Hi,

when booting 2.6.25-rc3 on powerpc64 with SLUB_DEBUG_ON and 64k page size, it drops into xmon 
during boot with the following :

Console: colour dummy device 80x25
Dentry cache hash table entries: 262144 (order: 5, 2097152 bytes)
Inode-cache hash table entries: 131072 (order: 4, 1048576 bytes)
freeing bootmem node 0
freeing bootmem node 1
Memory: 2037184k/2097152k available (4864k kernel code, 59968k reserved, 768k data, 419k bss, 384k init)
SLUB: Genslabs=17, HWalign=128, Order=0-2, MinObjects=8, CPUs=4, Nodes=16
Mount-cache hash table entries: 4096
BUG: scheduling while atomic: kthreadd/2/0x00056ef8
Call Trace:
[c00000003c187b68] [c00000000000f140] .show_stack+0x70/0x1bc (unreliable)
[c00000003c187c18] [c000000000052d0c] .__schedule_bug+0x64/0x80
[c00000003c187ca8] [c00000000036fa84] .schedule+0xc4/0x6b0
[c00000003c187d98] [c0000000003702d0] .schedule_timeout+0x3c/0xe8
[c00000003c187e68] [c00000000036f82c] .wait_for_common+0x150/0x22c
[c00000003c187f28] [c000000000074868] .kthreadd+0x12c/0x1f0
[c00000003c187fd8] [c000000000024864] .kernel_thread+0x4c/0x68
------------[ cut here ]------------
kernel BUG at /home/auto/jens/kernels/linux-2.6.25-rc3/kernel/sched.c:4532!
cpu 0x0: Vector: 700 (Program Check) at [c00000003c187bc8]
    pc: c000000000051f8c: .sched_setscheduler+0x5c/0x48c
    lr: c0000000000748b0: .kthreadd+0x174/0x1f0
    sp: c00000003c187e48
   msr: 9000000000029032
  current = 0xc00000007e0808a0
  paca    = 0xc0000000004cf880
    pid   = 2, comm = kthreadd
kernel BUG at /home/auto/jens/kernels/linux-2.6.25-rc3/kernel/sched.c:4532!
enter ? for help
[c00000003c187f28] c0000000000748b0 .kthreadd+0x174/0x1f0
[c00000003c187fd8] c000000000024864 .kernel_thread+0x4c/0x68
0:mon> 

In the code this corresponds to

int sched_setscheduler(struct task_struct *p, int policy,
                       struct sched_param *param)
{
        int retval, oldprio, oldpolicy = -1, on_rq, running;
        unsigned long flags;
        const struct sched_class *prev_class = p->sched_class;
        struct rq *rq;

        /* may grab non-irq protected spin_locks */
        BUG_ON(in_interrupt());
recheck:
        /* double check policy once rq lock held */
        if (policy < 0)
                policy = oldpolicy = p->policy;
        else if (policy != SCHED_FIFO && policy != SCHED_RR &&
                        policy != SCHED_NORMAL && policy != SCHED_BATCH &&
                        policy != SCHED_IDLE)
                return -EINVAL;

With slub_debug=- on the kernel command line, the problem is gone.
With 4k page size the problem also does not occur.

Any ideas on why this occurs and how to debug this further ?

Gruß,
	Jens