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

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Date:	Sun, 5 Jun 2011 10:17:47 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Arne Jansen <lists@...-jansens.de>,
	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()

* 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.

Arne, you can turn this on via one of these:

  earlyprintk=vga,keep
  earlyprintk=serial,ttyS0,115200,keep

(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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