[2.6.37-rc5] Timer or ICE1724 issues, HZ=250, NO_HZ=y

[Simon Kirby]

With Linus git HEAD just compiled earlier today, I noticed a weird issue
just now while in the middle of some coding.  I think this may have
happened before (maybe in 2.6.36), but not as obvious as this time.

The music that was playing stopped, and the key I had just pressed did
not show up in rxvt.  I figured the machine had hung, but I hit another
key and the music played again for another second and my _previous_
keystroke output appeared.

Looking at /proc/interrupts, "timer" had completely stopped.  "perf top"
showed "snd_pcm_hwsync" was at the top of the profile with about 80%
usage.  Spinning in "snd_pcm_stream_lock_irq" from other CPUs, probably..

dmesg showed:

[13995.498393] Clocksource tsc unstable (delta = -8589909196 ns)
[13995.511452] Switching to clocksource hpet
[14039.525074] NOHZ: local_softirq_pending 100
[14039.882183] hrtimer: interrupt took 30800 ns
[14134.540724] NOHZ: local_softirq_pending 100

I tried a few things:

# cd /sys/devices/system/clocksource/clocksource0
# grep . *
available_clocksource:hpet acpi_pm 
current_clocksource:hpet
# echo acpi_pm > current_clocksource

[14249.256629] Switching to clocksource acpi_pm

Still problems, so I tried again:

# echo tsc > current_clocksource

[14258.808333] Override clocksource tsc is not HRT compatible. Cannot switch while in HRT/NOHZ mode
[14258.808343] Switching to clocksource hpet

...but after this, things seemed to return to normal.  /proc/interrupts
shows "timer" increasing once again.

Full boot dmesg: http://0x.ca/sim/ref/2.6.37/oof.dmesg
.config: http://0x.ca/sim/ref/2.6.37/oof.config

Simon-
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