Re: missing doorbell interrupt when onlining cpu

Nathan Lynch [off-list ref] writes:

I'm hoping for some help investigating a behavior I see when doing cpu
hotplug under load on P9 and P8 LPARs. Occasionally, while coming online
a cpu will seem to get "stuck" in idle, with a pending doorbell
interrupt unserviced (cpu 12 here):

cpuhp/12-70    [012] 46133.602202: cpuhp_enter:          cpu: 0012 target: 205 step: 174 (0xc000000000028920s)
 load.sh-8201  [014] 46133.602248: sched_waking:         comm=cpuhp/12 pid=70 prio=120 target_cpu=012
 load.sh-8201  [014] 46133.602251: smp_send_reschedule:  (c000000000052868) cpu=12
  <idle>-0     [012] 46133.602252: do_idle:              (c000000000162e08)
 load.sh-8201  [014] 46133.602252: smp_muxed_ipi_message_pass: (c0000000000527e8) cpu=12 msg=1
 load.sh-8201  [014] 46133.602253: doorbell_core_ipi:    (c00000000004d3e8) cpu=12
  <idle>-0     [012] 46133.602257: arch_cpu_idle:        (c000000000022d08)
  <idle>-0     [012] 46133.602259: pseries_lpar_idle:    (c0000000000d43c8)

I should be more explicit that given my tracing configuration I would
expect to see doorbell events etc here e.g.

         <idle>-0     [012] 46133.602086: doorbell_entry:       pt_regs=0xc000000200e7fb50
         <idle>-0     [012] 46133.602087: smp_ipi_demux_relaxed: (c0000000000530f8)
         <idle>-0     [012] 46133.602088: scheduler_ipi:        (c00000000015e4f8)
         <idle>-0     [012] 46133.602091: sched_wakeup:         cpuhp/12:70 [120] success=1 CPU:012
         <idle>-0     [012] 46133.602092: sched_wakeup:         migration/12:71 [0] success=1 CPU:012
         <idle>-0     [012] 46133.602093: doorbell_exit:        pt_regs=0xc000000200e7fb50

but instead cpu 12 goes to idle.