And another one from the debug kernel on EC2, with a slightly different call stack:

[ 4389.480352] ------------[ cut here ]------------
[ 4389.480884] kernel BUG at /home/smb/precise-amd64/ubuntu-2.6/kernel/sched_fair.c:1239!
[ 4389.480894] invalid opcode: 0000 [#1] SMP 
[ 4389.480902] CPU 0 
[ 4389.480905] Modules linked in: ipt_REJECT xt_tcpudp nf_conntrack_ipv4 nf_defrag_ipv4 xt_state nf_conntrack iptable_filter ip_tables x_tables bnep rfcomm bluetooth parport_pc ppdev lp parport isofs acpiphp
[ 4389.480945] 
[ 4389.480949] Pid: 24612, comm: ohai Not tainted 3.2.0-24-virtual #37+lp999755dbg1  
[ 4389.480958] RIP: e030:[<ffffffff81050cd5>]  [<ffffffff81050cd5>] pick_next_entity+0x105/0x110
[ 4389.480972] RSP: e02b:ffff8801d0cdd818  EFLAGS: 00010092
[ 4389.480977] RAX: 000000000000002c RBX: ffff8801d1106e00 RCX: 000000000003ffff
[ 4389.480983] RDX: 0000000000000000 RSI: ffffffff81e000a0 RDI: 0000000000000004
[ 4389.480988] RBP: ffff8801d0cdd838 R08: 000000000000000a R09: 0000000000000000
[ 4389.480994] R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801dff866c0
[ 4389.481000] R13: ffff8801d1107a00 R14: 0000000000000008 R15: ffff8801d1e97800
[ 4389.481009] FS:  00007f09fcc8a700(0000) GS:ffff8801dff73000(0000) knlGS:0000000000000000
[ 4389.481015] CS:  e033 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 4389.481021] CR2: 00000000012dbf30 CR3: 00000001d0702000 CR4: 0000000000002660
[ 4389.481028] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 4389.481035] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 4389.481041] Process ohai (pid: 24612, threadinfo ffff8801d0cdc000, task ffff8801d2905b80)
[ 4389.481047] Stack:
[ 4389.481051]  ffff8801d1106e00 ffff8801dff866c0 ffff8801d1107a00 0000000000000008
[ 4389.481063]  ffff8801d0cdd868 ffffffff810544e8 ffff8801d0cdd868 ffff8801dff866c0
[ 4389.481075]  0000000000000000 ffff8801d2905f28 ffff8801d0cdd8e8 ffffffff81652f3c
[ 4389.481086] Call Trace:
[ 4389.481093]  [<ffffffff810544e8>] pick_next_task_fair+0x38/0x70
[ 4389.481101]  [<ffffffff81652f3c>] __schedule+0x14c/0x6f0
[ 4389.481108]  [<ffffffff816535af>] schedule+0x3f/0x60
[ 4389.481114]  [<ffffffff816546cd>] schedule_hrtimeout_range_clock+0x14d/0x170
[ 4389.481124]  [<ffffffff8100aa1f>] ? xen_restore_fl_direct_reloc+0x4/0x4
[ 4389.481131]  [<ffffffff8165564e>] ? _raw_spin_unlock_irqrestore+0x1e/0x30
[ 4389.481139]  [<ffffffff8108935d>] ? add_wait_queue+0x4d/0x60
[ 4389.481146]  [<ffffffff81654703>] schedule_hrtimeout_range+0x13/0x20
[ 4389.481154]  [<ffffffff811877d9>] poll_schedule_timeout+0x49/0x70
[ 4389.481160]  [<ffffffff81188356>] do_select+0x4d6/0x600
[ 4389.481167]  [<ffffffff811878e0>] ? poll_freewait+0xe0/0xe0
[ 4389.481173]  [<ffffffff811879d0>] ? __pollwait+0xf0/0xf0
[ 4389.481179]  [<ffffffff81005191>] ? __raw_callee_save_xen_pte_val+0x11/0x1e
[ 4389.481186]  [<ffffffff81054c3e>] ? update_curr+0x21e/0x230
[ 4389.481192]  [<ffffffff8103cc65>] ? pvclock_clocksource_read+0x55/0xf0
[ 4389.481199]  [<ffffffff810553db>] ? check_preempt_wakeup+0x15b/0x230
[ 4389.481207]  [<ffffffff8104ed74>] ? check_preempt_curr+0x84/0xa0
[ 4389.481214]  [<ffffffff8104edcd>] ? ttwu_do_wakeup+0x3d/0x120
[ 4389.481222]  [<ffffffff8130fd49>] ? put_dec+0x59/0x60
[ 4389.481228]  [<ffffffff81310c1f>] ? number.isra.2+0x31f/0x350
[ 4389.481236]  [<ffffffff81323436>] ? nla_parse+0x86/0xe0
[ 4389.481242]  [<ffffffff8165564e>] ? _raw_spin_unlock_irqrestore+0x1e/0x30
[ 4389.481250]  [<ffffffff8105e610>] ? try_to_wake_up+0x190/0x200
[ 4389.481257]  [<ffffffff81188641>] core_sys_select+0x1c1/0x330
[ 4389.481263]  [<ffffffff8100aa32>] ? check_events+0x12/0x20
[ 4389.481269]  [<ffffffff8100a25d>] ? xen_force_evtchn_callback+0xd/0x10
[ 4389.481276]  [<ffffffff8100aa32>] ? check_events+0x12/0x20
[ 4389.481282]  [<ffffffff8100aa1f>] ? xen_restore_fl_direct_reloc+0x4/0x4
[ 4389.481289]  [<ffffffff81004c62>] ? xen_mc_flush+0xb2/0x1c0
[ 4389.481295]  [<ffffffff8100aa1f>] ? xen_restore_fl_direct_reloc+0x4/0x4
[ 4389.481302]  [<ffffffff811889eb>] sys_select+0xbb/0x100
[ 4389.481308]  [<ffffffff8105edf7>] ? schedule_tail+0x27/0xb0
[ 4389.481314]  [<ffffffff8165da42>] system_call_fastpath+0x16/0x1b
[ 4389.481319] Code: 89 df e8 69 c1 5e 00 48 8b 73 38 48 c7 c7 7b ef 9f 81 31 c0 e8 98 c9 5e 00 48 8b 73 10 48 c7 c7 98 ef 9f 81 31 c0 e8 86 c9 5e 00 <0f> 0b 66 0f 1f 84 00 00 00 00 00 55 48 89 e5 48 83 ec 10 48 89 
[ 4389.481407] RIP  [<ffffffff81050cd5>] pick_next_entity+0x105/0x110
[ 4389.484006]  RSP <ffff8801d0cdd818>
[ 4389.484006] ---[ end trace 7ee7cea7516c9821 ]---