Suraj Jitindar Singh's on March 29, 2019 3:20 pm: > On Wed, 2019-03-27 at 17:51 +0100, Cédric Le Goater wrote: >> On 3/27/19 5:37 PM, Cédric Le Goater wrote: >> > On 3/27/19 1:36 PM, Sebastian Andrzej Siewior wrote: >> > > With qemu-system-ppc64le -machine pseries -smp 4 I get: >> > > >> > > > # 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 >> > > >> > > … >> > > > 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 >> > > >> > > … >> > > > 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? >> > > >> > > 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