[BUG][PPC64] BUG in 2.6.26.5-rt9 causing Hang

[Chirag Jog <chirag@linux.vnet.ibm.com>]

Hi Gregory,
We see the following BUG followed by a hang on the latest kernel 2.6.26.5-rt9 on a Power6 blade (PPC64)
It is easily recreated by running the async_handler or sbrk_mutex (realtime tests from ltp) tests.

login: cpu 0x2: Vector: 700 (Program Check) at [c0000000e8e875d0]
    pc: c00000000005110c: .pick_next_pushable_task+0x54/0x9c
    lr: c000000000059f50: .push_rt_task+0x44/0x2b4
    sp: c0000000e8e87850
   msr: 8000000000021032
  current = 0xc0000000ea5bb2e0
  paca    = 0xc000000000608700
    pid   = 2811, comm = async_handler
kernel BUG at kernel/sched_rt.c:1041! <---------------------
enter ? for help
[link register   ] c000000000059f50 .push_rt_task+0x44/0x2b4
[c0000000e8e87850] c0000000e8e878f0 (unreliable)
[c0000000e8e87900] c00000000005a1dc .push_rt_tasks+0x1c/0x38
[c0000000e8e87980] c00000000005a21c .post_schedule_rt+0x24/0x44
[c0000000e8e87a10] c000000000057cbc .finish_task_switch+0xd0/0x180
[c0000000e8e87ab0] c0000000003b6e88 .__schedule+0x6e0/0x798
[c0000000e8e87b90] c0000000003b7148 .schedule+0xec/0x11c
[c0000000e8e87c10] c0000000003b7a40 .do_nanosleep+0x6c/0xcc
[c0000000e8e87c90] c000000000080738 .hrtimer_nanosleep+0x7c/0x100
[c0000000e8e87d90] c000000000080830 .sys_nanosleep+0x74/0x94
[c0000000e8e87e30] c0000000000086ac syscall_exit+0x0/0x40
--- Exception: c00 (System Call) at 0000008026449844
SP (400014185f0) is in userspace


This is generated by the BUG_ON lines in the pick_next_pushable function
introduced by the sched-only-push-once-per-queue.patch .

The -rt kernel prior to this patch didnot give such BUGes.

All this was tried with
CONFIG_GROUP_SCHED=N
CONFIG_RT_GROUP_SCHED=N


Setting the options
CONFIG_GROUP_SCHED=y
CONFIG_RT_GROUP_SCHED=Y,
seems to solve the problem.



-- 
-Thanks,Chirag