Real-Time Preemption: Magic Sysrq p doesn't work

["kus Kusche Klaus" <kus@keba.com>]

While testing, I noticed that Sysrq p is silently ignored on current RT
kernels with RT preemption: The syslog contains a message that Sysrq p
was pressed, but no registers are printed. 

A vanilla 2.6.11 with the same config (except for the RT-specific
settings) on the same hardware (i386) gives correct Sysrq p output.

The keyboard is connected via USB.

I checked the code: The sysrq p function is passed a "struct pt_regs *"
arg, and only produces output if this pointer is non-NULL. For RT
kernels, it is always NULL (I added a printk).

This "struct pt_regs *" travels all the way from the actual keyboard
interrupt handler (it is passed as an arg to "usb_kbd_irq"), but I was
not able to trace it back any further in the source.

Is this a known problem?

Is there any workaround? (Sysrq p would be very helpful for debugging!)

Thanks in advance for any hints!

-- 
Klaus Kusche                 (Software Development - Control Systems)
KEBA AG             Gewerbepark Urfahr, A-4041 Linz, Austria (Europe)
Tel: +43 / 732 / 7090-3120                 Fax: +43 / 732 / 7090-6301
E-Mail: kus@keba.com                                WWW: www.keba.com
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