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    SubjectRe: RCU hang on cpu re-hotplug with 2.6.27rc8
    On Thu, Oct 09, 2008 at 06:56:46AM +0200, Andi Kleen wrote:
    > [fix up Thomas' address to not bounce]
    >
    > On Wed, Oct 08, 2008 at 06:33:21PM -0700, Paul E. McKenney wrote:
    > > The attached patch (similar to one in -tip, but set up for mainline and
    > > tweaked to make stall-checking on by default) should get you a stack
    > > trace of any CPUs holding up RCU grace periods for more than about
    > > three seconds.
    > >
    > > On the off-chance that this helps.
    >
    > It actually does. The stall detector makes the online echo return
    > after three seconds, although it's not 100% clear to me why.

    Interesting. This behavior would be consistent with the CPU entering
    dyntick-idle mode without RCU's being aware of this. Except that your
    earlier .config file says "# CONFIG_NO_HZ is not set". And that would
    mean that the CPU really should be invoking RCU's state machine every
    scheduling tick.

    I confess confusion.

    Thanx, Paul

    > here's the backtrace
    >
    > RCU detected CPU 14 stall (t=4295149800/5928 jiffies)
    > Pid: 0, comm: swapper Not tainted 2.6.27-rc9 #5
    >
    > Call Trace:
    > <IRQ> [<ffffffff8025d188>] __rcu_pending+0x6e/0x1d9
    > [<ffffffff8025d329>] rcu_pending+0x36/0x6e
    > [<ffffffff8023b480>] update_process_times+0x37/0x5b
    > [<ffffffff8024be72>] tick_periodic+0x68/0x74
    > [<ffffffff8024be9f>] tick_handle_periodic+0x21/0x66
    > [<ffffffff8021bcd2>] smp_apic_timer_interrupt+0x8a/0xa8
    > [<ffffffff8020bfe6>] apic_timer_interrupt+0x66/0x70
    > <EOI> [<ffffffff803adb39>] ? acpi_safe_halt+0x2b/0x3e
    > [<ffffffff803adbfa>] ? acpi_idle_enter_c1+0xae/0x102
    > [<ffffffff804ffdd6>] ? cpuidle_idle_call+0x70/0xa2
    > [<ffffffff8020a097>] ? cpu_idle+0x7e/0x9c
    > [<ffffffff805bef4a>] ? start_secondary+0x157/0x15c
    >
    > Timer issue?
    >
    >
    > -Andi


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