time accounting problem (powerpc only?)

[Johannes Berg <johannes@sipsolutions.net>]

Hi,

On my powerbook, with NO_HZ and HIGH_RES_TIMERS, I observed recently
that powernowd would not ever switch between CPU speeds.

Doing some scripting to read /proc/stat every half second and print the
differences, I get output like this on a mostly idle system:

# for reference:
# [user, nice, system, idle, iowait, irq, softirq, steal, guest]

[4, 0, 3, 46, 0, 264, 0, 0, 0]
[5, 0, 4, 41, 0, 266, 0, 0, 0]
[4, 0, 7, 37, 0, 299, 0, 0, 0]

Now starting an empty while (1) loop, I see:

[53, 0, 1, 0, 0, 243, 0, 0, 0]
[53, 0, 2, 0, 0, 224, 0, 0, 0]
[45, 0, 7, 0, 0, 249, 0, 0, 0]

As you can see, the "irq" time is very high so that "user" (mostly my
CPU burning program) only accounts for maybe 15%. This leads to
powernowd thinking that all is fine and no switching is required. Also,
the whole stuff leads to top(1) displaying about 80-90% "hi" (hard irq?
calculated as irq - softirq?) time.

Does anybody have an idea why the irq time is so high? And no, I don't
think it can actually be true, I'm seeing maybe 500 interrupts/second
(most due to USB).

johannes