Re: [3.14] core onlining/hotplug regression

[Daniel J Blueman <daniel@numascale.com>]

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@cn.fujitsu.com>
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@cn.fujitsu.com>
     Signed-off-by: Peter Zijlstra <peterz@infradead.org>
     Cc: Andrew Morton <akpm@linux-foundation.org>
     Cc: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
     Cc: Linus Torvalds <torvalds@linux-foundation.org>
     Cc: Michael wang <wangyun@linux.vnet.ibm.com>
     Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
     Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
     Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
     Cc: Toshi Kani <toshi.kani@hp.com>
     Link: http://lkml.kernel.org/r/53758B12.8060609@cn.fujitsu.com
     Signed-off-by: Ingo Molnar <mingo@kernel.org>
-- 
Daniel J Blueman
Principal Software Engineer, Numascale