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

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Date:	Wed, 15 May 2013 10:37:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	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>, dzickus@...hat.com
Subject: Re: [PATCH 1/2] nohz: Disable LOCKUP_DETECTOR when NO_HZ_FULL is
 enabled

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

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