linux-kernel - Re: rcu: INFO: rcu_preempt detected stalls on CPUs/tasks on v3.6

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Message-ID: <501FFD2A.4010905@us.ibm.com>
Date:	Mon, 06 Aug 2012 10:21:46 -0700
From:	John Stultz <johnstul@...ibm.com>
To:	Sasha Levin <levinsasha928@...il.com>
CC:	Avi Kivity <avi@...hat.com>, paulmck@...ux.vnet.ibm.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	mingo@...nel.org, a.p.zijlstra@...llo.nl, prarit@...hat.com,
	tglx@...utronix.de
Subject: Re: rcu: INFO: rcu_preempt detected stalls on CPUs/tasks on v3.6

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().
>
> At that point I've bisected it, and got the following commit (parties Cc'ed):
>
> commit 5baefd6d84163443215f4a99f6a20f054ef11236
> Author: John Stultz <johnstul@...ibm.com>
> Date:   Tue Jul 10 18:43:25 2012 -0400
>
>      hrtimer: Update hrtimer base offsets each hrtimer_interrupt
>
>
> I've also confirmed that reverting the commit above on top of linux-next indeed fixes the issue.
Hey Sasha,
     Thanks for the heads up.  I don't have a clear sense of what could 
be wrong here yet, but if you see this with 3.6-rc but not 3.5,  could 
you try the fix(1d17d17484d40f2d5b35c79518597a2b25296996) Ingo just made 
on tip/timers/urgent?

thanks
-john



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