[Next] CPU Hotplug test failures on powerpc

[Sachin Sant]

While executing cpu_hotplug(from autotest) tests against latest
next on a power6 box, the machine locks up. A soft reset shows
the following trace

cpu 0x0: Vector: 100 (System Reset) at [c00000000c9333d0]
    pc: c0000000003433d8: .find_next_bit+0x54/0xc4
    lr: c000000000342f10: .cpumask_next_and+0x4c/0x94
    sp: c00000000c933650
   msr: 8000000000089032
  current = 0xc00000000c173840
  paca    = 0xc000000000bc2600
    pid   = 2602, comm = hotplug06.top.s
enter ? for help
[link register   ] c000000000342f10 .cpumask_next_and+0x4c/0x94
[c00000000c933650] c0000000000e9f34 .cpuset_cpus_allowed_locked+0x38/0x74 (unreliable)
[c00000000c9336e0] c000000000090074 .move_task_off_dead_cpu+0xc4/0x1ac
[c00000000c9337a0] c0000000005e4e5c .migration_call+0x304/0x830
[c00000000c933880] c0000000005e0880 .notifier_call_chain+0x68/0xe0
[c00000000c933920] c00000000012a92c ._cpu_down+0x210/0x34c
[c00000000c933a90] c00000000012aad8 .cpu_down+0x70/0xa8
[c00000000c933b20] c000000000525940 .store_online+0x54/0x894
[c00000000c933bb0] c000000000463430 .sysdev_store+0x3c/0x50
[c00000000c933c20] c0000000001f8320 .sysfs_write_file+0x124/0x18c
[c00000000c933ce0] c00000000017edac .vfs_write+0xd4/0x1fc
[c00000000c933d80] c00000000017efdc .SyS_write+0x58/0xa0
[c00000000c933e30] c0000000000085b4 syscall_exit+0x0/0x40
--- Exception: c01 (System Call) at 00000fff9fa8a8f8
SP (fffe7aef200) is in userspace
0:mon> e
cpu 0x0: Vector: 100 (System Reset) at [c00000000c9333d0]
    pc: c0000000003433d8: .find_next_bit+0x54/0xc4
    lr: c000000000342f10: .cpumask_next_and+0x4c/0x94
    sp: c00000000c933650
   msr: 8000000000089032
  current = 0xc00000000c173840
  paca    = 0xc000000000bc2600
    pid   = 2602, comm = hotplug06.top.s

Last few messages from the dmesg log shows

0:mon> 
<4>IRQ 17 affinity broken off cpu 0
<4>IRQ 18 affinity broken off cpu 0
<4>IRQ 19 affinity broken off cpu 0
<4>IRQ 264 affinity broken off cpu 0
<4>cpu 0 (hwid 0) Ready to die...
<7>clockevent: decrementer mult[83126e97] shift[32] cpu[0]
<4>Processor 0 found.
<4>IRQ 17 affinity broken off cpu 1
<4>IRQ 18 affinity broken off cpu 1
<4>IRQ 19 affinity broken off cpu 1
<4>IRQ 264 affinity broken off cpu 1
<4>cpu 1 (hwid 1) Ready to die...
<7>clockevent: decrementer mult[83126e97] shift[32] cpu[1]
<4>Processor 1 found.
<4>cpu 1 (hwid 1) Ready to die...
<7>clockevent: decrementer mult[83126e97] shift[32] cpu[1]
<4>Processor 1 found.
<4>cpu 1 (hwid 1) Ready to die...
<6>process 2423 (bash) no longer affine to cpu1
<7>clockevent: decrementer mult[83126e97] shift[32] cpu[1]
<4>Processor 1 found.
<4>cpu 1 (hwid 1) Ready to die...
<7>clockevent: decrementer mult[83126e97] shift[32] cpu[1]
<4>Processor 1 found.
<4>cpu 1 (hwid 1) Ready to die...
<7>clockevent: decrementer mult[83126e97] shift[32] cpu[1]
<4>Processor 1 found.
<4>cpu 1 (hwid 1) Ready to die...
<3>INFO: RCU detected CPU 0 stall (t=1000 jiffies)
<3>INFO: RCU detected CPU 0 stall (t=4000 jiffies)
0:mon>

After some debugging a possible suspect seems to be commit
6ad4c18.. : sched: Fix balance vs hotplug race

If i revert this patch i am able to execute the tests on this
power6 without any issues. 

But at the same time the above patch is required to solve the
cpu hotplug related race on x86_64(as a side note this same
x86_64 issue can be recreated against latest Linus git as well)
that i reported here :

http://marc.info/?l=linux-kernel&m=125802682922299&w=2

I will try few more iterations with and without the above
patch just to make sure i have the correct results.

If someone has a suggestion let me know.

Thanks
-Sachin


-- 

---------------------------------
Sachin Sant
IBM Linux Technology Center
India Systems and Technology Labs
Bangalore, India
---------------------------------