Subject: Badness in xics_ipi_dispatch

From: Brian King <[email-masked]>
Date: Tue, 20 Apr 2010 17:17:12 -0500

In stress testing enabling and disabling of SMT, we are regularly
seeing the badness warning below. Looking through the cpu offline
path, this is what I see:

1. stop_cpu: IRQ's get disabled
2. pseries_cpu_disable: set cpu offline (no barriers after this)
3. xics_migrate_irqs_away: Remove ourselves from the GIQ, but still allow
    IPIs
4. stop_cpu: IRQ's get enabled again (local_irq_enable)

It looks to me like there is plenty of opportunity between 1 and 2 for
an IPI to get queued, resulting in the badness below. Is there something
in xics_migrate_irqs_away that should clear any pending IPIs? If there
is, maybe the solution is as simple as adding a barrier after marking
the cpu offline. Or is the warning bogus and we should just remove it?

Thanks,

Brian

<3>Badness at arch/powerpc/platforms/pseries/xics.c:511
<4>NIP: c0000000000673f0 LR: c00000000010fc6c CTR: 000000000171b0b0
<4>REGS: c00000000f2e3b30 TRAP: 0700   Tainted: G           X  (2.6.32.11-0.3.1.bk2-ppc64)
<4>MSR: 8000000000021032 <ME,CE,IR,DR>  CR: 28000024  XER: 00000001
<4>TASK = c000000079216ce0[421] 'kstop/9' THREAD: c00000003fe30000 CPU: 9
<4>GPR00: 0000000000000001 c00000000f2e3db0 c000000000ea4008 0000000000000009
<4>GPR04: 0000000000000000 0000000000000000 fffffffffffffff0 0000000000000000
<4>GPR08: 0000000000000009 0000000000000000 c000000000600930 c000000000f55ba8
<4>GPR12: 0000000000000100 c000000000f73800 00000000000532b3 0000000000053191
<4>GPR16: 00000000000532ab 000000000004003c 0000000000055138 0000000000054c46
<4>GPR20: 00000000000547bc 00000000000655a8 0000000000065594 0000000000000000
<4>GPR24: 0000000000000004 0000000000000010 0000000000000001 0000000000000010
<4>GPR28: 0000000000000000 0000000000000000 c000000000e1c7a0 c00000007e023500
<4>NIP [c0000000000673f0] .xics_ipi_dispatch+0x50/0x1f8
<4>LR [c00000000010fc6c] .handle_IRQ_event+0x9c/0x1d8
<4>Call Trace:
<4>[c00000000f2e3db0] [c0000000000674f4] .xics_ipi_dispatch+0x154/0x1f8 (unreliable)
<4>[c00000000f2e3e50] [c00000000010fc6c] .handle_IRQ_event+0x9c/0x1d8
<4>[c00000000f2e3f00] [c000000000112b0c] .handle_percpu_irq+0x74/0xf8
<4>[c00000000f2e3f90] [c000000000030720] .call_handle_irq+0x1c/0x2c
<4>[c00000003fe33860] [c00000000000e380] .do_IRQ+0x118/0x208
<4>[c00000003fe33910] [c000000000004c98] hardware_interrupt_entry+0x18/0x1c
<4>--- Exception: 501 at .raw_local_irq_restore+0x70/0xc8
<4>    LR = .stop_cpu+0xfc/0x1c0
<4>[c00000003fe33c00] [c00000003fe33c90] 0xc00000003fe33c90 (unreliable)
<4>[c00000003fe33c90] [c000000000100b64] .stop_cpu+0xfc/0x1c0
<4>[c00000003fe33d40] [c0000000000c6f0c] .run_workqueue+0xf4/0x1e0
<4>[c00000003fe33e00] [c0000000000c70b8] .worker_thread+0xc0/0x180
<4>[c00000003fe33ed0] [c0000000000cce74] .kthread+0xb4/0xc0
<4>[c00000003fe33f90] [c0000000000309fc] .kernel_thread+0x54/0x70
<4>Instruction dump:
<4>fb61ffd8 e96a0000 fb81ffe0 79291f24 fba1ffe8 fbc1fff0 fbe1fff8 f821ff61
<4>7c0b482a 7c004436 68000001 540007fe <0b000000> 7c0004ac e802a2d8 78633e24

-- 
Brian King
Linux on Power Virtualization
IBM Linux Technology Center