linux-kernel - Re: [PATCH 3/8] watchdog: Boot-disable by default on full dynticks

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Date:	Mon, 20 May 2013 20:14:26 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Don Zickus <dzickus@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>
Subject: Re: [PATCH 3/8] watchdog: Boot-disable by default on full dynticks

On Mon, May 20, 2013 at 01:52:29PM -0400, Don Zickus wrote:
> On Mon, May 20, 2013 at 06:01:51PM +0200, Frederic Weisbecker wrote:
> > When the watchdog runs, it prevents the full dynticks
> > CPUs from stopping their tick because the hard lockup
> > detector uses perf events internally, which in turn
> > rely on the periodic tick.
> > 
> > Since this is a rather confusing behaviour that is not
> > easy to track down and identify for those who want to
> > test CONFIG_NO_HZ_FULL, let's default disable the
> > watchdog on boot time when full dynticks is enabled.
> > 
> > The user can still enable it later on runtime using
> > proc or sysctl.
> 
> I thought Peter committed a patch to perf so that this isn't needed any
> more?

There is the patchset that converts perf_event_task_tick() to an hrtimer.
But I'm not sure it covers everything. Also this will be for the next merge
window. And in the end this moves the problem elsewhere: we still have timer
interrupts, they just come as specific hrtimers, although may be less often,
I haven't looked yet at the patchset.
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