9 hangs with idletick = 0

I have a box running a GENERIC kernel from September 25th that freezes
completely for a few seconds, up to a minute or so, at random intervals.
Sooner or later it freezes permanently (or at least longer than I am
willing to wait for it to unfreeze) and must be power-cycled, as it does
not respond to Ctrl+Alt+Esc.  Network activity (such as downloading ISO
images over a 6 Mbps DSL line) seems to aggravate the issue.  The
problem goes away if I enable idletick.

I had the same problems with a slightly customized kernel (basically
GENERIC with unused drivers removed) built from September 10th sources;
I switched to GENERIC to eliminate the possibility that my kernel config
was to blame.

Prior to that, the machine ran a kernel with the same custom config
built from April 27th sources without any trouble whatsoever, so the
issue must have been introduced sometime between April 27th and
September 10th.

The machine is an Intel Core 2 Duo E6600 with 4 GB RAM.

Relevant sysctls:

kern.eventtimer.choice: HPET(450) HPET1(440) HPET2(440) LAPIC(400) i8254(100) RTC(0)
kern.eventtimer.et.LAPIC.flags: 15
kern.eventtimer.et.LAPIC.frequency: 0
kern.eventtimer.et.LAPIC.quality: 400
kern.eventtimer.et.HPET.flags: 3
kern.eventtimer.et.HPET.frequency: 14318180
kern.eventtimer.et.HPET.quality: 450
kern.eventtimer.et.HPET1.flags: 3
kern.eventtimer.et.HPET1.frequency: 14318180
kern.eventtimer.et.HPET1.quality: 440
kern.eventtimer.et.HPET2.flags: 3
kern.eventtimer.et.HPET2.frequency: 14318180
kern.eventtimer.et.HPET2.quality: 440
kern.eventtimer.et.i8254.flags: 1
kern.eventtimer.et.i8254.frequency: 1193182
kern.eventtimer.et.i8254.quality: 100
kern.eventtimer.et.RTC.flags: 17
kern.eventtimer.et.RTC.frequency: 32768
kern.eventtimer.et.RTC.quality: 0
kern.eventtimer.periodic: 0
kern.eventtimer.timer: HPET
kern.eventtimer.idletick: 1
kern.eventtimer.singlemul: 2
kern.timecounter.tick: 1
kern.timecounter.choice: TSC-low(1000) i8254(0) HPET(950) ACPI-fast(900) dummy(-1000000)
kern.timecounter.hardware: TSC-low
kern.timecounter.stepwarnings: 0
kern.timecounter.tc.ACPI-fast.mask: 16777215
kern.timecounter.tc.ACPI-fast.counter: 6800584
kern.timecounter.tc.ACPI-fast.frequency: 3579545
kern.timecounter.tc.ACPI-fast.quality: 900
kern.timecounter.tc.HPET.mask: 4294967295
kern.timecounter.tc.HPET.counter: 3991486344
kern.timecounter.tc.HPET.frequency: 14318180
kern.timecounter.tc.HPET.quality: 950
kern.timecounter.tc.i8254.mask: 65535
kern.timecounter.tc.i8254.counter: 27169
kern.timecounter.tc.i8254.frequency: 1193182
kern.timecounter.tc.i8254.quality: 0
kern.timecounter.tc.TSC-low.mask: 4294967295
kern.timecounter.tc.TSC-low.counter: 4021161618
kern.timecounter.tc.TSC-low.frequency: 9375191
kern.timecounter.tc.TSC-low.quality: 1000
kern.timecounter.smp_tsc: 1
kern.timecounter.invariant_tsc: 1

Let me know if you need anything else.  I can try to bisect, but it will
take time since the freezes are tricky to reproduce.

DES
-- 
Dag-Erling Smørgrav - des_at_des.no