linux-kernel - Re: [PATCH 4/6] watchdog: Boot-disable by default on full dynticks

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Date:	Wed, 12 Jun 2013 13:03:16 -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>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Anish Singh <anish198519851985@...il.com>
Subject: Re: [PATCH 4/6] watchdog: Boot-disable by default on full dynticks

On Wed, Jun 12, 2013 at 04:02:36PM +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 we had a conversation awhile ago, where we agreed this was going
to be fixed for 3.11?  Didn't Peter find the patch and apply it to his
tree?  I am confused why this is still needed?

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>
> Cc: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
> Cc: Anish Singh <anish198519851985@...il.com>
> ---
>  kernel/watchdog.c |    8 ++++++++
>  1 files changed, 8 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 52c9a9b..277d713 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -553,6 +553,14 @@ void __init lockup_detector_init(void)
>  {
>  	set_sample_period();
>  
> +#ifdef CONFIG_NO_HZ_FULL
> +	if (watchdog_enabled) {
> +		watchdog_enabled = 0;
> +		pr_warning("Disabled lockup detectors by default for full dynticks\n");
> +		pr_warning("You can reactivate it with 'sysctl -w kernel.watchdog=1'\n");
> +	}
> +#endif
> +
>  	if (watchdog_enabled)
>  		watchdog_enable_all_cpus();
>  }
> -- 
> 1.7.5.4
> 
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