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

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Date:	Wed, 15 May 2013 18:59:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Don Zickus <dzickus@...hat.com>,
	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:
> 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.
> 

Hmm.. Stephane had a bunch of patches converting the rotation thing to
an hrtimer. I seem to have forgotten what happened to them but I can't
seem to find them merged.

I'll go look.

That leaves the frequency stuff, but the watchdog doesn't use that.

At which point we could run the watchdog without perf_event_task_tick().
> 
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