linux-kernel - Re: [tip:sched/locking] sched: Add p->pi_lock to task_rq

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Date:	Sun, 05 Jun 2011 12:22:16 +0200
From:	Arne Jansen <lists@...-jansens.de>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	efault@....de, npiggin@...nel.dk, akpm@...ux-foundation.org,
	frank.rowand@...sony.com, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/locking] sched: Add p->pi_lock to task_rq_lock()

On 05.06.2011 11:55, Ingo Molnar wrote:
>
> * Arne Jansen<lists@...-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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