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SubjectRe: PREEMPT_RT vs ADEOS: the numbers, part 1

* Karim Yaghmour <karim@opersys.com> wrote:

> Ingo Molnar wrote:
> > how were interrupt response times measured, precisely? What did the
> > target (measured) system have to do to respond to an interrupt? Did you
> > use the RTC to measure IRQ latencies?
>
> The logger used two TSC values. One prior to shooting the interrupt to
> the target, and one when receiving the response. Responding to an
> interrupt meant that a driver was hooked to the target's parallel port
> interrupt and simply acted by toggling an output pin on the parallel
> port, which in turn was hooked onto the logger's parallel port in a
> similar fashion. [...]

FYI, there's a new feature in the -V0.7.48-25 (and later) -RT patches
that implements this: CONFIG_LPPTEST. It is a simple standalone driver
and userspace utility from Thomas Gleixner that can be used to measure
the IRQ-latency of the system over a null-modem-parallel-cable.

to use it, enable CONFIG_LPPEST in the .config [disable CONFIG_PARPORT
first], boot the kernel on both the target and the host systems, and
then run the scripts/testlpp utility on the host system which will
measure latencies and will do a maximum-search.

(the driver assumes normal LPT1 PC layout - 0x378/IRQ7)

Ingo
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