nohz_mode disabled with NO_HZ_IDLE=y

[Rian Quinn]

I am working on the Boxy hypervisor and we have our own custom
compiled Linux kernel. I was doing some performance tests and noticed
that our guest "hlt" handler which implements a guest idle was only
idling for 10ms at a time (or 100Hz) which is the rate we have the
periodic timer set to in the kernel config. The thing is, we have
NO_HZ_IDLE enabled as well and the test that I am running simply
outputs the time to the console and then sleeps for a second, so I
expected the idle time to be closer to 1 second.

It appears that the kernel is acting like it is tick based, even
though it was configured for tickless idle. Here is the output of
/proc/timer_list. Right now, we require an invariant TSC and we
provide an emulated LAPIC timer (we have no intention of ever
supporting legacy hardware so our goal is to keep the amount of
emulation that we have to provide to a minimum). From the output,
nohz_mode is disabled. Another thing that is interesting about this is
I have the high-resolution timing enabled as well, but from the
output, it would appear that high-resolution timing is not working
either.

Any idea on what the issue could be?

------------------

[    0.020043] [U] Timer List Version: v0.8
[    0.020083] [U] HRTIMER_MAX_CLOCK_BASES: 8
[    0.020121] [U] now at 18467102 nsecs
[    0.020157] [U] cpu: 0
[    0.020180] [U]  clock 0:
[    0.020206] [U]   .base:       (____ptrval____)
[    0.020248] [U]   .index:      0
[    0.020279] [U]   .resolution: 10000000 nsecs
[    0.020320] [U]   .get_time:   ktime_get
[    0.020357] [U]   .offset:     0 nsecs
[    0.020392] [U] active timers:
[    0.020421] [U]  clock 1:
[    0.020447] [U]   .base:       (____ptrval____)
[    0.020489] [U]   .index:      1
[    0.020520] [U]   .resolution: 10000000 nsecs
[    0.020561] [U]   .get_time:   ktime_get_real
[    0.020602] [U]   .offset:     1553443224994997061 nsecs
[    0.020651] [U] active timers:
[    0.020680] [U]  clock 2:
[    0.020706] [U]   .base:       (____ptrval____)
[    0.020748] [U]   .index:      2
[    0.020779] [U]   .resolution: 10000000 nsecs
[    0.020820] [U]   .get_time:   ktime_get_boottime
[    0.020864] [U]   .offset:     0 nsecs
[    0.020899] [U] active timers:
[    0.020928] [U]  clock 3:
[    0.020954] [U]   .base:       (____ptrval____)
[    0.020996] [U]   .index:      3
[    0.021027] [U]   .resolution: 10000000 nsecs
[    0.021068] [U]   .get_time:   ktime_get_clocktai
[    0.021112] [U]   .offset:     1553443224994997061 nsecs
[    0.021161] [U] active timers:
[    0.021190] [U]  clock 4:
[    0.021216] [U]   .base:       (____ptrval____)
[    0.021260] [U]   .index:      4
[    0.021291] [U]   .resolution: 10000000 nsecs
[    0.021331] [U]   .get_time:   ktime_get
[    0.021368] [U]   .offset:     0 nsecs
[    0.021404] [U] active timers:
[    0.021433] [U]  clock 5:
[    0.021459] [U]   .base:       (____ptrval____)
[    0.021501] [U]   .index:      5
[    0.021532] [U]   .resolution: 10000000 nsecs
[    0.021572] [U]   .get_time:   ktime_get_real
[    0.021613] [U]   .offset:     1553443224994997061 nsecs
[    0.021662] [U] active timers:
[    0.021691] [U]  clock 6:
[    0.021717] [U]   .base:       (____ptrval____)
[    0.021759] [U]   .index:      6
[    0.021790] [U]   .resolution: 10000000 nsecs
[    0.021830] [U]   .get_time:   ktime_get_boottime
[    0.021874] [U]   .offset:     0 nsecs
[    0.021909] [U] active timers:
[    0.021939] [U]  clock 7:
[    0.021964] [U]   .base:       (____ptrval____)
[    0.022006] [U]   .index:      7
[    0.022037] [U]   .resolution: 10000000 nsecs
[    0.022078] [U]   .get_time:   ktime_get_clocktai
[    0.022122] [U]   .offset:     1553443224994997061 nsecs
[    0.022171] [U] active timers:
[    0.022200] [U]   .expires_next   : 9223372036854775807 nsecs
[    0.022253] [U]   .hres_active    : 0
[    0.022287] [U]   .nr_events      : 0
[    0.022322] [U]   .nr_retries     : 0
[    0.022357] [U]   .nr_hangs       : 0
[    0.022391] [U]   .max_hang_time  : 0
[    0.022444] [U]   .nohz_mode      : 0
[    0.022479] [U]   .last_tick      : 0 nsecs
[    0.022518] [U]   .tick_stopped   : 0
[    0.022553] [U]   .idle_jiffies   : 0
[    0.022587] [U]   .idle_calls     : 0
[    0.022622] [U]   .idle_sleeps    : 0
[    0.022656] [U]   .idle_entrytime : 0 nsecs
[    0.022695] [U]   .idle_waketime  : 0 nsecs
[    0.022734] [U]   .idle_exittime  : 0 nsecs
[    0.022773] [U]   .idle_sleeptime : 0 nsecs
[    0.022813] [U]   .iowait_sleeptime: 0 nsecs
[    0.022852] [U]   .last_jiffies   : 0
[    0.022887] [U]   .next_timer     : 0
[    0.022921] [U]   .idle_expires   : 0 nsecs
[    0.022960] [U] jiffies: 4294937297
[    0.022994] [U] Tick Device: mode:     0
[    0.023031] [U] Broadcast device
[    0.023062] [U] Clock Event Device: <NULL>
[    0.023100] [U] tick_broadcast_mask: 0
[    0.023135] [U] tick_broadcast_oneshot_mask: 0
[    0.023177] [U] Tick Device: mode:     0
[    0.023214] [U] Per CPU device: 0
[    0.023246] [U] Clock Event Device: lapic-deadline
[    0.023290] [U]  max_delta_ns:   1514478803259
[    0.023331] [U]  min_delta_ns:   1000
[    0.023366] [U]  mult:           12180259
[    0.023404] [U]  shift:          25
[    0.023437] [U]  mode:           3
[    0.023469] [U]  next_event:     25002939 nsecs
[    0.023511] [U]  set_next_event: lapic_next_deadline
[    0.023557] [U]  shutdown: lapic_timer_shutdown
[    0.023599] [U]  periodic: lapic_timer_set_periodic
[    0.023644] [U]  oneshot:  lapic_timer_set_oneshot
[    0.023689] [U]  oneshot stopped: lapic_timer_shutdown
[    0.023736] [U]  event_handler:  tick_handle_periodic
[    0.023783] [U]  retries:        0