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

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Date:	Sun, 05 Jun 2011 10:53:43 +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 10:17, Ingo Molnar wrote:
>
> * Peter Zijlstra<peterz@...radead.org>  wrote:
>
>> On Fri, 2011-06-03 at 12:02 +0200, Arne Jansen wrote:
>>> On 03.06.2011 11:15, Peter Zijlstra wrote:
>>
>>>> Anyway, Arne, how long did you wait before power cycling the box? The
>>>> NMI watchdog should trigger in about a minute or so if it will trigger
>>>> at all (its enabled in your config).
>>>
>>> No, it doesn't trigger,
>>
>> Bummer.
>
> Is there no output even when the console is configured to do an
> earlyprintk? That will allow the NMI watchdog to punch through even a
> printk or scheduler lockup.
>

Just to be clear, I have no boot problems whatsoever. And I have no
problems with the serial console. It's just the regular printk locking
up when e.g. I load the test module.

> Arne, you can turn this on via one of these:
>
>    earlyprintk=vga,keep

I don't have access to vga as it is a remote machine.

>    earlyprintk=serial,ttyS0,115200,keep

I'll try that.

>
> (the ',keep' portion is important to have it active even after the
> regular console has been switched on.)
>
> Could you also please check with the (untested) patch below applied?
> This will turn off *all* printk done by the NMI watchdog and switches
> it to do pure early_printk() - which does not use any locking so it
> should never lock up.
>
> [ If you keep seeing 'NMI watchdog tick' messages periodically
>    occuring after the lockup then i'll send a more complete patch that
>    shuts off the regular printk path and makes sure that all output is
>    early_printk() based only. ]
>
> earlyprintk=,keep with such a patch has let me down only on the
> rarest of occasions.
>
> ( 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. )
>
> Thanks,
>
> 	Ingo
>
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 3d0c56a..7c7e33f 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -234,15 +234,12 @@ static void watchdog_overflow_callback(struct perf_event *event, int nmi,
>   		if (__this_cpu_read(hard_watchdog_warn) == true)
>   			return;
>
> -		if (hardlockup_panic)
> -			panic("Watchdog detected hard LOCKUP on cpu %d", this_cpu);
> -		else
> -			WARN(1, "Watchdog detected hard LOCKUP on cpu %d", this_cpu);
> -
>   		__this_cpu_write(hard_watchdog_warn, true);
>   		return;
>   	}
>
> +	early_printk("NMI watchog tick %ld\n", jiffies);
> +
>   	__this_cpu_write(hard_watchdog_warn, false);
>   	return;
>   }

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