Subject: Re: missing doorbell interrupt when onlining cpu

From: Nathan Lynch <[email-masked]>
Date: Tue, 10 Sep 2019 18:04:53 -0500

Nathan Lynch <[email-masked]> writes:

> Nathan Lynch <[email-masked]> 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.

Another clue is that I've occasionaly provoked this warning:

WARNING: CPU: 7 PID: 9045 at arch/powerpc/kernel/irq.c:282 arch_local_irq_restore+0xdc/0x150
Modules linked in:
CPU: 7 PID: 9045 Comm: offliner Not tainted 5.3.0-rc2-00190-g9b123d1ea237-dirty #45
NIP:  c00000000001d91c LR: c000000001988210 CTR: 0000000000334ee8
REGS: c00000000e19f390 TRAP: 0700   Not tainted  (5.3.0-rc2-00190-g9b123d1ea237-dirty)
MSR:  800000010282b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE,TM[E]>  CR: 44444424  XER: 20040000
CFAR: c00000000001d884 IRQMASK: 0 
GPR00: c000000001988210 c00000000e19f620 c0000000032f6200 0000000000000000 
GPR04: c00000000e589f10 0000000000000006 c00000000e19f664 c00000000395f260 
GPR08: 000000000000003b 0000000000008000 0000000000000009 0000000000000000 
GPR12: 0000000000000001 c00000001eca7780 000000000000005c 00000100106c7de0 
GPR16: 0000000000000000 0000000000000000 00000000100c0a48 0000000000000001 
GPR20: 00000000100c5748 00000001fc710000 0000000000000078 c000000003345c78 
GPR24: c0000003ffd99a00 c000000003349de0 0000000000000000 c0000003fb086c10 
GPR28: 0000000000000000 000000000000000f c0000003fb086c10 0000000000000000 
NIP [c00000000001d91c] arch_local_irq_restore+0xdc/0x150
LR [c000000001988210] _raw_spin_unlock_irqrestore+0xa0/0xd0
Call Trace:
[c00000000e19f6a0] [c000000001988210] _raw_spin_unlock_irqrestore+0xa0/0xd0
[c00000000e19f6d0] [c0000000001be920] try_to_wake_up+0x330/0xf30
[c00000000e19f7a0] [c0000000001bf5b0] wake_up_q+0x70/0xc0
[c00000000e19f7e0] [c0000000002b5a08] cpu_stop_queue_work+0xc8/0x140
[c00000000e19f850] [c0000000002b5bac] queue_stop_cpus_work+0xdc/0x160
[c00000000e19f8b0] [c0000000002b5c98] __stop_cpus+0x68/0xc0
[c00000000e19f950] [c0000000002b65ec] stop_cpus+0x5c/0x90
[c00000000e19f9a0] [c0000000002b6924] stop_machine_cpuslocked+0x194/0x1f0
[c00000000e19fa10] [c00000000016c768] takedown_cpu+0x98/0x260
[c00000000e19fad0] [c00000000016cea4] cpuhp_invoke_callback+0x114/0xf40
[c00000000e19fb60] [c00000000017194c] _cpu_down+0x19c/0x320
[c00000000e19fbd0] [c00000000016ff58] do_cpu_down+0x68/0xb0
[c00000000e19fc10] [c000000000ccccd4] cpu_subsys_offline+0x24/0x40
[c00000000e19fc30] [c000000000cc2860] device_offline+0x100/0x140
[c00000000e19fc70] [c000000000cc2a00] online_store+0x70/0xf0
[c00000000e19fcb0] [c000000000cbcee8] dev_attr_store+0x38/0x60
[c00000000e19fcd0] [c00000000059c970] sysfs_kf_write+0x70/0xb0
[c00000000e19fd10] [c00000000059afa8] kernfs_fop_write+0xf8/0x280
[c00000000e19fd60] [c0000000004b436c] __vfs_write+0x3c/0x70
[c00000000e19fd80] [c0000000004b8700] vfs_write+0xd0/0x220
[c00000000e19fdd0] [c0000000004b8abc] ksys_write+0x7c/0x140
[c00000000e19fe20] [c00000000000bbd8] system_call+0x5c/0x68

i.e. in arch_local_irq_restore():
		/*
		 * We should already be hard disabled here. We had bugs
		 * where that wasn't the case so let's dbl check it and
		 * warn if we are wrong. Only do that when IRQ tracing
		 * is enabled as mfmsr() can be costly.
		 */
		if (WARN_ON_ONCE(mfmsr() & MSR_EE))
			__hard_irq_disable();

Anyway, I've proposed a fix:

https://patchwork.ozlabs.org/patch/1160572/