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

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Message-ID: <20130520175229.GI133453@redhat.com>
Date:	Mon, 20 May 2013 13:52:29 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.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 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?

Cheers,
Don

> 
> Reported-by: Steven Rostedt <rostedt@...dmis.org>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Li Zhong <zhong@...ux.vnet.ibm.com>
> Cc: Don Zickus <dzickus@...hat.com>
> ---
>  kernel/watchdog.c |    6 ++++++
>  1 files changed, 6 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 05039e3..7e1a021 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -543,6 +543,12 @@ static struct smp_hotplug_thread watchdog_threads = {
>  
>  void __init lockup_detector_init(void)
>  {
> +#ifdef CONFIG_NO_HZ_FULL
> +	watchdog_enabled = 0;
> +	watchdog_disabled = 1;
> +	pr_warning("Disabled lockup detectors by default because of full dynticks\n");
> +	pr_warning("You can overwrite that with 'sysctl -w kernel.watchdog=1'\n");
> +#endif
>  	set_sample_period();
>  	if (smpboot_register_percpu_thread(&watchdog_threads)) {
>  		pr_err("Failed to create watchdog threads, disabled\n");
> -- 
> 1.7.5.4
> 
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