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

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Date:	Sun, 5 Jun 2011 11:45:50 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Arne Jansen <lists@...-jansens.de>
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()


* Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Arne Jansen <lists@...-jansens.de> wrote:
> 
> > 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.
> 
> Yes.
> 
> > > 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.
> 
> Please don't forget:
> 
> > > 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
> 
> if you get a lockup somewhere within printk then the NMI watchdog 
> will lock up.

Please use the updated patch below - the first one wasnt informative 
enough and it would stop 'ticking' after a hard lockup - not good :-)

With the patch below applied you should get periodic printouts from 
the NMI watchdog both before and after the hard lockup.

If the NMI watchdog does not stop ticking after the lockup i'll send 
a more complete patch that allows the printout of a backtrace on 
every CPU, after the lockup.

Thanks,

	Ingo
--
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 3d0c56a..d335bc7 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -216,6 +216,8 @@ static void watchdog_overflow_callback(struct perf_event *event, int nmi,
 	/* Ensure the watchdog never gets throttled */
 	event->hw.interrupts = 0;
 
+	early_printk("CPU #%d NMI watchdog tick %ld\n", smp_processor_id(), jiffies);
+
 	if (__this_cpu_read(watchdog_nmi_touch) == true) {
 		__this_cpu_write(watchdog_nmi_touch, false);
 		return;
@@ -234,11 +236,6 @@ 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;
 	}
--
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