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    Subject[BUG] cpu hotplug vs scheduler
    I'm testing host cpu hotplug with kvm.  Basically running 7 guests on a 
    4 core machine, offlining and onlining host cpus at random. Eventually
    I hit this:

    [4298303.496645] Booting processor 3/7 ip 6000
    [4298303.506116] Initializing CPU#3
    [4298303.506116] Calibrating delay using timer specific routine..
    5319.66 BogoMIPS (lpj=2659833)
    [4298303.506116] CPU: L1 I cache: 32K, L1 D cache: 32K
    [4298303.506116] CPU: L2 cache: 4096K
    [4298303.506116] CPU: Physical Processor ID: 3
    [4298303.506116] CPU: Processor Core ID: 1
    [4298303.506116] x86 PAT enabled: cpu 3, old 0x7040600070406, new
    0x7010600070106
    [4298303.582937] CPU3: Intel(R) Xeon(R) CPU 5150 @ 2.66GHz
    stepping 06
    [4298303.585087] checking TSC synchronization [CPU#0 -> CPU#3]: passed.
    [4298303.707287] Switched to high resolution mode on CPU 3
    [4298303.712943] kvm: enabling virtualization on CPU3
    [4298303.713955] CPU0 attaching sched-domain:
    [4298303.713901] BUG: unable to handle kernel NULL pointer dereference
    at 0000000000000158
    [4298303.713901] IP: [<ffffffff8022e722>] pick_next_task_fair+0x55/0x7c
    [4298303.713901] PGD 0
    [4298303.713901] Oops: 0000 [1] PREEMPT SMP
    [4298303.713901] CPU 3
    [4298303.713901] Modules linked in: kvm_intel kvm netconsole autofs4 nfs
    lockd nfs_acl sunrpc bridge llc acpi_cpufreq backlight sg e1000 button
    serio_raw rtc_cmos rtc_core rtc_lib ata_piix dm_snapshot dm_mod ahci
    libata dock sd_mod scsi_mod [last unloaded: kvm]
    [4298303.713901] Pid: 15115, comm: migration/3 Not tainted 2.6.26-rc2 #723
    [4298303.713901] RIP: 0010:[<ffffffff8022e722>] [<ffffffff8022e722>]
    pick_next_task_fair+0x55/0x7c
    [4298303.713901] RSP: 0018:ffff81004fdfbe20 EFLAGS: 00010046
    [4298303.713901] RAX: 0000000000000000 RBX: ffff81000103df80 RCX:
    0000000000000000
    [4298303.713901] RDX: ffff81000103e038 RSI: 000000003b9aca00 RDI:
    ffff81000103df00
    [4298303.713901] RBP: ffff81004fdfbe40 R08: ffff81004fdfbdd0 R09:
    ffff81000103a0a0
    [4298303.713901] R10: 0000000000000000 R11: 0000000000000003 R12:
    0000000000000000
    [4298303.713901] R13: ffff81000103df00 R14: ffffffff8060a140 R15:
    0000000000000003
    [4298303.713901] FS: 0000000000000000(0000) GS:ffff81007f806a80(0000)
    knlGS:0000000000000000
    [4298303.713901] CS: 0010 DS: 0018 ES: 0018 CR0: 000000008005003b
    [4298303.713901] CR2: 0000000000000158 CR3: 0000000000201000 CR4:
    00000000000026a0
    [4298303.713901] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
    0000000000000000
    [4298303.713901] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7:
    0000000000000400
    [4298303.713901] Process migration/3 (pid: 15115, threadinfo
    ffff81004fdfa000, task ffff81003f5f0700)
    [4298303.713901] Stack: ffff81004fdfbe40 ffff81000103df00
    ffffffff804543c0 ffff810020985bf8
    [4298303.713901] ffff81004fdfbee0 ffffffff804373fe ffff81004fdfbe80
    ffffffff8023060a
    [4298303.713901] ffff81000103df00 ffff81003f5f0700 0000000000000000
    ffff81003f5f0a60
    [4298303.713901] Call Trace:
    [4298303.713901] [<ffffffff804373fe>] schedule+0x414/0x6ab
    [4298303.713901] [<ffffffff8023060a>] ? hrtick_set+0x9d/0xe8
    [4298303.713901] [<ffffffff8043772f>] ? thread_return+0x9a/0xbf
    [4298303.713901] [<ffffffff80231652>] migration_thread+0x185/0x22d
    [4298303.713901] [<ffffffff802314cd>] ? migration_thread+0x0/0x22d
    [4298303.713901] [<ffffffff8024afe6>] kthread+0x49/0x77
    [4298303.713901] [<ffffffff8020d228>] child_rip+0xa/0x12
    [4298303.713901] [<ffffffff8024af9d>] ? kthread+0x0/0x77
    [4298303.713901] [<ffffffff8020d21e>] ? child_rip+0x0/0x12
    [4298303.713901]
    [4298303.713901]
    [4298303.713901] Code: c0 74 28 48 8b 7b 58 4c 8d 60 f0 48 85 ff 74 10
    4c 89 e6 e8 df cc ff ff 85 c0 75 04 4c 8b 63 58 4c 89 e6 48 89 df e8 4a
    e5 ff ff <49> 8b 9c 24 58 01 00 00 48 85 db 75 bf 49 83 ec 38 4c 89 ef 4c
    [4298303.713901] RIP [<ffffffff8022e722>] pick_next_task_fair+0x55/0x7c

    This seems to be the assignment to cfs_rq after pick_next_entity().

    I'm running kvm.git, which is currently 2.6.26-rc2 plus a few kvm
    patches. It could be kvm's fault, but it doesn't appear so from the traces.

    --
    error compiling committee.c: too many arguments to function



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