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

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Message-ID: <20130515165542.GD13916@laptop.home>
Date:	Wed, 15 May 2013 18:55:42 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Don Zickus <dzickus@...hat.com>
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 11:06:53AM -0400, Don Zickus wrote:
> 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.

Yah, right :-) So I think what happens is that when we enter kernel
space for whatever reason (say NMIs for a watchdog), we kill the magic
NO_HZ state that allows a single task that's stuck in userspace to
effectively have the tick disabled.

But yeah, the initial patch was a tad light on detail, so I'll have to
let Steve and Frederic expand / correct.
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