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Date:	Wed, 08 Aug 2012 10:20:03 +0800
From:	Michael Wang <>
To:	Sasha Levin <>
CC:	John Stultz <>, Avi Kivity <>,,
	"" <>,,,,, Dave Jones <>
Subject: Re: rcu: INFO: rcu_preempt detected stalls on CPUs/tasks on v3.6

On 08/07/2012 06:24 PM, Sasha Levin wrote:
> On 08/07/2012 07:40 AM, John Stultz wrote:
>> On 08/06/2012 11:28 AM, Sasha Levin wrote:
>>> On 08/06/2012 08:20 PM, John Stultz wrote:
>>>> On 08/06/2012 10:21 AM, John Stultz wrote:
>>>>> On 08/05/2012 09:55 AM, Sasha Levin wrote:
>>>>>> On 07/30/2012 03:17 PM, Avi Kivity wrote:
>>>>>>> Possible causes:
>>>>>>>    - the APIC calibration in the guest failed, so it is programming too
>>>>>>> low values into the timer
>>>>>>>    - it actually needs 1 us wakeups and then can't keep up (esp. as kvm
>>>>>>> interrupt injection is slowing it down)
>>>>>>> You can try to find out by changing
>>>>>>> arch/x86/kvm/lapic.c:start_lapic_timer() to impose a minimum wakeup of
>>>>>>> (say) 20 microseconds which will let the guest live long enough for you
>>>>>>> to ftrace it and see what kind of timers it is programming.
>>>>>> I've kept trying to narrow it down, and found out It's triggerable using adjtimex().
>>>> Sorry, one more question: Could you provide details on how is it trigger-able using adjtimex?
>>> It triggers after a while of fuzzing using trinity of just adjtimex ('./trinity --quiet -l off -cadjtimex').
>>> Trinity is available here: .
>>> Let me know if I can help further with reproducing this, I can probably copy over my testing environment to some other host if you'd like.
>> Ok. Finally I *think* got it reproduced.  (Had some trouble initially, as I think since the first time I ran it as a normal user, the socket cache isn't the same as if you run it the first time as root? Anyway, after doing a make clean and rebuilding it started to trigger).
>> I'm not seeing the rcu stall message, but I do manage to trigger two other behaviors: a hard hang and a sort of zombie state where memory isn't properly being freed & everything starts segfaulting.   So this may not be the exact same issue, but it triggers quickly as you described (within a few seconds of running trinity as root).
>> It looks like both of these issues are caused by adjtimex(ADJ_SETOFFSET), which  adds or subtracts a huge offset and that either goes negative or gets clamped to a ktime_t at KTIME_MAX (if you get clamped the system hangs, if it goes negative, the system barely functions, but sort of drags along).
>> An updated version of my KTIME_MAX sanity checking patch to handle both of these conditions is below.
>> Would you mind giving this patch a shot and letting me know if you still see problems?
> This fixes the problem for me. Thanks!

Could you please also try to reduce the CONFIG_RCU_CPU_STALL_TIMEOUT
from 60 to 3 and see whether the rcu stall info will show again?(sorry I
can't do it by my self since the stall info never appear in my sight...)

That will be very helpful as I suppose it is time to recover
CONFIG_RCU_CPU_STALL_TIMEOUT to a reasonable value now ;-)

Michael Wang

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