purpose
> operating systems seems pointless for the vast majority of the devices
> plugged into boxes these days. Not to mention possible starvation issues
> from high frequency long running interrupts.

Well, the idea is that long running interrupts get low prios ;-)

But yes, we're fairly non-deterministic, apart from the timer interrupt, etc
which are at one end of the scale, though I forget which ;-)

M.

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