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Date: Mon, 30 Mar 2015 15:12:45 -0400
From: Don Zickus <dzickus@redhat.com>
To: cmetcalf@ezchip.com
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Andrew Jones <drjones@redhat.com>,
        chai wen <chaiw.fnst@cn.fujitsu.com>, Ingo Molnar <mingo@kernel.org>,
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        Gilad Ben-Yossef <gilad@benyossef.com>,
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        open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] watchdog: nohz: don't run watchdog on nohz_full cores
Message-ID: <20150330191245.GO162412@redhat.com>
References: <1427741465-15747-1-git-send-email-cmetcalf@ezchip.com>
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On Mon, Mar 30, 2015 at 02:51:05PM -0400, 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.

Hi Chris,

It seems like the correct solution would be to hook into the idle_loop
somehow.  If the cpu is idle, then it seems unlikely that a lockup could
occur.

My fear with this apporach is a lockup would occur on the nohz cpu and it
would go undetected because that cpu is disabled.  Further no printk is
thrown out to even indicate a cpu is disabled making it more difficult to
debug.

Cheers,
Don

> 
> 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);
>  
> -- 
> 2.1.2
>