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

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Message-ID: <20130515162654.GR23604@redhat.com>
Date:	Wed, 15 May 2013 12:26:54 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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, May 15, 2013 at 11:27:02AM -0400, Steven Rostedt 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.
> 
> 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.

Ok.  Thanks.  I don't know what the rotate list is for (nor what it does
in general).  But I'll poke around.

Cheers,
Don
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