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

[Chris Metcalf <cmetcalf@ezchip.com>]

On 03/31/2015 03:25 AM, Ingo Molnar wrote:
> * 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.
>>
>> [...]
> 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.

That's certainly a reasonable thing to do; it looks like just an #ifdef
at the top of watchdog.c would suffice.  Does this look right?

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 8a46d9d8a66f..c8555c211e65 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -25,7 +25,11 @@
  #include <linux/kvm_para.h>
  #include <linux/perf_event.h>
  
+#ifdef CONFIG_NO_HZ_FULL
+int watchdog_user_enabled = 0;
+#else
  int watchdog_user_enabled = 1;
+#endif
  int __read_mostly watchdog_thresh = 10;
  #ifdef CONFIG_SMP
  int __read_mostly sysctl_softlockup_all_cpu_backtrace;

It doesn't look like I need to do anything else special to disable
HARDLOCKUP_DETECTOR, and khungtaskd can happily run on
a non-nohz core, so that should be OK.

What I was trying to achieve with my proposed patch was kind
of orthogonal: to allow the watchdog to run on standard cores,
but not run on nohz cores, so we could benefit from it on the
cores where it was safe for it to run.  Do you see value in this,
or better to just enable/disable all watchdog threads collectively?

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com