linux-kernel - Re: [PATCH] watchdog: nohz: don't run watchdog on nohz

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Message-ID: <20150331072502.GA16754@gmail.com>
Date:	Tue, 31 Mar 2015 09:25:02 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	cmetcalf@...hip.com
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Don Zickus <dzickus@...hat.com>,
	Andrew Jones <drjones@...hat.com>,
	chai wen <chaiw.fnst@...fujitsu.com>,
	Ulrich Obergfell <uobergfe@...hat.com>,
	Fabian Frederick <fabf@...net.be>,
	Aaron Tomlin <atomlin@...hat.com>,
	Ben Zhang <benzh@...omium.org>,
	Christoph Lameter <cl@...ux.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] watchdog: nohz: don't run watchdog on nohz_full cores


* cmetcalf@...hip.com <cmetcalf@...hip.com> wrote:

> From: Chris Metcalf <cmetcalf@...hip.com>
> 
> Running watchdog can be a helpful debugging feature on regular
> cores, but it's incompatible with nohz_full, since it forces
> regular scheduling events.  Accordingly, just exit out immediately
> from any nohz_full core.
> 
> An alternate approach would be to add a flags field or function to
> smp_hotplug_thread to control on which cores the percpu threads
> are created, but it wasn't clear that much mechanism was useful.
> 
> Signed-off-by: Chris Metcalf <cmetcalf@...hip.com>
> ---
>  kernel/watchdog.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 3174bf8e3538..8a46d9d8a66f 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -19,6 +19,7 @@
>  #include <linux/sysctl.h>
>  #include <linux/smpboot.h>
>  #include <linux/sched/rt.h>
> +#include <linux/tick.h>
>  
>  #include <asm/irq_regs.h>
>  #include <linux/kvm_para.h>
> @@ -431,6 +432,10 @@ static void watchdog_enable(unsigned int cpu)
>  	hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
>  	hrtimer->function = watchdog_timer_fn;
>  
> +	/* nohz_full cpus do not do watchdog checking. */
> +	if (tick_nohz_full_cpu(cpu))
> +		do_exit(0);
> +
>  	/* Enable the perf event */
>  	watchdog_nmi_enable(cpu);

So what happens if someone wants to enable the lockup detector, with a 
long timeout, even on nohz-full CPUs? This patch makes that 
impossible.

A better solution would be to tweak the defaults:

 - to default the watchdog(s) to disabled when nohz-full is
   enabled, even if HARDLOCKUP_DETECTOR=y or DETECT_HUNG_TASK=y, and
   allow it to be re-enabled via its sysctl.

Thanks,

	Ingo
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