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

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Date:	Wed, 15 May 2013 11:27:02 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.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, 2013-05-15 at 11:06 -0400, Don Zickus wrote:

> > 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*.

Well, when NO_HZ_FULL is set, it covers no code :-)

> > 
> > 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

Yeah, just disabling it via run time might work.


> 
> 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.

Right now perf requires a tick, not sure exactly why, but you can look
at the code in perf_event_task_tick(). Thus if NO_HZ_FULL sees that a
perf tick is pending, it won't disable ticks. Unfortunately, the
watchdogs, both NMI and soft lockup, use the perf infrastructure to
trigger NMIs or interrupts. This adds a perf element on the rotate list
and keeps NO_HZ_FULL from *ever* activating.

-- Steve


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