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SubjectRe: 2.6.16-rt10

* Simon Derr <Simon.Derr@bull.net> wrote:

> On Tue, 28 Mar 2006, Ingo Molnar wrote:
>
> >
> > * Simon Derr <simon.derr@bull.net> wrote:
> >
> > > On Mon, 27 Mar 2006, Ingo Molnar wrote:
> > >
> > > > i've released -rt10
> > >
> > > Is anyone working on a port of this patch to the IA64 architecture ?
> >
> > not that i know of. If someone wants to do that, take a look at the
> > x86_64 changes (or ppc/mips/i386 changes) to get an idea. These are the
> > rough steps needed:
> > [snip]
>
>
> Work in progress... (based on -rt11)
> So far I have a kernel that almost boots, but not quite.
>
> First issue: 'BUG: udev:45 task might have lost a preemption check!'
>
> When looking at the code in preempt_enable_no_resched(), why is the
> value of preempt_count() checked to be non-zero _after_ calling
> dec_preempt_count() ?
>
> I saw several posts on this list claiming that this message is
> harmless, but I'd like to figure what's going on.

the warning means that doing a preempt_enable_no_resched() while being
in a preemptible section is most likely a bug, and that you could lose a
need_resched() check. (and introduce a scheduling latency) What's the
backtrace? This could be the sign of a not fully/correctly converted
arch/*/kernel/process.c (but i'm only guessing here).
> My boot process is stuck later when insmod loads the driver for the
> MPT Fusion SCSI adapter. It's waiting for a second interrupt to
> arrive, and that never happens.
>
> I see that the -rt patch touches many drivers by changing calls to
> local_irq_save (and friends), changing the type of the semaphores, but
> the MPT driver makes no use of these.
>
> Any ideas ?

you should first check the PREEMPT_NONE kernel with PREEMPT_SOFTIRQS and
PREEMPT_HARDIRQS enabled. I.e. first check whether IRQ threading works.
Then enable all the other PREEMPT options one by one: PREEMPT_DESKTOP,
PREEMPT_RCU, PREEMPT_BKL. Only when all these work switch to PREEMPT_RT.

Ingo
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