Re: [PATCH v9 2/3] watchdog: add watchdog_cpumask sysctl to assist nohz

[Andrew Morton]

On Fri, 17 Apr 2015 14:37:17 -0400 Chris Metcalf <cmetcalf@xxxxxxxxxx> 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.
> 
> If we allowed the watchdog to run on nohz_full cores, the timer
> interrupts and scheduler work would prevent the desired tickless
> operation on those cores.  But 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.

Could you please expand on the patch motivation?  "would prevent the
desired tickless operation on those cores" is quite vague.

Exactly what userspace-visible problem is the current implementation
causing and how does the patchset improve things?

If any of this is quantifiable (wakeups/sec, joules/hour etc) then some
attempt to perform those measurements would also be useful.



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