Oops in find_busiest_group(): 2.6.8-rc1-mm1

[Dave Hansen <haveblue@us.ibm.com>]

Looks like 'find_busiest_group()::this' is null:

cpu 0x1: Vector: 380 (Data SLB Access) at [c0000002ffe0b420]
    pc: c000000000046644: .find_busiest_group+0x24c/0x470
    lr: c00000000004681c: .find_busiest_group+0x424/0x470
    sp: c0000002ffe0b6a0
   msr: 8000000000001032
   dar: 10
  current = 0xc0000002fff70da0
  paca    = 0xc00000000033c900
    pid   = 0, comm = swapper
...
1:mon> r
R00 = 0000000000000080   R16 = 0000000000000080
R01 = c0000002ffe0b6a0   R17 = 0000000000000080
R02 = c0000000004a5470   R18 = 0000000000000080
R03 = 0000000000000046   R19 = c00000000adfb408
R04 = c00000000050dd27   R20 = 0000000000000001
R05 = c00000000052dd50   R21 = 0000000000000000
R06 = c0000000003b7828   R22 = 0000000000000000
R07 = fffffffffffe0cb8   R23 = c0000002ffe0b710
R08 = c00000000050d180   R24 = c0000000004a2008
R09 = c00000000050d918   R25 = c000000000330c38
R10 = 0000000000000000   R26 = c000000000330c38
R11 = 0000000000000001   R27 = 0000000000000001
R12 = 0000000000000010   R28 = c00000000050d198
R13 = c00000000033c900   R29 = 0000000000000080
R14 = 0000000000000000   R30 = c0000000003c29e8
R15 = c000000000330c38   R31 = c0000002ffe0b6a0
pc  = c000000000046644 .find_busiest_group+0x24c/0x470
lr  = c00000000004681c .find_busiest_group+0x424/0x470
msr = 8000000000001032   cr  = 88428428
ctr = c0000000001527a8   xer = 0000000000000000   trap =      380

I put a little printk in:

        /* How much load to actually move to equalise the imbalance */
        if (!busiest || !this)
                printk("%s() busiest: %p this: %p\n", __func__, busiest, this);
        *imbalance = (*imbalance * min(busiest->cpu_power, this->cpu_power))
                                / SCHED_LOAD_SCALE;

And sure enough, this showed up on the console:

find_busiest_group() busiest: c00000000050d180 this: 0000000000000000

This code also looks funny to begin with:

>                 if (local_group) {
>                         this_load = avg_load;
>                         this = group;
>                         goto nextgroup;
>                 } else if (avg_load > max_load) {
>                         max_load = avg_load;
>                         busiest = group;
>                 }
> nextgroup:
>                 group = group->next;
>         } while (group != sd->groups);

Why bother with the 'goto nextgroup;'?  Shouldn't the first if block
just fall through to the target of the goto anyway?

-- Dave