Re: [PATCH 1/3] Separate IRQ-stacks from 4K-stacks option

[Zwane Mwaikambo]

yOn Sun, 12 Sep 2004, Martin J. Bligh wrote:

> --Andrea Arcangeli <andrea@xxxxxxxxxx> 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.

	Zwane

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