linux-kernel - Re: [PATCH 1/2] nohz: Disable LOCKUP_DETECTOR when NO_HZ

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Date:	Wed, 15 May 2013 11:06:53 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/2] nohz: Disable LOCKUP_DETECTOR when NO_HZ_FULL is
 enabled

On Wed, May 15, 2013 at 10:37:29AM +0200, Peter Zijlstra wrote:
> On Tue, May 14, 2013 at 06:02:51PM +0200, Frederic Weisbecker wrote:
> > From: Steven Rostedt <rostedt@...dmis.org>
> > 
> > Trying to test the nohz_full code, I was not able to get it to work.
> > Finally I enabled the tick_stop tracepoint and it showed:
> > 
> >  tick_stop:            success=no msg=perf events running
> > 
> > I talked to Frederic Weisbecker about this and he informed me that
> > perf is used by the lockup detector. I checked the code, and sure
> > enough it is.
> > 
> > As perf is always running when LOCKUP_DETECTOR is enabled, which
> > will always disable nohz_full from working, instead of confusing
> > users, disable LOCKUP_DETECTOR when NO_HZ_FULL is enabled.
> > 
> > When perf is changed such that it does not prevent nohz_full from
> > working, then we can and should remove this constraint.
> 
> That's a bit contradictory in function, you want the NMI watchdog to
> cover all code, so disabling whilst entering NO_HZ state is going to
> make it not cover some code - *fail*.
> 
> Rather I would suggest disabling the NMI watchdog's runtime default; so
> you can still enable it with something like: 
> 
>   echo 1 > /proc/sys/kernel/nmi_watchdog

Coming into the middle of the thread is always hard, but why/how does perf
disable nohz_full?  I didn't think the hardware events of perf would cause
problems as they are no different than an irq.  Curious.

Cheers,
Don
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