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SubjectRe: [PATCH] local_irq_disable removal
On Sat, 2005-06-11 at 22:23 +0200, Esben Nielsen wrote:
> >
> No because it correctly leaves irqs on but not preemption on.

I see your worries now. See below.

> > > The worry is local_irq_disable() (and preempt_disable()). I can undefine
> > > it and therefore find where it is used. I can then look at the code, make
> > > it into raw_local_irq_disable() or try to make a lock.
> > > In many cases the raw-irq disable is the best and simplest when I am only
> > > worried about task-latencies. But now Daniel and Sven wants to use the
> > > distingtion between raw_local_irq_disable() and local_irq_disable() to
> > > make irqs fast.
> >
> > We aim to make IRQ latencies deterministic. This affects preemption
> > latency in a positive way.
> >
> No. If you leave preemption off but irqs on, which is what is done here,
> you get good, deterministic IRQ latencies but nothing for task-latencies -
> actually slightly (unmeassureable I agree) worse due to the extra step
> you have to go from the physical interrupt to the task-switch is
> completed.

If its unmeasurable, what is the BIG deal? Don't make me say again that
we need real data here.

> > Anywhere in the kernel that IRQs are disabled, preemption is impossible.
> > (you can't interrupt the CPU when irqs are disabled)
> >
> For me it is the _same_ thing. Equally bad. If preemption is off I don't
> care if irqs are off.

So if its the same thing, why are you concerned with the improvement as

> > But you said you are worried about overhead. You have to incur overhead
> > to make task response deterministic.
> >
> > Are you sure you are not just trying to make it FAST?
> >
> Certainly not. I was pressing for priority inheritance forinstance. That
> thing does certainly not make it fast, but it makes use of locks
> deterministic.

PI is already in there. I think you are missing some basic concepts here,
for example that IRQs can happen ANYTIME, not just when we happen to enable
interrupts where they have previously been disabled.

I am going to stop responding to this thread until you back up your concerns
with real data, or throw some code out there, that you can back up with real data.



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