Re: [PATCH] watchdog: nohz: don't run watchdog on nohz_full cores

Ingo Molnar
2015-03-31T07:25:10Z

* 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