Re: [PATCH] local_irq_disable removal

[Mika PenttilÃ]

Sven-Thorsten Dietrich wrote:

> On Sat, 2005-06-11 at 18:00 +0300, Mika Penttilà wrote:
>  
>
> > Ingo Molnar wrote:
> >
> >    
> >
> > > i've done two more things in the latest patches:
> > >
> > > - decoupled the 'soft IRQ flag' from the hard IRQ flag. There's
> > > basically no need for the hard IRQ state to follow the soft IRQ state. 
> > > This makes the hard IRQ disable primitives a bit faster.
> > >
> > > - for raw spinlocks i've reintroduced raw_local_irq primitives again.
> > > This helped get rid of some grossness in sched.c, and the raw
> > > spinlocks disable preemption anyway. It's also safer to just assume
> > > that if a raw spinlock is used together with the IRQ flag that the
> > > real IRQ flag has to be disabled.
> > >
> > > these changes dont really impact scheduling/preemption behavior, they 
> > > are cleanup/robustization changes.
> > >
> > > 	Ingo
> > > -
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> > >
> > >
> > >
> > >      
> > With the soft IRQ flag local_irq_disable() doesn't seem to protect 
> > against soft interrupts (via SA_NODELAY interrupt-> invoke_softirq()). 
> > Could this be a problem?
> >    
>
> Only if you run SOFT IRQs as SA_NODELAY, which is going to KILL all your
> preemption gains with the first arriving network packet.
>
> And that is, if you don't get buried in "scheduling while atomic" printk
> messages first.
>
> SA_NODELAY is not generally allowed in PREEMPT_RT, except for code
> designed to take advantage of the IRQ void that has been created. 
>
> This code must follow a new set of rules, which people who design RT
> apps are really happy to accet, they have to accept worse compromises
> with the alternatives (subkernel or ANOTHER OS (ugh))
>
> Sven
>
>
>
>  
The timer irq is run as NODELAY, so soft irqs are run against 
local_irq_disable sections all the time.

--Mika


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