Re: [3.14] core onlining/hotplug regression

[Daniel J Blueman]

On 07/25/2014 05:05 PM, Thomas Gleixner wrote:
> On Fri, 25 Jul 2014, Daniel J Blueman wrote:
> > On a larger x86 system with 1728 cores, 3.15(.6) asserts on
> > smpboot_thread_fn's td->cpu != smp_processor_id() consistently after ~1500
> > cores are online.
> >
> > Reverting the only directly related changes I could find [1,2] doesn't help.
> > Debugging indicates there is a race where the created thread is quickly
> > migrated to core 0 when this occurs, since smp_processor_id returns 0 in these
> > cases. Thomas introduced a thread parked state to fix related issues a year
> > back. Linux 3.14(.13) boots just nice.
>
> Weird. Commits [1,2] are definitely not the culprits.
>
> > Full boot output is at:
> > https://resources.numascale.com/linux-315-thread-mig.txt
>
> Not really helpful, as we don't see what causes it. We just see the
> wreckage.
>
> > Any theories so far? I'll start bisecting when I have full access to the
> > system again in a week and I'll do some more debugging with intermittent
> > access before then.
>
> One thing you could try is enabling tracing.
>
>      "ftrace=function ftrace_dump_on_oops"
>
> It'll take a looooong time to spill out the traces, but that should
> give us the root cause precisely.

It turns out that bisecting led to Lai's patch "Fix hotplug vs. 
set_cpus_allowed_ptr()" [1]. Reverting it prevents the smpboot.c 
BUG_ON(td->cpu != smp_processor_id()) in smpboot_thread_fn from tripping.

-- [1]

commit 6acbfb96976fc3350e30d964acb1dbbdf876d55e
Author: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
Date:   Fri May 16 11:50:42 2014 +0800

    sched: Fix hotplug vs. set_cpus_allowed_ptr()

    Lai found that:

      WARNING: CPU: 1 PID: 13 at arch/x86/kernel/smp.c:124 
native_smp_send_reschedule+0x2d/0x4b()
      ...
      migration_cpu_stop+0x1d/0x22

    was caused by set_cpus_allowed_ptr() assuming that cpu_active_mask is
    always a sub-set of cpu_online_mask.

    This isn't true since 5fbd036b552f ("sched: Cleanup cpu_active 
madness").

    So set active and online at the same time to avoid this particular
    problem.

    Fixes: 5fbd036b552f ("sched: Cleanup cpu_active madness")
    Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
    Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
    Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
    Cc: Gautham R. Shenoy <ego@xxxxxxxxxxxxxxxxxx>
    Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
    Cc: Michael wang <wangyun@xxxxxxxxxxxxxxxxxx>
    Cc: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
    Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
    Cc: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
    Cc: Toshi Kani <toshi.kani@xxxxxx>
    Link: http://lkml.kernel.org/r/53758B12.8060609@xxxxxxxxxxxxxx
    Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
--
Daniel J Blueman
Principal Software Engineer, Numascale
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