Re: [PATCH v10 2/3] watchdog: add watchdog_cpumask sysctl to assist nohz

Don Zickus
2015-04-30T20:01:40Z

On Thu, Apr 30, 2015 at 03:39:25PM -0400, Chris Metcalf wrote:
> Change the default behavior of watchdog so it only runs on the
> housekeeping cores when nohz_full is enabled at build and boot time.
> Allow modifying the set of cores the watchdog is currently running
> on with a new kernel.watchdog_cpumask sysctl.
>
> In the current system, the watchdog subsystem runs a periodic timer
> that schedules the watchdog kthread to run. However, nohz_full cores
> are designed to allow userspace application code running on those cores
> to have 100% access to the CPU. So the watchdog system prevents the
> nohz_full application code from being able to run the way it wants to,
> thus the motivation to suppress the watchdog on nohz_full cores,
> which this patchset provides by default.
>
> However, if we disable the watchdog globally, then the housekeeping
> cores can't benefit from the watchdog functionality. So we allow
> disabling it only on some cores. See Documentation/lockup-watchdogs.txt
> for more information.
>
> Acked-by: Don Zickus <dzickus@redhat.com>
> Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
> ---
> Documentation/lockup-watchdogs.txt | 18 +++++++++++
> Documentation/sysctl/kernel.txt | 21 +++++++++++++
> include/linux/nmi.h | 3 ++
> kernel/sysctl.c | 7 +++++
> kernel/watchdog.c | 64 +++++++++++++++++++++++++++++++++++---
> 5 files changed, 108 insertions(+), 5 deletions(-)
>

<snip>

> @@ -697,8 +704,12 @@ static int watchdog_enable_all_cpus(void)
> err = smpboot_register_percpu_thread(&watchdog_threads);
> if (err)
> pr_err("Failed to create watchdog threads, disabled\n");
> - else
> + else {
> + if (smpboot_update_cpumask_percpu_thread(
> + &watchdog_threads, &watchdog_cpumask))
> + pr_err("Failed to set cpumask for watchdog threads\n");

Stupid nitpick, this error message tells us the 'watchdog' threads caused
the cpumask failure, but ....

> watchdog_running = 1;
> + }
> } else {
> /*
> * Enable/disable the lockup detectors or
> @@ -869,12 +880,55 @@ out:
> mutex_unlock(&watchdog_proc_mutex);
> return err;
> }
> +
> +/*
> + * The cpumask is the mask of possible cpus that the watchdog can run
> + * on, not the mask of cpus it is actually running on. This allows the
> + * user to specify a mask that will include cpus that have not yet
> + * been brought online, if desired.
> + */
> +int proc_watchdog_cpumask(struct ctl_table *table, int write,
> + void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> + int err;
> +
> + mutex_lock(&watchdog_proc_mutex);
> + err = proc_do_large_bitmap(table, write, buffer, lenp, ppos);
> + if (!err && write) {
> + /* Remove impossible cpus to keep sysctl output cleaner. */
> + cpumask_and(&watchdog_cpumask, &watchdog_cpumask,
> + cpu_possible_mask);
> +
> + if (watchdog_enabled && watchdog_thresh) {
> + /*
> + * Failure would be due to being unable to allocate
> + * a temporary cpumask, so we are likely not in a
> + * position to do much else to make things better.
> + */
> + if (smpboot_update_cpumask_percpu_thread(
> + &watchdog_threads, &watchdog_cpumask) != 0)
> + pr_err("cpumask update failed\n");

This one does not. :-( If there is a respin, I would suggest copying the
above message down here.

Cheers,
Don