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Date: Tue, 31 Mar 2015 09:25:02 +0200
From: Ingo Molnar <mingo@kernel.org>
To: cmetcalf@ezchip.com
Cc: Andrew Morton <akpm@linux-foundation.org>, Don Zickus <dzickus@redhat.com>,
        Andrew Jones <drjones@redhat.com>,
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Subject: Re: [PATCH] watchdog: nohz: don't run watchdog on nohz_full cores
Message-ID: <20150331072502.GA16754@gmail.com>
References: <1427741465-15747-1-git-send-email-cmetcalf@ezchip.com>
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* cmetcalf@ezchip.com <cmetcalf@ezchip.com> wrote:

> From: Chris Metcalf <cmetcalf@ezchip.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@ezchip.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