Suraj Jitindar Singh's on March 29, 2019 3:20 pm:

On Wed, 2019-03-27 at 17:51 +0100, Cédric Le Goater wrote:

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On 3/27/19 5:37 PM, Cédric Le Goater wrote:

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On 3/27/19 1:36 PM, Sebastian Andrzej Siewior wrote:

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With qemu-system-ppc64le -machine pseries -smp 4 I get:

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#  chrt 1 hackbench
Running in process mode with 10 groups using 40 file
descriptors each (== 400 tasks)
Each sender will pass 100 messages of 100 bytes
Oops: Exception in kernel mode, sig: 4 [#1]
LE PAGE_SIZE=64K MMU=Hash PREEMPT SMP NR_CPUS=2048 NUMA pSeries
Modules linked in:
CPU: 0 PID: 629 Comm: hackbench Not tainted 5.1.0-rc2 #71
NIP:  c000000000046978 LR: c000000000046a38 CTR:
c0000000000b0150
REGS: c0000001fffeb8e0 TRAP: 0700   Not tainted  (5.1.0-rc2)
MSR:  8000000000089033 <SF,EE,ME,IR,DR,RI,LE>  CR:
42000874  XER: 00000000
CFAR: c000000000046a34 IRQMASK: 1
GPR00: c0000000000b0170 c0000001fffebb70 c000000000a6ba00
0000000028000000

…

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NIP [c000000000046978] doorbell_core_ipi+0x28/0x30
LR [c000000000046a38] doorbell_try_core_ipi+0xb8/0xf0
Call Trace:
[c0000001fffebb70] [c0000001fffebba0] 0xc0000001fffebba0
(unreliable)
[c0000001fffebba0] [c0000000000b0170]
smp_pseries_cause_ipi+0x20/0x70
[c0000001fffebbd0] [c00000000004b02c]
arch_send_call_function_single_ipi+0x8c/0xa0
[c0000001fffebbf0] [c0000000001de600]
irq_work_queue_on+0xe0/0x130
[c0000001fffebc30] [c0000000001340c8]
rto_push_irq_work_func+0xc8/0x120

…

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Instruction dump:
60000000 60000000 3c4c00a2 384250b0 3d220009 392949c8 81290000
3929ffff
7d231838 7c0004ac 5463017e 64632800 <7c00191c> 4e800020
3c4c00a2 38425080
---[ end trace eb842b544538cbdf ]---

This is unusual and causing powerpc code to crash because the rt
scheduler is telling irq_work_queue_on to queue work on this CPU.
Is that something allowed? There's no warnings in there but it must
be a rarely tested path, would it be better to ban it?

Steven is this queue_work_on to self by design?

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and I was wondering whether this is a qemu bug or the kernel is
using an
opcode it should rather not. If I skip doorbell_try_core_ipi() in
smp_pseries_cause_ipi() then there is no crash. The comment says
"POWER9
should not use this handler" so…

I would say Linux is using a msgsndp instruction which is not
implemented
in QEMU TCG. But why have we started using dbells in Linux ? 

Yeah the kernel must have used msgsndp which isn't implemented for TCG
yet. We use doorbells in linux but only for threads which are on the
same core.
And when I try to construct a situation with more than 1 thread per
core (e.g. -smp 4,threads=4), I get "TCG cannot support more than 1
thread/core on a pseries machine".

So I wonder why the guest thinks it can use msgsndp...

IPI to self evidently. Under TCG it really should implement the
instruction or remove the DBELL feature.

Thanks,
Nick