0.000000! Initializing cgroup subsys cpuset
     0.000000! Initializing cgroup subsys cpu
     0.000000! Linux version 2.6.28.7 (root@aragorn) (gcc version 4.2.4 (Debian 4.2.4-4)) #66 SMP Thu Mar 12 16:56:40 CET 2009
     0.000000! We are running native (31 bit mode)
     0.000000! This machine has an IEEE fpu
     0.000000! debug: ignoring loglevel setting.
     0.000000! Zone PFN ranges:
     0.000000!   Normal   0x00000000 -> 0x00010000
     0.000000! Movable zone start PFN for each node
     0.000000! early_node_map 1! active PFN ranges
     0.000000!     0: 0x00000000 -> 0x00010000
     0.000000! On node 0 totalpages: 65536
     0.000000!   Normal zone: 512 pages used for memmap
     0.000000!   Normal zone: 0 pages reserved
     0.000000!   Normal zone: 65024 pages, LIFO batch:15
     0.000000!   Movable zone: 0 pages used for memmap
     0.000000! Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 65024
     0.000000! Kernel command line: ro vmpoff="LOGOFF" root=/dev/disk/by-path/ccw-0.0.0120-part1 ignore_loglevel BOOT_IMAGE=4
     0.000000! PID hash table entries: 1024 (order: 10, 4096 bytes)
     0.000000! Registering new clock device comparator
     0.000000! New clock device comparator registered
     0.000633! console  ttyS0! enabled
     0.002854! Dentry cache hash table entries: 32768 (order: 5, 131072 bytes)
     0.004258! timekeeping: clock source changed from none to jiffies (shift: 8)

     0.013649! Inode-cache hash table entries: 16384 (order: 4, 65536 bytes)
     0.137013! Memory: 251008k/262144k available (2144k kernel code, 0k reserved, 897k data, 152k init)
     0.137365! Write protected kernel read-only data: 0x12000 - 0x2d8fff
     0.139484! Calibrating delay loop (skipped)... 171.00 BogoMIPS preset
     0.140962! Security Framework initialized
     0.141155! SELinux:  Disabled at boot.
     0.142142! Mount-cache hash table entries: 512
     0.148444! Initializing cgroup subsys ns
     0.148631! Initializing cgroup subsys cpuacct
     0.149045! Initializing cgroup subsys devices
     0.150337! init: calling smp_prepare_cpus
     0.183775! CPUs: 2 configured, 0 standby
     0.183947! s390_smp: smp_detect_cpus calling get_online_cpus
     0.184162! s390_smp: smp_detect_cpus calling __smp_rescan_cpus
     0.184381! s390_smp: smp_rescan_cpus_sigp starting loop
[!!! Hang is here !!!]

[... With 5cd1c9c5 and 6c9bacb4 reverted, the boot continues as follows ...]
[... Next two messages are from Hercules ...]
CPU0000: SIGP Set prefix (0D) CPU0001, PARM 0FEC5000: CC 0
CPU0000: SIGP Restart (06) CPU0001, PARM 00000000: CC 0
[... A correct boot shows similar time lapse since last message ...]
     0.525049! s390_smp: smp_rescan_cpus_sigp loop done
     0.525310! s390_smp: smp_detect_cpus calling put_online_cpus
     0.525555! s390_smp: smp_detect_cpus done
     0.526037! cpu 0 phys_idx=0 vers=00 ident=002623 machine=3090 unused=0000
     0.526408! s390_smp: start loop smp_create_idle
     0.531784! s390_smp: loop smp_create_idle done
     0.532009! init: do_pre_smp_initcalls
     0.533334! init: start_boot_trace
     0.533499! init: smp_init
     0.535875! Registering new clock device tod
     0.536040! New clock device tod registered
     0.536254! cpu 1 phys_idx=1 vers=00 ident=102623 machine=3090 unused=0000
     0.545159! Brought up 2 CPUs
     0.545323! init: sched_init_smp
     0.545724! CPU0 attaching sched-domain:
     0.545952!  domain 0: span 0-1 level MC
     0.546182!   groups: 0 1
     0.546498! CPU1 attaching sched-domain:
     0.546717!  domain 0: span 0-1 level MC
     0.546943!   groups: 1 0
     0.548670! init: cpuset_init_smp
     0.548847! init: do_basic_setup
     0.589620! net_namespace: 524 bytes
     0.599256! NET: Registered protocol family 16
     0.715112! Switched to high resolution mode on CPU 0
     0.715262! Switched to high resolution mode on CPU 1
     0.718291! timekeeping: clock source changed from jiffies to tod (shift: 12)
     0.731545! NET: Registered protocol family 2
     0.787094! IP route cache hash table entries: 2048 (order: 1, 8192 bytes)
     0.800004! TCP established hash table entries: 8192 (order: 4, 65536 bytes)
     0.804289! TCP bind hash table entries: 8192 (order: 4, 65536 bytes)
     0.807714! TCP: Hash tables configured (established 8192 bind 8192)
     0.807993! TCP reno registered
     0.831294! NET: Registered protocol family 1
     0.837270! checking if image is initramfs...

In arch/s390/kernel/smp.c:
static int smp_rescan_cpus_sigp(cpumask_t avail)
{
        int cpu_id, logical_cpu;

        logical_cpu = first_cpu(avail);
        if (logical_cpu == NR_CPUS)
                return 0;
printk("s390_smp: smp_rescan_cpus_sigp starting loop\n");
        for (cpu_id = 0; cpu_id <= 65535; cpu_id++) {
                if (cpu_known(cpu_id))
                        continue;
                __cpu_logical_map[logical_cpu] = cpu_id;
                smp_cpu_polarization[logical_cpu] = POLARIZATION_UNKNWN;
                if (!cpu_stopped(logical_cpu))
                        continue;
                cpu_set(logical_cpu, cpu_present_map);
                smp_cpu_state[logical_cpu] = CPU_STATE_CONFIGURED;
                logical_cpu = next_cpu(logical_cpu, avail);
                if (logical_cpu == NR_CPUS)
                        break;
        }
printk("s390_smp: smp_rescan_cpus_sigp loop done\n");
        return 0;
}