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    SubjectRe: RCU hang on cpu re-hotplug with 2.6.27rc8
    On Tue, Oct 07, 2008 at 01:28:37AM +0200, Andi Kleen wrote:
    > [modifying subject]
    >
    > On Mon, Oct 06, 2008 at 04:12:20PM +0200, Andi Kleen wrote:
    > > [Rafael, something for the regression list]
    > >
    > > While testing cpu hotunplug/hotreplug (first
    > > setting two CPUs to offline and then to online again) on a 16 thread machine
    > > with 2.6.27rc8 the first
    > >
    > > # echo 1 > ./devices/system/cpu/cpu14/online
    > >
    > > after hotunplug deadlocked somewhere in the scheduler:
    >
    > I let it run for longer and I ended up with more and more processes
    > stuck in synchronize_rcu(). No more backtraces because the system
    > has no console and is now not able to write to disk anymore.
    >
    > So it seems like there's something broken with RCU & cpu hotplug
    > in 2.6.28rc8. cc Paul.
    >
    > It's probably not the scheduler, sorry for blaming it earlier.

    Could you please try the patch at the following URL (from Thomas
    Gleixner)?

    http://www.rdrop.com/users/paulmck/patches/2.6.27-rc7-tglx-timer-1.patch

    This fixed some CPU hotplug hangs that I was seeing in 2.6.27-rc7 and
    -rc8. Alternatively, try 2.6.27-rc9, which seems to include Thomas's
    patch.

    Thanx, Paul

    > -Andi
    >
    > > bash D 00000000ffffcb5b 0 4683 4671
    > > ffff8804bc583c68 0000000000000086 ffff8804bc9d8640 0000000000000296
    > > ffff8804bdd34730 ffff8804be6fc090 ffff8804bdd34978 0000000c805a1e2a
    > > ffff8804be4fd780 ffffffff802298b4 ffffffff808acd98 ffff88027d0b1168
    > > Call Trace:
    > > [<ffffffff802298b4>] __dequeue_entity+0x25/0x68
    > > [<ffffffff805a1b4b>] schedule_timeout+0x1e/0xad
    > > [<ffffffff8022a11f>] __disable_runtime+0x57/0x155
    > > [<ffffffff8025cc47>] cpupri_set+0xbe/0xcd
    > > [<ffffffff805a19b3>] wait_for_common+0xcd/0x131
    > > [<ffffffff8022c918>] default_wake_function+0x0/0xe
    > > [<ffffffff80241f3a>] synchronize_rcu+0x30/0x36
    > > [<ffffffff80241fac>] wakeme_after_rcu+0x0/0xc
    > > [<ffffffff8022dab6>] partition_sched_domains+0x9b/0x1dd
    > > [<ffffffff8022dc26>] update_sched_domains+0x2e/0x35
    > > [<ffffffff805a5297>] notifier_call_chain+0x29/0x4c
    > > [<ffffffff8059ef76>] _cpu_up+0xd0/0x10a
    > > [<ffffffff8059f004>] cpu_up+0x54/0x61
    > > [<ffffffff805837d5>] store_online+0x43/0x67
    > > [<ffffffff802c51e9>] sysfs_write_file+0xd2/0x110
    > > [<ffffffff8028656b>] vfs_write+0xad/0x136
    > > [<ffffffff802869f3>] sys_write+0x45/0x6e
    > > [<ffffffff8020b22b>] system_call_fastpath+0x16/0x1b
    > >
    > > It just hung forever, but the machine was otherwise fully functional.
    > >
    > > This was without frame pointers so the backtrace presumably has
    > > some garbage. Haven't looked too closely.
    > >
    > > -Andi
    > >
    > > --
    > > ak@linux.intel.com
    >
    > --
    > ak@linux.intel.com


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