Message-ID: <4DEB58D8.4000805@die-jansens.de>
Date: Sun, 05 Jun 2011 12:22:16 +0200
From: Arne Jansen <lists@die-jansens.de>
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To: Ingo Molnar <mingo@elte.hu>
CC: Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>, mingo@redhat.com,
        hpa@zytor.com, linux-kernel@vger.kernel.org, efault@gmx.de,
        npiggin@kernel.dk, akpm@linux-foundation.org, frank.rowand@am.sony.com,
        tglx@linutronix.de, linux-tip-commits@vger.kernel.org
Subject: Re: [tip:sched/locking] sched: Add p->pi_lock to task_rq_lock()
References: <4DE674EB.1000200@die-jansens.de> <1306951751.2497.626.camel@laptop> <1306953870.2497.627.camel@laptop> <4DE6936F.7090700@die-jansens.de> <BANLkTimXXR1mJDtdCsbXF410sd3irFSitQ@mail.gmail.com> <1307092535.2353.2973.camel@twins> <4DE8B13D.9020302@die-jansens.de> <1307097052.2353.3061.camel@twins> <20110605081747.GA17920@elte.hu> <4DEB4FA7.3050400@die-jansens.de> <20110605095555.GA22058@elte.hu>
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On 05.06.2011 11:55, Ingo Molnar wrote:
>
> * Arne Jansen<lists@die-jansens.de>  wrote:
>
>>> ( Arne, please also double check on a working bootup that the NMI
>>>    watchdog is actually ticking, by checking the NMI counts in
>>>    /proc/interrupts go up slowly but surely on all CPUs. )
>>
>> It does, but _very_ slowly. Some CPUs do not count up for tens of
>> minutes if the machine is idle. If I generate some load like 'make
>> tags', the counters go up quite quickly.
>> After 4 minutes and one 'make cscope' it looks like this:
>> NMI:          8         13         43          5          2
>> 3        22          1   Non-maskable interrupts
>>
>> But I never see a single tick on console or in dmesg, even when I
>> replace the early_printk with a printk.
>
> hm, that might be because the NMI watchdog uses halted cycles to
> tick.
>
> That's not a problem (the kernel cannot lock up while there are no
> cycles ticking) but nevertheless could you work this around please
> by starting 8 infinite shell loops:
>
>     for ((i=0; i<8; i++)); do while : ; do : ; done&  done
>
> ?
>
> This will saturate all cores and makes sure the NMI watchdog is
> ticking everywhere.
>
> Hopefully this wont make the bug go away :-)
>

OK, now we get going. I get the ticks, the bug is still there, and
all CPUs still tick after the lockup. I also added an early_printk
inside the lockup-if, and it reports hard lockups. At first for only
one or 2 CPUs, and after some time all CPUs are locked up.

-Arne
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